Women and Exercise - Physiology and Sports Medicine

Endurance Training 77 letes at the same level of competition, the • Changes in running performance with variance in maximal aerobic power be- training occur without equivalent changes tween them is small. in Vo2max. • Oxygen consumption increases as a func- Noakes's data suggest that a good predic- tion of velocity in all endurance events. Although athletes vary somewhat in their tor of endurance performance is peak tread- efficiencies, the variance between them is mill velocity. He hypothesized that maxi- small. mum speed may be related to the muscles' Even though a high Vo2max is important for capacity for high cross-bridge cycling and achieving superior levels of endurance, it is respiratory adaptations. Respiratory adap- not the only requirement for success. tations may make it possible to prevent the Noakes36 has questioned the validity of onset of exercise-induced dyspnea. Vo2max as a predictor of endurance perfor- mance. His reservations are based on these Mltochondrial Density observations: Mitochondrial density is a better predic- • Much of the evidence of an oxygen limi- tation during exercise is circumstantial. tor of endurance capacity than Vo2max. En- Noakes analyzed the data of the classic durance is the ability to sustain a particular studies that established Vo2max as a lab- submaximal level of physical effort. Davies oratory benchmark for cardiovascular and co-workers43 showed that cytochrome performance.40–42 He found that most sub- oxidase activity (which is directly depen- jects did not show that Vo2leveled off with dent upon mitochondrial mass) had a cor- increasing intensity of exercise at maxi- relation coefficient of 0.92 with running en- mum. durance but only 0.70 with Vo2max. With • Studies have used transfusion or O2 training, Vo2max increases by less than20% breathing in an attempt to show that O2 in most people, but the ability to sustain a transport is limiting. None of these stud- given submaximal exercise intensity may in- ies have demonstrated that their subjects crease by much more. Endurance perfor- reached a plateau in Vo2 during normalex- mance by athletes in sports such as cycling, ercise. There was no evidence of an O2 running, swimming, and cross-country ski- transport limitation before the experi- ing requires intense effort and maintenance mental intervention. of that intensity for a long time. Increased • In blood doping studies, there is a disso- mitochondrial density may be the key factor ciation between changes in Vo2max and in endurance. It may allow some athletes to performance. Performance changes last run, cycle, or swim at high velocities for only a few days, while changes in Vo2max longer than others, even though their maxi- last longer. mal oxygen uptakes are similar to those of • Exercise at extreme altitudes is not lim- slower athletes. ited by high blood lactate levels or by in- dications of central limitations in cardiac Endurance training results in an in- or respiratory function. creased mitochondrial density in both fast- • Exhaustion during maximal exercise oc- twitch and slow-twitch muscle fibers.44 This curs at a lower oxygen consumptiondur- probably plays a major role in improving en- ing cycling than during running in the durance. There are several possible mecha- same subjects. nisms. Increased mitochondrial mass may • Blood lactate levels at exhaustion during increase fat utilization during exercise and progressive treadmill exercise testing are thus spare muscle glycogen. It also may im- lowest in elite athletes. prove muscle lactic acid clearance capacity, allowing exercise at a higher intensity.44,45 A fundamental purpose of energy metab-

78 Basic Concepts of Exercise Physiology olism during exercise is to generate ATP to • During sustained exercise, lactate produc- meet the demands of the exercise intensity. tion and removal occur simultaneously A deficit in ATP causes the athlete to fatigue within active muscle. quickly. The rate of ATPformation is critical. Fat provides the most energy per gram.Car- • Most lactic acid produced during exercise bohydrate is the most important fuel for is oxidized. high-intensity endurance exercise, how- ever, because it provides the most ATP per • During endurance exercise, the turnover liter of oxygen. Thus, carbohydrate pro- and oxidation rates of lactate exceed vides ATP more quickly than does fat.46 those of glucose. At least two problems are associated with • Lactate production during both rest and the use of carbohydrates during endurance exercise is not necessarily associated exercise: with muscle anaerobiosis. • Training mainly affects the rate of lactate removal rather than its production. • The supply of carbohydrates islimited. The effects of the increased mitochon- • The rapid use of carbohydrates during drial mass with trainingare complex but el- egant. Glycogen is the critical fuel for endur- high-intensity exercise results in a rate of ance exercise. However, its use increases lactic acid production greater than its rate the risk of its own depletion and lactic acid of clearance. Accumulation of lactic acid accumulation due to an excess of lactic acid may interfere with muscle contraction production over clearance. The increased and energy metabolism.47,48 mitochondrial mass that results from train- Increasing muscle mitochondrial mass may ing prevents lactic acid accumulation. It help the body to cope with both of these does this by providing the muscles with an problems. increased capacity for lactic acid oxidation. The glycogen content of muscle is impor- It also prevents glycogen depletion by al- tant in endurance capacity. When glycogen lowing an increased use of fats as fuel. is depleted, fatigue results. During sustained exercise, muscle glycogen is the muscle's Nevertheless, training is probably not as principal source of carbohydrate.45 In addi- important as genetics for obtaining a high tion, the rate of glycogen utilization in- mitochondrial mass in the muscles required creases as a function of exercise intensity. It for endurance exercise.50 Studies of success- is very important, then, for the athlete to ful male endurance athletes have shown that conserve glycogen to maintainthe intensity they often have a high percentage of slow- of exercise at the desired level. Endurance twitch muscle fibers. A high mitochondrial training, which results in an increased mi- density is a characteristic of these fibers. tochondrial mass, increases the capacity of Tesch and Karlsson51 suggest, however, that the muscle to oxidize fat.49 This slows the the greater percentage of slow-twitch fibers rate of glycolysis and the catabolism of glu- in the active muscles of endurance athletes cose and glycogen. Thus, glycogen is spared may be an adaptive response. As discussed, and fatigue delayed. Vo2max and mitochondrial density are The increased mitochondrial mass ac- highly related. Athleteswhose muscles have companying training may also increase the a high mitochondrial mass also have high muscle's ability to remove lactate through Vo2max values. oxidation. For more than 50 years, lactic acid has been thought of by many as a met- Performance Efficiency abolic pariah. However, research using ra- Although exercise intensity is the most dioactive tracer methodology has demon- strated that lactate is an important substrate important determinant of metabolic rate, in- during exercise:45,50 dividual differences in performance effi- ciency can be responsible for the difference

Endurance Training 79 between winning and losing. When power ulating ventilation, changing body compo- output can be measured accurately, effi- sition, improving training status, and im- ciency can be calculated with the following proving running style.55 equation (Eq. 5-3):52 Other than metabolic considerations, Efficiency = Change in power output (100) Change in caloric equivalent of O2 consumption Efficiency is decreased by energy lost as technique is probably the most important heat, by wasted movement, and by mechan- factor affecting performance efficiency. In ical factors such as wind resistance, friction, swimming, athletes should develop good and drag. The efficiency of walking and cycle hydrodynamics, using strokes that employ ergometry is slightly less than 30%.52,53 It is efficient propulsive force and minimize probable that the efficiency of running,cy- drag. This may contribute almost as much to cling, swimming, and cross-country skiingat success as improving the physiologic as- competitive exercise intensities is less than pects of endurance. Likewise, the frequent that. use of \"skating\" in cross-country skiing has revolutionized the sport. Efficient runners High-intensity exercise is not performed are thought to have a lower vertical compo- at a steady rate. Vo2does not account for all nent in their technique. Efficient cyclists of the ATP supplied during exercise; a por- pedal smoothly at high revolutions per min- tion is supplied through anaerobic glycoly- ute without engaging muscle groups that do sis. Consequently, efficiency cannot be ac- not contribute to pedaling speed.57,58 Wind curately calculated even when power output resistance is also a factor in running and cy- can be measured. cling. It is reduced by wearing clothing that enhances aerodynamics. The relative change in efficiency can be estimated by measuring changes in oxygen Body Composition consumption under different conditions. Vo2 The importance of body composition for measurements can measure the effects of wind resistance, mechanical aids (e.g., toe endurance varies with the sport. In distance clips in cyclingand wax in cross-country ski- running, gravity places a greater load on the ing), and technique. A fundamental problem athlete than in swimming or cycling. Run- is determininghow much of the efficiency is ners are usually leaner than other endur- due to mechanical factors (i.e., technique ance athletes, and there is less variance in and equipment) and how much to physio- body fat among elite performers.59–62Typi- logic factors (i.e., mitochondrial density). cal fat percentages for female endurance For example, if one runner seems more effi- athletes are shown in Table 4-1. Although cient than another, it is difficult to identify the data are limited, all categories of female whether the greater efficiency is due to a endurance athletes are leaner than seden- more efficient running style or to a superior tary women of the same age. Swimmers have lactic acid clearance capacity. more body fat than runners, cyclists, and cross-country skiers. In long-distance swim- In women, running economy (the oxygen mers, a slightly higher fat percentage de- cost of running at a specific speed) has not creases drag in the water and provides in- been shown to be a good predictor of per- sulation against the cold. formance.54 However, when the subject pop- ulation is homogeneous, running economy Tanaka and Matsuura60 reported that an- aids in the prediction of running perfor- thropometric factors accounted for 20% to mance.55,56 At present, the effect of running 40% of the variance in male distance run- economy on performance is not well under- ners. This is comparable to the importance stood. The most promising methods for improving running economy may be manip-

80 Basic Concepts of Exercise Physiology Table 4-1. BODY COMPOSITION OF ELITE fects may include endocrine and reproduc- FEMALE ENDURANCE ATHLETES tive function and bone metabolism. These problems are discussed elsewhere in this Sport Percent Fat volume. Distance running65 15.2 It is possible that the higher percentage of Distance running62 body fat found in female swimmers com- Distance running38 16.9 pared with that of other endurance athletes Distance running54 15.3 may be an advantage. When swimming at Cross-country skiing66 15.4 comparable velocities, women demonstrate Cross-country skiing67 a lower body drag than men,probably due to Cycling20 21.8 more subcutaneous fat. This makes women Swimming68 more efficient at the sport. The ideal fat per- Swimming69 16.1 centage of the female swimmer is also af- Swimming70 15.4 fected by fitness and stroke mechanics, Swimming71 however. Rennie and co-workers64 have hy- Swimming59 18.1 pothesized that women could swim faster Swimming72 17.8 than men if they could develop comparable 13.7 physical capacities. The difference between the sexes in the world record in the 1500- 15.6 meter run is 10%,but there is only a 6% dif- 16.6 ference between them in the 400-meter 21.7 swim. The lower drag among women swim- mers may account for the reduced sex dif- of maximal oxygen consumption. However, ference. Top women swimmers today are remember that correlation coefficients de- swimming faster than did 1972 Olympic scribe relationships. They do not mean that champion Mark Spitz. one factor causes another. These investiga- tors did not study female athletes. Most SEX DIFFERENCES IN studies have found that female distance run- ENDURANCE PERFORMANCE ners average 16% fat. Levels as low as 6% have been reported. Christensen and Women's performance times are 6% to Ruhling38 have found that female marathon 15% slower than men's in most endurance runners continue to become leaner the sports73,74 (Table 4-2). However, there is longer they participate in the sport. Novice considerable variance in performance in marathon runners were found to have 18% specific events. As mentioned, in the 400- fat, experienced marathoners had 16.3%, meter swim, the difference between the and elite marathoners had 15.3%. The aver- men's and women's world record is slightly age body fat percentage of a young adult more than 6%. The difference in the 80-km woman in the United States is 25%. run is almost 44%. Men rode longer dis- In running, cycling,and cross-country ski- tances in the 1988 Olympic cycling road race ing, excess fat increases the energy cost of competition (82 km for women and 196.8 km exercise. The ideal lower limit of body fat is for men). Yet the average velocity of the win- not known. There is a 40%to 60% difference ning man was only 5%faster than that of the between men and women in Vo2max ex- winning woman.There are slightly larger dif- pressed in liters per minute, but these sex ferences between the sexes in upper-body differences are reduced to less than 10% endurance events, such as canoeing.75Men when Vmoa2sms.a6x3 is expressed per kilogram lean have relatively more muscle in the upper body Although it appears that low body, which allows them to generate more levels of body fat are desirable for peak en- power. durance performance in women, world dis- tance running records have been set by women with greater than 15% fat. Extremely low levels of body fat in female endurance athletes may affect other aspects of physi- ology. Related are the training and dietary habits necessary to achieve low body fat. Ef-

Endurance Training 81 Table 4-2. COMPARISON BETWEEN MALE same training stimuli.79,80 At elite levels, the AND FEMALE GOLD MEDAL ENDURANCE training programs of men and women may PERFORMANCE TIMES IN THE 1992 be closer to each other in intensity than OLYMPICS those of lower-level athletes. With years of Performance Time training, men and women get closer to their Event Male Female absolute potential. As they approach abso- lute potential, it becomes possible to make realistic comparisons of true sex differ- Track ences. 800-m run 1:43.66 1:55.54 Absolute maximal oxygen consumption 1500-m run 3:40.12 3:55.30 (L.min–1) is typically more than 40% 10,000-mrun 27:46.70 31:06.02 greater in men than in women. This differ- Marathon 2:13.23 2:32.41 Swimming ence is reduced to approximately 20% when 200-m freestyle wVoei2gmhatx.77 is expressed per kilogram body 400-m freestyle 1:46.70 1:57.90 It decreases further to less than 200-m butterfly stroke 3:45.00 4:07.18 10% when expressed per kilogram of lean 200-m breaststroke 1:56.26 2:08.67 body weight. Although excess fat is a hand- 200-m backstroke 2:10.16 2:26.65 icap to women endurance athletes, it does 1:58.47 2:07.06 not appear to account for all sex differences in performance. Cureton and Sparling78 Some events, such as the 80-km run, are added extra weight to men in an attempt ex- not contested very often by women. This perimentally to equalize fat masses. They makes it difficult to determine true sex dif- were able to completely abolish the differ- ferences from performance comparisons. ences between men and women in relative Sex differences in the physiologic responses Vo2max, but the following sex differences re- to exercise are often unclear from the liter- mained: 30% in distance run in 12 minutes, ature, since many studies have compared 31% in maximum treadmill run time, and physically fit male subjects with sedentary 20% in running efficiency after the experi- female subjects. mental intervention. They estimated that fat percentage accounts for 74% of the sex dif- Organ size and body mass are probably ferences in running performance. The the most important factors determining the higher Vo2max of men (mL•kg LBM–1) ac- sex differences in endurance performance. counted for 20%. Greater size provides a greater power-out- The average man has a larger heart size put capacity. Men have more muscle mass, and heart volume than the average woman both in relative and absolute terms, while (in both absolute and relative terms). This women have more fat. Greater lean body results in a greater stroke volume during mass is an asset, while more fat weight is a maximal exercise and contributes to the sex hindrance. Although muscle fiber composi- differences in Vo2max. Even though women tion is similar between the sexes, both fast- have a higher relative heart rate during ex- twitch and slow-twitch muscle fibers are ercise, it is not enough to compensate for usually larger in men.76 their lower stroke volume. The resultant smaller cardiac output of women contrib- Sex differences in enduranceperformance utes to their lower aerobic capacity. The increase as sport levels decrease.75 Thus, amount and concentration of hemoglobin there are fewer sex differences between also are higher in men, giving male blood male and female elite athletes than between greater oxygen-carrying capacity. Women those of lesser standing. Strength and power average about 13.7 g Hb.100 mL–1,whereas differences are major reasons for sexual di- men average 15.8 g Hb.100 m L – 1 . The differ- morphism in performance. Males and fe- ence is attributed to the stimulating effect of. males make the same relative gains in strength when they are subjected to the

82 Basic Concepts of Exercise Physiology androgens on hemoglobin production and extreme fatigue that occurs late in the race to the effects of menstrual blood loss and dif- and is probably related to glycogen deple- ferences in dietary intake3 (see Chapter 6). tion. There are few sex differences among the Costill and co-workers89 did not support factors that account for individual differ- this hypothesis. They used equally trained ences in endurance performance. In com- male and female subjects who ran for 1 hour paratively trained men and women, the en- on a treadmilland found that the capacity to ergy cost of running is similar.8 Bosco and use fat as fuel duringexercise was similar in colleagues82 have shown that the energy men and women. Muscle succinate dehy- cost of running is related to the percentage drogenase and carnitine palmitoyl transfer- of fast-twitch fibers. They have hypothe- ase activities were higher in the men, sug- sized that many women runners have a gesting that the muscle mitochondrial higher proportion of slow-twitch fibers than density in the male subjects may have been most men. Women thus may have a predis- greater. position for a higher running economy dur- ing submaximal exercise.83 As discussed, TRAINING FOR ENDURANCE other investigators have found no difference between men and women in the distribution Training is an adaptive process. Unfortu- of muscle fiber types. nately, athletes often forget this simple fact. They attempt overzealous training pro- There are differences in running economy grams with no real thought as to how their in different subject populations. This may bodies will respond to them. Consequently, partially explain some of the variability in they often become overtrained. They fail to running performance not explained by improve at a desirable rate, or they become Vo2max. Most studies show no sex differ- injured. ednucriensginextheercpiseer.c84e,8n5taAglethoofuVgoh2mthaexresuisstasionmede disagreement among researchers, there do Selye90 formulated a theory of stress ad- not seem to be any appreciable sex differ- aptation, which has implications for condi- ences in performance efficiency in running tioning endurance athletes. Selye called his or cycling.86 theory the general adaptation syndrome (GAS). He described three processes in- To date, there are no definitive studies on volved in the response to a stressor: (1) sex differences in lactate production and alarm reaction, (2) resistance development, clearance rates. No large sex differences in and (3) exhaustion. temperature regulation capacity have been found when researchers have made a seri- In the alarm reaction, the body mobilizes ous attempt to use subjects of equal fitness. its resources. During exercise, cardiac out- Finally, there are no sex differences in the put increases, blood is directed to active ability to improve Vo2max through training muscle, and metabolic rate increases. Body or in the ability to improve endurance per- balance is upset. formance through interval and continuous exercise programs.87 The resistance development stage can also be called the adaptive stage. It occurs Ullyot88 hypothesized that the higher when fitness is increased. It is the goal and body fat of women could be an advantage purpose of the endurance trainingprogram. during marathon and ultramarathon endur- The athlete must exercise at a threshold in- ance events, because they may have a tensity to get an adaptive response. This greater capacity for fat metabolism. Ullyot threshold is individual and is much higher observed that, unlike male runners, many in elite athletes than in sedentary people. women runners do not \"hit the wall\"during the marathon. \"Hitting the wall\" is sudden, When a stress cannot be tolerated, the athlete enters the stage of exhaustion. This

Endurance Training 83 stress can be either acute or chronic. Symp- will probably have to be decreased and rest, toms of acute exhaustion include fractures, increased. The application of each factor de- sprains, and strains. Chronic exhaustion is pends on variables such as experience, time characterized by stress fractures, staleness, of year, health, goals, and environment. and emotional stress. The basic purpose of the trainingprogram is to train hard enough Intensity is perhaps the most critical of to get an adaptive response and improvefit- the basic overload factors. As discussed, the ness, but not so hard as to become injured. optimum intensity during endurance exer- cise is tied to carbohydrate metabolism. If The body adapts specifically to the stress the intensity is too high, lactic acid produc- of exercise.91 Athletes should develop the tion exceeds clearance capacity. The athlete type of fitness required in their sport; run- fatigues very quickly, and recovery is more ners should run and weight lifters should lift difficult. In addition, valuable glycogen weights. The training program should also stores are rapidly depleted. However, if the reflect the various components of the activ- pace is too slow, then the athlete does not ity. For example, if a runner or cyclist must perform up to potential. She will probably go up hills in competition, then she should lose the race or will not reach the desired include hill-running or hill-cycling in her level of physical conditioning. program. There have been many attempts by re- The varying force requirements encoun- searchers to identify physiologic markers of tered during exercise are met by recruiting the ideal exercise intensity. Markersinclude the number of motor units needed to per- blood lactate, heart rate, ventilation, per- form the task. Because a motor unit is ceived exertion, and percentage of maxi- trained in proportion to its recruitment,92it mum effort. Esoteric physiologic measures is critical that the motor units that will be such as lactate inflection point have not used in competition be trained regularly. been very useful. Good measures of training Therefore, a runner who hopes to run re- load are exercise heart rate, percentages of peated 6-minute miles in competition must race pace, and perceived exertion. Exercise include a portion of her trainingat race pace heart rate helps select a pace that is propor- or faster. This will condition the motor units tional to oxygen consumption. Training at that will be recruited during the race. The different speeds helps to train more motor frequency of different types of training de- units, since different motor units are re- pends upon the relative importance of their cruited when running fast or slow. Perceived target motor units. So, while repeated short exertion helps the athlete to adjust the train- sprints may be the central component for a ing program. She can better respond to in- 100-meter runner, they would be much less jury, illness, glycogen depletion, overtrain- valuable for a distance runner. ing, and environmental stress. The most effective programs are those that work the Components of Overload athlete through a range of distances and in- The amount of overload (training stimu- tensities according to the requirements of the sport. lus) in the trainingprogram can be varied by manipulating intensity, volume, duration, The program should consist of over-dis- and rest. Intensity is the speed at which the tance training and interval training. The pur- activity is carried out. Volume is the number pose of over-distance training (long, slow of repetitions. Duration is the distance of distance) is to increase or maintainVo2max. each repetition. Rest is the amount of time It also increases tissue respiratory capacity between repetitions; each factor is affected by increasing muscle mitochondrial den- by the others. For example, if the intensity sity. As discussed, mitochondrial density is (speed) is increased, volume and duration better correlated with endurance capacity than is Vo2max. It is probably the major ben-

84 Basic Concepts of Exercise Physiology eficiary of over-distance conditioning. Be- \"Listen to your body,\" the thirdprinciple, cause of the principle of specificity, how- is familiar to anyone who has ever read a ever, a segment of this distance training book or article on exercise. While the ex- should be conducted close to race pace. pression is a bit weathered, it is true never- theless. The athlete should not adhere to Interval training involves periods of in- her planned program too dogmatically. tense exercise interspersed with rest (see Sometimes her body needs rest more than Chapter 1). The nature of the interval train- exercise. Most studies show that the abso- ing program varies with the distance ofcom- lute intensity is perhaps the most important petition. Athletes who run shorter races will factor in improving fitness. An overtrained run shorter, faster distances in training than athlete is typically not recovered enough to those who run longer, slower races. Interval train at the optimal intensity. A few days' training increases Vo2max.It does this by in- rest sometimes will allow her to recover creasing maximal cardiac output and speed. enough to train more intensely. Onthe other Interval training also teaches pace, builds hand, she should still try to follow a struc- speed, and improves lactate removal. It also tured program. increases mitochondrial density but is less effective than over-distance training. Endurance athletes should train first for distance and only later for speed. Soft tis- Principles of Training sues need time to adjust to the rigors of training. Ligamentsand tendons adjust very Nine principles of endurance training are slowly to the stresses of exercise.93 The ath- listed in Table 4-3. They are explicit instruc- lete must prepare her body for heavy train- tions for applying the general adaptation ing, or injury may result. syndrome to the training of endurance ath- letes and will result in improved perfor- The fifth training principle suggests that mance with a minimum risk of injury. athletes should cycle the volume and inten- sity of their workouts. The practice of alter- The first principle is to train all year nating between hard and easy training days round. Athletes lose much fitness through is an application of cycle training (also deconditioning. They are much more sus- called periodization of training).3 Cycle ceptible to injury if they try to get in shape training allows the body to recover more rapidly during the competitive season. The fully and to train hard when hard trainingis next principle is related to the first: Get in required. shape gradually.The athlete should give her body time to adapt to the stress of exercise. Athletes should incorporate base and Overzealous training leads to injury and peak cycles (workouts) into the competitive overtraining. strategy. These cycles are groups of work- outs practiced to improve fitness gradually Table 4-3. PRINCIPLES OF ENDURANCE (base) or to increase sharpness for compe- TRAINING tition (peak). Base or load cycles are char- • Train allyear round. acterized by high volume with varying in- • Get in shape gradually. tensity. Peak cycles employ lowvolume and • Listen to your body. high-intensity workouts with plenty of rest. • Begin with over-distance training before Peak cycles are designed to produce maxi- mum performance.The base or load cycle is progressing to intervaltraining. the foundation for peak performance. How- • Cycle your training: Incorporate load, peak, and ever, peak fitness can be maintained for only a short time, and every peak is gained at the recovery cycles. price of deconditioning. Both cycles are • Do not overtrain; rest the day before competition. thus important. The successes of the peaks • Train systematically. make the hard work of the base period • Train the mind. worthwhile. • Putsport in its proper perspective.

Endurance Training 85 A difficult training principle to adhere to must also have time for her family and other is the sixth, \"do not overtrain.\" It contra- aspects of life that are important to her. dicts the work ethic that is ingrained in so many athletes. The athlete should think of SUMMARY conditioning for endurance events as amul- tiyear process. Adaptations to training take The determining factors of endurance place very gradually. Excessive training performance include maximal oxygen con- tends to lead to overtraining and overuse in- sumption, mitochondrial density, perfor- juries rather than to accelerated develop- mance efficiency, and body composition. ment of fitness. Similarly, athletes should Maximal oxygen consumption is the body's avoid excessive competition because nu- maximum ability to transport and use oxy- merous studies have shown that consider- gen and is largely determined by the cardiac able muscle damage occurs during long-dis- output capacity. It is improved by about20% tance races.74 Competing too frequently through training. A high initial value is im- results in an inability to recover, which de- portant for success in endurance events. Mi- creases the overall level ofconditioning. tochondrial density is highly related to en- durance capacity. It provides a high The seventh training principle tells the oxidative capacity and the abilityto use fats athlete to train systematically. The athlete as fuel during exercise. Efficiency is deter- should plan an approximate workout sched- mined by physiologic factors such as mito- ule for the coming year (or even the next 4 chondrial density. Mechanical factors, such years), month, and week. Of course, she as technique and wind resistance, are also should not be so rigid that she cannot important. The importance of body compo- change the program owing to unforeseen sition for endurance varies with the sport. In circumstances. She should train in a manner sports such as running, cycling, and cross- that will produce a consistent increase in fit- country skiing, additional fat increases the ness. Coaching, training partners, and a energy cost of exercise. The ideal lower limit training diary will help her workouts be- of body fat is not known. In long-distance come more systematic. Coaching helps the swimmers, a slightly higher fat percentage athlete meet her competitive goals. A good decreases drag in the water and provides in- coach, who is knowledgeable and experi- sulation against the cold. enced, can keep her from repeating common training mistakes made by others. The Sex differences exist in endurance perfor- coach will also help motivate the athlete. mance. The relative changes that occur with Training partners are important for motiva- training and the basic underlying mecha- tion and competition. The trainingdiary will nisms that determine performance are the help the athlete to formulate her goals and same in men and women. Women trail men to identify effective training techniques. by 6%to 15% in most endurance sports, but there is considerable variance in perfor- Training the mind is as important astrain- mance in specific events. It is difficult to ing the body. Successful athletes believe in summarize and quantify physiologic sex dif- themselves and their potential; they have ferences reported in the literature; physi- goals and know how to achieve them. In en- cally fit male subjects were often compared durance training in particular, the athlete with sedentary female subjects. must be patient and be content with contin- uous small improvements over many years. Training is an adaptive process. Athletes should not become involved in overzealous Finally, sports should be put in their training programs that often lead to injury. proper perspective. Too often, athletes Because the body adapts specificallyto the think of themselves solely as runners, cy- stress of exercise, the training program clists, or swimmers rather than as human should reflect the various components of beings who participate in those activities. Although sports are important, the athlete

86 Basic Concepts of Exercise Physiology the activity. Trainingoverload can be varied cytochrome oxidase activity and its relation- by manipulating intensity, volume,duration, ship to maximal oxygen consumption in man. and rest. Intensity is most important for Pflugers Arch 349:319,1974. achieving high levels of performance. Good 13. Ivy JL, Costill DL, and Maxwell BD: Skeletal measures of intensity are exercise heart muscle determinants of maximal aerobic rate, percentages of race pace, and per- power in man. Eur J Appl Physiol 44:1,1980. ceived exertion. Endurance athletes should 14. Klissouras V, Pirnay F, and Petit J-M: Adap- use a combination of interval and over-dis- tations to maximal effort: Genetics and age. J tance training techniques. Appl Physiol 35:288,1973. 15. Bouchard C, and Lortie G: Heredity and en- REFERENCES durance performance. Sports Med 1:38,1984. 1. Brooks GA (ed): Perspectives on the Aca- 16. Komi PV, and Karlsson J: Physical perfor- mance, skeletal muscle enzyme activities, demic Discipline of Physical Education. and fiber types in monozygous and dizygous Human Kinetics, Champaign 1L, 1981. twins of both sexes. Acta Physiol Scand 2. American College of Sports Medicine: Ency- 462(Suppl):462,1979. clopedia of Sport Sciences and Medicine. 17. Bouchard C: Discussion: Heredity, fitness, Macmillan, New York, 1971. and health. In Bouchard C, Shephard RJ, Ste- 3. Brooks GA, and Fahey TD: Exercise Physiol- phens T, et al (eds): Exercise, Fitness, and ogy: Human Bioenergetics and Its Applica- Health. Human Kinetics, Champaign, IL, tions. Macmillan, New York, 1984. 1990, p 147. 4. Braunwald E, Ross J, and Sonnenblick EH: 18. Blumenthal JA, Emery CF, Madden DJ, et al: Mechanisms of the normal and failing heart. Cardiovascular and behavioral effects of aer- N Engl J Med 277:794,1967. obic exercise training in healthy older men 5. Bhan A, and Scheuer J: Effects of physical and women. J Gerontol 44:M147, 1989. training on cardiac myosin ATPase activity. J 19. Kearney JJ, Stull GA, Ewing JL, et al: Cardio- Physiol228:1178, 1975. respiratory responses of sedentary college 6. Scheuer J, and Tipton CM: Cardiovascular ad- women as a function of training intensity. J aptations to physical training. Ann Rev Phys- Appl Physiol 41:822,1976. iol39:221,1977. 20. Burk EJ: Physiological effects of similar train- 7. Stromme SB, and Ingjer F: The effect of regu- ing programs in males and females. Res Q lar physical training on the cardiovascular 48:510, 1977. system. Scand J Soc Med 29(Suppl):37, 21. Hanson JS, and Nedde WH:Long-term phys- 1982. ical training effect in sedentary females. J 8. Zeldis SM, Morganroth J, and Rubier S:Car- Appl Physiol 37:112,1974. diac hypertrophy in response to dynamic 22. Hickson RC, Bromze HA, and Holloszy JO: conditioning in female athletes. J Appl Phys- Linear increase in aerobic power induced by iol 44:849,1978. strenuous exercise. J Appl Physiol 42:372, 9. Roost R: The athlete's heart: What we did 1977. learn from Henschen, what Henschen could 23. Lewis S, HaskellWL,Wood PD, et al: Effects of have learned from us! J Sports Med Phys Fit physical activity on weight reduction in 30:339, 1990. obese middle-aged women. Am J Clin Nutr 10. Tesch PA: Short- and long-term histochemi- 29:151, 1976. cal and biochemical adaptations in muscle. 24. Schaible TF, and Scheuer J: Response of the In Komi PV (ed): Strength and Power in heart to exercise training. In Zak R (ed): Sport. BlackwellScientific Publications, Lon- Growth of the Heart in Health and Disease. don, 1992, p 239. Raven Press, New York, 1984, p 381. 11. Gohil K, Jones DA, Corbucci, GG, et al: Mito- 25. Longhurst JC, Kelly AR, Gonyea WJ, et al: chondrial substrate oxidation, muscle com- Echocardiographic left ventricular mass in position, and plasma metabolite levels in distance runners and weight lifters. J Appl marathon runners. In Knuttgen HG, Vogel GA, Physiol 48:154, 1980. and Poortsman J (eds): Biochemistry of Ex- 26. Ekblom B, GoldbargAN, and Gullbring B: Re- ercise. Human Kinetics, Champaign, II, 1982, sponse to exercise after blood loss and rein- p286. fusion. J Appl Physiol 33:175,1972. 12. Booth FW,and Narahara KA:Vastus lateralis 27. Gledhill N: Blood doping and related issues: A brief review. Med Sci Sports 14:183,1982. 28. Fagraeus L: Cardiorespiratory and metabolic functions during exercise in the hyperbaric environment. Acta Physiol Scand 92(Suppl 414):1, 1974.

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88 Basic Concepts of Exercise Physiology 64. Rennie DW, Pendergast DR, and diPrampero 79. Cureton KJ, Collins MA, Hill DW, and Mc- PE: Energetics of swimming in man. In Clarys Elhannon FM: Muscle hypertrophy in men JP, and Lewillie L (eds): Swimming II. Uni- and women. Med Sci Sports Exerc 20:338, versity Park Press, Baltimore, 1975, p 97. 1988. 65. Wilmore JH, and Behnke AR: An anthropo- 80. Holloway JB, and Baechle TR: Strength train- metric estimation of body density and lean ing for female athletes. Sports Med 9:216, body weight in young women. Am J Clin Nutr 1990. 23:7,1970. 81. Brunc V, and Heller J: Energy cost ofrunning 66. Rusko H, Hara M; and Karvinen E: Aerobic in similarly trained men and women. Eur J performance in athletes. Eur J Appl Physiol Appl Physiol 59:178, 1989. 38:151, 1978. 82. Bosco C, Montanari G, Ribacchi P, et al: Re- 67. SinningWE, Cunningham LN, Racaniello AP, lationship between the efficiency of muscular et al: Body composition and somatotype of work during jumping and the energetics of male and female Nordic skiers. Res Q 48:741, running. Eur J Appl Physiol 56:138, 1987. 1977. 83. Bosco C, Komi PV,and SinkkonenK: Mechan- 68. Tittle K, and Wutscherk H: Sportanthropo- ical power, net efficiency, and muscle struc- metrie. Johann Ambrosius Barth, Leipzig, ture in male and female middle distance run- 1964, p 1. ners. Scand J Sports Sci 2:47, 1980. 69. Farmosi I:Az usz6nOk testalkatanak es teljes- 84. Davies CTM, and Thompson MW: Aerobic itmenyenek osszefuggese. In Lasl6 N (ed): A performance of female marathon and male Sport es Testneveles IdoszerU Kerdesei—23. ultra-marathon athletes. Eur J Appl Physiol Sport, Budapest, 1980, p 77. 41:233, 1979. 70. Dessein M:Studie van enkele zwemtechnisch 85. Conley DL, KrahenbuhlGS, Burkett LN, et al: gebonden componenten en in net bijzonder Physiological correlates of female road rac- van somatische karakteristieken. Licentiaat. ing performance. Res Q 52:544, 1981. Katholieke Universitait te Leuven, Leuven, 1981, p 66. 86. Pate RR, and KriskaA: Physiological basis of the sex difference in cardiorespiratory en- 71. Meleski BW, Shoup RF, and Malina RM:Size, durance. Sports Med 1:87, 1984. physique and body composition of competi- tive female swimmers 11 through 20 years of 87. Eddy DO, Sparks KL,and Adelizi DA:The ef- age. Hum Biol 54:609, 1982. fects of continuous and interval training in women and men. Eur J Appl Physiol 37:83, 72. Vallieres F, Tremblay A, and St-Jean L: Study 1977. of the energy balance and the nutritional sta- tus of highly trained female swimmers. Nutr 88. Ullyot J: Women's secret weapon. In Van Res 9:699, 1989. Aaken E: Van Aaken Method. World Publica- tions, Mountain View, CA,1976. 73. International Olympic Committee: Games of the XXIIIrd Olympiad Los Angeles 1984 Com- 89. Costill DL, Fink WJ, Getchell LH, et al: Lipid memorative Book. International Sport Publi- metabolism in skeletal muscle of endurance- cations, Salt Lake City, 1984. trained males and females. J Appl Physiol 47:787, 1979. 74. Noakes T: Lore of Running. Oxford Univer- sity Press, Capetown, 1985. 90. Selye H: The Stress of Life. McGraw-Hill, New York, 1976. 75. Drabik J: Sexual dimorphism and sports re- sults. J Sports Med Phys Fit 28:287, 1988. 91. Henry FM: The evolution of the memory drum theory of neuromotor reaction. In 76. Wells CL,and Plowman SA:Sex differences in Brooks GA (ed): Perspectives on the Aca- athletic performance: Biological and behav- demic Discipline of Physical Education. ioral. Phys Sports Med 11:52,1983. Human Kinetics, Champaign, IL, 1981. 77. Sparling PB: A meta-analysis of studies com- 92. Edgerton VR: Mammalianmuscle fiber types paring maximal oxygen uptake in men and and their adaptability. Am Zool 18:113, 1976. women. Res Q 51:542,1980. 93. Zernicke RF, and Loitz BJ: Exercise-related 78. Cureton KJ, and Sparling PB: Distance run- adaptations in connective tissue. In Komi PV ning performance and metabolic responses (ed): Strength and Power in Sport. Blackwell to running in men and women with excess Scientific Publications, London, 1992, p 77. weight experimentally equated. Med Sci Sports Exerc 12:288, 1980.

5CHAPTER Bone Concerns EVERETT L. SMITH, Ph.D., and CATHERINE GILLIGAN, B.A. INCIDENCE AND COST OF EFFECTSOF EXERCISE OSTEOPOROSIS ATHLETIC AMENORRHEA AND EFFECTS OF CALCIUM INTAKE BONE MECHANISM OF EXERCISE PROBLEMS IN STUDYING EXERCISE BENEFITS EFFECTS EFFECTS OF INACTIVITY Ttie skeleton is a dynamic tissue, constantly responding to conditions relative to its two major functions: providing structural support and acting as a mineral reservoir. Two interacting homeostatic mechanisms control plasma calcium and skeletal mineral: hormones and mechanical stress. The structural support func- tion of the skeleton permits movement and protects vital organs. As a reservoir, the skeleton responds to changes in hormone levels and helps to maintain blood calcium at about 9.8 mg/dL (Table 5-1).1 Because of the skeleton's dual role, structural integrity is jeopardized when the demands on the reservoir to main- tain serum homeostasis are too high. When dietary calcium is inadequate, cal- cium is mobilized from the bone to maintain serum calcium. If the dietary inad- equacy is chronic, calcium will be pulled continually from the bone reservoir, resulting in a negative calcium balance and a net loss of calcium and phosphorus. Mechanical strain through weight bearing and muscle contraction play a signif- icant role in maintainingskeletal structural integrity, as bone mineral content (BMC) changes in response to the mechanical stressors applied. Under balanced conditions, the hormonal and mechanical homeostatic mechanisms maintain both skeletal integrity and serum calcium. With aging, however, multiplefactors decline (involving diet, hormonal levels, and mechanical strain), precipitating bone involution that results in bone more susceptible to fracture and osteoporosis. Hormones and mechanical strain interact in maintaining body and skeletal functions. If stress to specific skeletal segments or to the skeleton as a whole is significantly reduced, bone mass declines. In severe disuse, such as in bed rest or spinal cord injury, the mobilizationof calcium from bone increases serum lev- els, decreases parathyroid levels and 1,25-(OH)2vitamin D, and thus decreases calcium absorption in the intestinal tract and increases calcium elimination from 89

90 Basic Concepts of Exercise Physiology Table 5-1. SERUM CALCIUM HOMEOSTASIS Metabolic Adaptation to Condition Increased fractional calcium absorption Condition Hormonal Response Decreased renal excretion ofcalcium Increased active form of vitamin D Low serum calcium Increased PTH Increased bone resorption Decreased fractional calcium absorption High serum calcium Decreased PTH Increased renal excretion ofcalcium Increased calcitonin Decreased active form of vitamin D Decreased bone resorption Decreased gonadal function Decreased gonadal hormones Decreased fractional calcium absorption Increased sensitivity of bone to PTH PTH = parathyroid. Source:Adapted from Smith and Raab,1 with permission. the kidneys. Generally, the decline in activ- causes 1.3 million fractures at a cost of 3.8 ity with age and the resultant bone and bio- billion dollars each year.2 A major cause of chemical changes are subtle. Over a long osteoporosis is age-related bone loss. Peak term, however, inactivity can significantly bone mass is reached at about age 35 in both reduce bone mass and threaten the integrity men and women. After age 35, women lose of skeletal structure. up to 1% per year, and they may lose as much as 4%to 6%per year during the first 4 INCIDENCE AND COST OF to 5 years after menopause (Fig. 5-1). Men OSTEOPOROSIS maintain bone mass until about age 50, after which they lose approximately 0.4%to 0.5% Osteoporosis is a major public health per year. Both peak bone mass and rate of problem, affecting more than 20 million peo- loss are involved in the likelihood of devel- ple in the United States. Osteoporosis oping osteoporosis. In cortical bone, loss occurs primarily on the endosteal surface, Figure 5-1. Cross-sections of long bones of women aged 30 and 70years. Note the enlarged medul- lary cavity and increased porosity of the cortical bone at age 70.

Bone Concerns 91 EFFECTS OF CALCIUM INTAKE Figure 5-2. A longitudinal section of the proximal end Both cross-sectional and longitudinal of a fernur, showingthe trabecularstructure within the studies of the effect of calcium intake on bone that provides maximum strength in the direction bone density or bone loss have produced of greatest applied pressure. The periosteumis a highly mixed results. In a cross-sectional study, vascular layer covering the surface of the bone;the end- Matkovic and colleagues3 compared bone osteum lines all interbone surfaces. (Adapted from Van mass and fracture incidence in twoYugosla- De Graff, KM: Human Anatomy, ed 2. Dubuque IA, Wil- vian populations, one with high (947 mg/d) liam C. Brown Publishers, 1988, p 158.) and one with low (424 mg/d) calcium intake based on dietary histories. The two groups with some loss on the periosteal surface were otherwise similar in heredity and en- (Fig. 5-2). In trabecular bone, the trabeculae vironment. The high-calcium group had a are thinned and may be entirely resorbed significantly greater skeletal mass at matu- (see Fig. 11-2). In the spine, horizontal sup- rity and a lower fracture incidence in old port trabeculae are lost preferentially, age. The loss of bone mass with age was sim- which reduces bone strength more than in- ilar in the two groups; therefore, the greater dicated by the density alone (Fig. 5-3). In incidence of fractures in the low-calcium conjunction with this decreased bone mass, group was attributed to lower peak bone the internal structure of bone also changes. mass. Other cross-sectional studies have re- Osteons are decreased in size and increased ported slight or nonsignificant correlations in number. Micropetrosis increases with la- of habitual calcium intakewith bone mass or cunae filled by calcium depositions. These fracture incidence.4,5 qualitative bone changes, in addition to the decreased BMC, contribute to a greater frac- Studies of the effects of calcium supple- ture potential. mentation on bone loss have not consis- tently demonstrated a positive effect. In 3- Although bone mass plays a significant and 4-year studies, we found that calcium role in determining bone strength, it is not supplementation reduced cortical bone loss the sole determinant. The geometric struc- in the arm (radius, ulna, and humerus) of el- ture of the tissue, determined by habitual derly and middle-aged postmenopausal stresses, collagen orientation, ligaments, women, but did not affect bone loss in and muscle tone, is also important. premenopausal women.6,7 Horsman and colleagues8 also reported that calcium sup- plementation reduced bone loss over 2 to 3 years at cortical forearm sites. In a 2-year study, Prince and associates9 found that cal- cium supplementation combined with exer- cise decreased forearm bone loss signifi- cantly compared to exercise alone. Other 2-year studies, however, failed to detect a significant difference in radius bone loss be- tween calcium-supplemented and control groups.10-12 Riis13 reported that calcium sup- plementation reduced cortical bone loss in the proximal forearm but did not retard tra- becular bone loss in the distal radius or spine in women who were recently post- menopausal. Similarly, Ettinger and co- workers14 found no effect of calcium supple- mentation for 1 year on spine bone loss in early postmenopausal women.

92 Basic Concepts of Exercise Physiology Figure 5-3. Varying degrees of osteoporosis in lumbar vertebrae. Upper left: Normal structure in a 63-year-old man. Lower left: The longitudinal trabeculae are narrowed and some broken ones are seen in the center of this vertebra, showing mild osteoporosis in a 65-year-old woman; the horizontal trabeculae are conspicuouslyreduced. Upper right: Pronounced osteoporosis in a 70-year-old woman. There is clear-cut breaking off of numerous longitudinal trabeculae on the right and the left. Lowerright: High-grade osteoporosis in a 71-year-oldwoman. The vertebral body has almost completely collapsed; there are several gaps in the trabecularstructure,and the restraining bone is transformed, with formation of new longitudinal trajectories. (From Remagen, W: Osteoporosis. Sandoz Ltd., Basle, Switzerland, 1989, Fig. 31, with permission.) Recent studies have investigated the ef- dius. In late postmenopausal women (>5 fects of calcium supplementation on bone years since menopause), however, theCCM loss in premenopausal and early and late group did not change significantly at any site postmenopausal women. Premenopausal measured; those taking CC declined signifi- women who increased their dietary calcium cantly in spine BMD; and the placebo group by an average of 610 mg/d (n = 20) did not declined significantly in spine and femur change significantly in spine bone mineral BMD. Radius BMD declined significantly density (BMD), while control subjects de- less in the CCM than in the placebo group. creased significantly (n = 17). Calcium sub- When subgroups of the late postmenopausal jects thus had significantly greater spine groups were formed on the basis ofcalcium BMD after 30 and 36 months than did control intake (<400 mg or 400 to 650 mg), differ- subjects.15 Dawson-Hughesand colleagues16 ences in change between groups were ap- studied 301 women with self-selected diets parent only for the lower-calcium subgroup. low in calcium (<650 mg/d), who were di- After 2 years, CCM had significantly reduced vided into placebo, calcium citrate malate loss in spine, femur, and radius BMDcom- (CCM) supplement, and calcium carbonate pared to the placebo group, and CC had sig- (CC) supplement groups and followed for 2 nificantly reduced loss in radius BMDcom- years. In early postmenopausal women (<5 pared to the placebo group. Elders and years since menopause), the groups did not associates17 found that calcium supplemen- differ in BMDloss in the spine, femur, or ra- tation retarded spine bone loss in early

Bone Concerns 93 postmenopausal women during the first ness. The degree of bone hypertrophy or at- year, but not the second year, of their trial. rophy is proportional to the difference in Finally, a team led by Nelson18 randomly as- magnitude and frequency of the mechanical signed postmenopausal women (mean age stimulus from normal. The habitual stimulus 60, mean years since menopause 11) to a to weight-bearing segments (legs and spine) high-calcium (831 mg) or placebo (41 mg is much greater than that to the non-weight- calcium) drink over a 1-year period. Half of bearing areas (ribs, arms, and skull). Forex- the subjects in each group participated in a ample, the impact of the heel during walking 1-year walking program. Calcium supple- (1.2 to 1.5 times body weight) is much mentation significantly reduced loss in greater than the stress applied by muscle femur BMD but did not affect loss in the contractions in the arm during activities of spine, radius, or total body calcium. daily living. Therefore, the calcaneus is nor- mally under greater stress than the radius, Some of the differences among calcium in- so when both bones are free of stress (as in tervention studies may be due to the wide the case of the astronauts in space), more variety of forms and doses of calcium used, bone is lost from the calcaneus than from along with differences in study length, sub- the radius.22,23 ject selection criteria, menopausal age, self- selected dietary intake, sites measured, and Numerous models of the mechanism by sample sizes. It is reasonable to hypothesize which bone responds to mechanical forces that calcium supplementation affects mainly have been proposed. Bassett24 indicated cortical bone in the early postmenopause19 that bone functions as a piezoelectric crys- but may affect other sites in premenopausal tal, generating an electric charge in propor- and late postmenopausal women. Calcium tion to the forces applied to the bone. supplementation may be most beneficial for Carter25 hypothesized that mechanical women with a self-selected diet low in cal- forces produce microfractures, which stim- cium and can be expected to avert only that ulate osteoclastic remodeling coupled with portion of bone loss due to inadequate cal- osteoblastic activity. A recent study, how- cium intake.20 ever, did not detect evidence of microfrac- ture in rats subjected to 20,000 loading cy- MECHANISM OF EXERCISE cles per day for 5 or 6 days.26 Other ways in BENEFITS which exercise may stimulate osteoclastic and osteoblastic activity include increased While evidence is accumulatingthat phys- hydrostatic pressure and streaming poten- ical activity increases bone mass, research tials. on the mechanisms by which bone is af- fected by mechanical stress is still in its Whereas dynamic loading produces hy- early stages. pertrophy, static loading of bone produces little or no hypertrophy.27 For a bone to hy- In 1892, Wolff21 hypothesized that in- pertrophy, dynamic stimuli must exceed creased weight bearing compresses and a threshold magnitude and frequency. bends the long bones, increases mineral Lanyon28 demonstrated that both the rate content, and consequently strengthens and magnitude of strain influenced bone re- bones, making them less liable to fracture modeling. He monitored BMCin the radiiof under similar loads. Weight bearing (grav- sheep under artificial stimulation. No ity) and muscle contraction are the two change occurred with a strain magnitude major mechanical forces applied to bone. less than that of the animal's normal walking Both hypodynamic and hyperdynamic load. With higher strain magnitude and nor- states affect bone balance. Bone mass in- mal strain rates, periosteal bone deposition creases with greater weight-bearing activity increased slightly on both convex and con- or muscle contraction or both and de- cave surfaces. When both magnitude and creases with immobilizationor weightless- rate were higher than in normal walking,

94 Basic Concepts of Exercise Physiology periosteal bone increased substantially. osteal surface than did untrained sows. Min- Other studies have applied more precisely eral apposition rate was also higher, at both quantified stress to bone. Rubin and the periosteal (76%) and intracortical oste- Lanyon29 applied controlled mechanical onal (23%) levels. Similarly, evidence of al- loads, using a pneumatically operated de- tered cellular activity was found in trained vice, to rooster ulnae isolated from muscular adult rats.34 Bone density and trabecular stress. Bone mass decreased if no load was number, thickness, and density were signif- applied, remained fairly constant at 4 cycles icantly higher in trained animals than in per day of normal (2000 microstrain) mag- controls after 18 and 26 weeks of exercise. nitude, and hypertrophied at a normal mag- The mineral apposition rate and bone for- nitude loading for 36 cycles per day (each mation rate were significantly higher in cycle about 2 seconds). The hypertrophy trained animals, while the percentage of from 3600 cycles per day and 36 cycles per eroded and labeled perimeter tended to be day did not differ. In a similar study, the lower in the trained animals. This study sup- number of loading cycles was held constant, ports the concept of Frost35 that increased and the magnitude of the strain varied.30 activity stimulates modeling and depresses Bone change was directly proportional to remodeling. the strain. Bone atrophied at strains below 1000 microstrain, and cross-sectional area Bone requires a specific magnitude and increased with strains over 1000 micro- rate of stimulus in order to hypertrophy. strain. It appears that the magnitude of the Within a normal range of stimulusspecific to strain is more important than the frequency the individual's activities and genotype, of application. bone neither atrophies nor hypertrophies. Beyond or below this range, bone will Recent studies have investigated the bio- change, as shown in Figure 5-4. Increased chemical and histologic sequelae of bone hypertrophy with increased stress will loading. The mechanism by which strain occur only to a point, however. Severe, re- produces an osteogenic cellular response petitive loading may result in fatigue dam- has not been delineated. Histomorphome- age such as that seen in the metatarsals, cal- tric data show increased osteoclastic func- caneus, tibia, and femur of some soldiers tion with disuse and increased osteoblastic and distance runners. Fatigue damage may function with increased activity. Neither also occur in untrained persons who in- mature osteoclasts nor osteoblasts, how- crease their activity levels more rapidly ever, seem to respond directly to changes in than the bone can adapt. At the other end of skeletal strain. Osteocytes (that number up the spectrum, bone atrophies with lessened to 20,000 per cubic millimeter), however, mechanical stress due to bed rest and may respond to changes in skeletal strain by weightlessness. the production of chemical transmitters act- ing on bone precursor cells. Using an in EFFECTS OF INACTIVITY vitro core biopsy model in the presence of [3H]uridine, El Haj and colleagues31 ob- Donaldson and associates36 observed served an increase in radioactive osteocyte three men for 30 to 36 weeks of bed rest, and RNA compared to nonloaded specimens. Hulley and colleagues37 observed five men Pead and Lanyon32 reported that within 5 for 24 to 30 weeks of bed rest. Calcium bal- days after a single period of skeletalloading, ance was negative throughout bed rest,with quiescent surface-lining cells were trans- 0.5% to 0.7%of total body calcium lost per formed into active, bone-forming osteo- month. In the weight-bearing calcaneus, blasts. bone loss was magnified; 25% to 45% was lost after 36 weeks. After remobilization, cal- Cellular activity at the femur midshaft in- cium balance became positive within a creased significantly in sows training on a month, and BMC was regained at a rate sim- motor-driven treadmill for 20 weeks.33 Trained sows had a 27%greater active peri-

Bone Concerns 95 Figure 5-4. Effect of mechanicalloading on bone mineral content. (Adapted from Carter.25) ilar to the rate of loss, reaching baseline lev- less severe than that of a patient at bed rest, els in about 36 weeks. Krolner and Toft38 ob- but extended over 10 to 20 years, the resul- served a 0.9%per week loss from the lumbar tant bone loss can be a major contributor to spine in individuals at bed rest for an aver- the development of osteoporosis. age of 27 days because of a disk protrusion. Human studies of immobilization are rare, LeBlanc and co-workers39 recently reported but a number of animal studies have been the effects of 17 weeks of bed rest and 6 performed. Kazarian and Von Gierke40 im- months of recovery at various skeletal sites. mobilized 16 rhesus monkeys in full body During bed rest, subjects lost significantly in casts for 60 days. Bone from the immobi- BMD at the calcaneus (10%), femur trochan- lized animals had fewer and thinner trabec- ter (5%), lumbar spine (4%), femur neck ulae, smaller trabecular plates, reduced tra- (4%), tibia (2%), and total body (1.4%). The becular surfaces, and reduced cortical lumbar spine, femur trochanter, and tibia thickness compared with bones of control BMD tended to increase duringthe recovery animals. Remodeling occurred in the trabec- period, but only calcaneus BMD increased ulae of the femoral neck, which \"corre- significantly. The distal and proximal radius sponded in position and curvature to the and ulna BMD did not decrease significantly lines of maximum compressive stress,\" so during bed rest or increase significantly dur- that only those trabeculae necessary for ing recovery. Regional analysis of total body structural integrity were retained. The com- scans showed significant decreases in lum- pressive strength of the immobilized bones bar spine, total spine, pelvis, trunk, and legs was two to three times less than that in con- during bed rest, with a significant increase trol animals. Cortical bone at the sites of in head BMD during bed rest. Pelvis and muscle and tendon attachments also was trunk BMDincreased significantly in the re- sigYnoifuincgan41tlyanwdeakNeirkltohwanitzi4n2 control animals. covery period. In the average woman, the and their co- usual decline in activity is more gradual and workers investigated changes in the tibias of

96 Basic Concepts of Exercise Physiology monkeys during 7 months of immobilization of the studies in which it was measured. Aer- and up to 40 months of recovery and remo- obic weight-bearing activities increased bilization. Remodeling was obvious within 1 spine bone density in middle-aged post- month of immobilization. After 10 weeks, menopausal women18,52 and women with os- they observed endosteal resorption, sub- teoporosis.53 In two of these studies,52,53 the periosteal loss, striations in the cortex (in- total BMD of L2 to L4 was increased signifi- dicative of resorptive cavities), surface ero- cantly compared to controls, while in the sion in the juxta-articularareas (patella and third study,18 trabecular (LI to L3) but not femoral condyles), and thin, irregular exter- total BMD was affected. Physical activity nal lamellar bone. During 6 months of im- programs incorporating arm exercises in- mobilization, BMC decreased 23% to 31% creased BMDor decreased bone loss in the and bending stiffness, 36% to 40%.Normal radius and ulna.6,54-56 In our study56 of mid- bending properties of the bone were re- dle-aged women, the response to exercise stored within 8V2months of recovery and re- appeared to be independent of menopausal mobilization, but BMC did not return to nor- status. Afew studies that used primarily aer- mal even after 15 months. New primary obic weight-bearing training reported no ef- haversian systems were generated during fects on radius9,57,58or spine59 bone density. that time, and by 40 months the cortex con- tained many secondary and tertiary osteons The bone hypertrophy observed in and approached normal BMC. weightlifters, along with animal studies showing a linear relationship between strain EFFECTS OF EXERCISE and bone hypertrophy, have led to several Numerous studies indicate that bone den- studies utilizing resistance training to pro- mote BMD. In studies comparing general sity is responsive to mechanical loading. aerobic trainingwith and withoutadditional The effect appears to be primarily local and resistance training, groups performing the proportional to the level of strain placed on strength training tended nonsignificantly to the bone. The most convincing evidence increase more in calcium bone index (bone that bone hypertrophy is localized appears mass of the central third of the skeleton ad- in studies of both young and old tennis play- justed for body size) and radius BMD than ers, whose dominant humerus was up to 35% those in aerobic training alone.54,60 Resist- higher in BMDthan was the nondominant ance training alone, however, has not con- arm.43- 45 In studies of athletes, the amountof sistently altered BMD. In studies of pre- hypertrophy was related to the loading ap- menopausal and early postmenopausal plied by the sports activity. For example, women, exercise subjects increased in ver- weightlifters had higher bone density in the tebral BMD relative to controls.61,62 Calca- spine and femur than aerobic athletes,46-49 neus, femur, and distal forearm BMD, how- whereas swimmers did not have signifi- ever, were not significantly affected in these cantly higher spine density than did seden- studies. Postmenopausal women receiving tary subjects.48-50 In one study, male swim- estrogen replacement therapy (ERT) and mers had significantly higher vertebral BMD assigned to a resistance-training group in- than sedentary subjects, but no difference creased significantly in spine, total body, could be detected between swimming and and radius BMD; only in the radius, how- sedentary women.51 ever, did the change differ significantly from that in the women receiving ERTalone.63 In Intervention studies have confirmed the contrast to these positive findings, pre- beneficial effect of exercise at various skel- menopausal women in a resistance-training etal sites, includingthe spine, radius, calca- program lost significantly more vertebral neus, and tibia. Femur BMD, however, was BMD than did controls.64 Postmenopausal not significantly affected by exercise in any women who performed back extensions with light weights for 2 years did not differ

Bone Concerns 97 significantly in vertebral BMD change from rheic controls; in fact, oarswomen tended to controls.65 be higher in vertebral BMC. Regular,oligo- menorrheic and amenorrheic oarswomen ATHLETIC AMENORRHEA AND did not differ significantly. This may have BONE been the result of small sample sizes, how- ever, since a nonsignificant trend toward Whereas exercise is associated with an in- lower density in groups with menstrual dis- crease in bone density, excessive exercise turbances was apparent. The authors spec- leading to amenorrhea is associated with a ulated that the increased muscular workof decrease in bone density. Investigators have the back involved in rowing and weightlift- found that unlike hyperprolactinemic, ano- ing exerted a protective effect on vertebral rexic, and premature menopausal women, bone. Wolman and colleagues,85 however, amenorrheic athletes do not have signifi- did not confirm this protective effect on ver- cantly lower cortical (radius) BMD,66-74 but tebral trabecular bone density. A two-way they are significantly lower than eumenor- analysis of variance incorporating men- rheic athletes in vertebral HMD67-69,73,75 and strual status and sports activity showed a vertebral trabecular density.72,75-79 Men- significant (negative) factor for amenorrhea strual history is an important determinant of and a significant (positive) factor for rowers vertebral density even among currently eu- but no significant interactive effect. menorrheic athletes.68,77 Athletes with men- strual disorders also appear more prone to The apparent immunity of the radius and injury and stress fractures.68,80-82 susceptibility of the spine to menstrual dis- orders indicates that there may be a differ- A few longitudinal studies have demon- ential responsiveness of cortical and trabec- strated that subjects with menstrual distur- ular bone. A recent study included bances lose more vertebral bone. Prior and additional sites of varying trabecular com- colleagues83 studied 66women without overt position.68 Ninety-seven athletes were menstrual irregularities, who varied widely graded for menstrual history, and BMD was in their exercise patterns. They found that measured at the spine (LI to L4),femur neck ovulatory disturbances accounted for 24% of and shaft, radius (10% and 20% distal), tibia, the variance in vertebral bone loss: spinal and fibula. Spine and femur shaft BMD were bone density tended to increase in women higher in subjects who had always been reg- with normal cycles but decreased signifi- ular than in subjects with some history of cantly in women with two or more short lu- menstrual irregularity. Currently amenor- teal phases and in those with anovulatory rheic subjects with a history of oligomenor- cycles. Cann and associates84 reported that rhea or amenorrhea were significantly lower loss of vertebral bone with amenorrhea was in vertebral BMDthan subjects with other biphasic. In one year, women who had been patterns of irregularity.No differences were amenorrheic 3 years or less lost 4.2% in ver- detected at other sites. The authors con- tebral trabecular mineral content, while cluded that deficits in bone density from those who had been amenorrheic for longer previous menstrual irregularityappeared to periods did not change significantly. be confined to the vertebrae. Weight was sig- nificantly correlated with BMD at all sites, In most of these studies of bone density, and the association became stronger as the the athletes were runners or involved in severity of menstrual disorder increased. other aerobic sports that did not particu- larly stress the back musculature. On the An important question is whether bone other hand, Snyder and co-workers74 re- deficits due to amenorrhea can be corrected ported that oarswomen, regardless of men- or averted. Seven subjects who regained strual status, did not differ significantly in menses following a reduction in trainingin- vertebral BMC from sedentary eumenor- creased significantly (6.3%) in vertebral BMD, while matched eumenorrheic subjects

98 Basic Concepts of Exercise Physiology did not change significantly over 1 year.86 tempting to delineate the exercise programs Bone density increased more slowly the fol- most effective in stimulatingbone hypertro- lowing year, and then plateaued. After 4 phy. years of normal menses, vertebral density SUMMARY remained well below normal. Further evi- dence of the persistence of vertebral loss Bone is a dynamic tissue performing two was provided by a study of 208 runners.Sub- functions: providing structural support and jects with past or current untreated amen- acting as a mineral reservoir. Two homeo- orrhea had significantly lower spinal den- static mechanisms act on bone at the same sity than subjects who had always been time: hormones and mechanical stress. regular. Women with current or past oligo- menorrhea but no amenorrhea, and women Researchers have evaluated the relation- with treated (by estrogen or oral contracep- ship of weight-bearing and non-weight- tives) amenorrhea for less than 3years were bearing forces on bone to bone mass and similar in bone density to the always-regular bone strength. Bone adjusts locally to sup- subjects. port the structural demands ofweight-bear- ing and muscular activity. Inactivity results PROBLEMS IN STUDYING in bone involution, whereas increased activ- EXERCISE EFFECTS ity induces bone hypertrophy. Subjects at bed rest or in weightless conditions lose In animal studies, exercise and mechani- bone rapidly. Conversely, athletes have cal loading have consistently benefitted greater bone mass than their sedentary bone density. Although the mechanisms counterparts. Exercise intervention slows or have not been fully elucidated, loading pro- reverses bone loss in middle-aged and el- duces increased cellular activity and bone derly women. Bone response is specific to formation rates. In humans, athletes had the area stressed, as seen in the selective greater bone density than sedentary sub- hypertrophy of the dominant arm in tennis jects, but few cross-sectional studies ofnon- players. Very intense levels of exercise cou- athletes have been able to detect a differ- pled with amenorrhea may reduce skeletal ence in bone density between moderately mass, especially in the spine. active and sedentary subjects. It could be that many moderately active subjects do not More research is needed to understand exceed the threshold for stimulating bone the precise mechanisms by which exercise hypertrophy. Intervention studies gener- affects bone, and the optimum type and in- ally, but not always, increased bone density tensity of physical activity for preventing os- or reduced bone loss. A confusing aspect of teoporosis. bone research is the failure of intervention with weight training to increase bone mass REFERENCES in some studies. Some of the negative results 1. Smith EL, and Raab DM: Osteoporosis and in human studies can be attributed to mea- surement sites not stressed by the exercise, exercise. In Astrand PO, and Grimby G (eds): exercise programs of insufficient intensity, Proceedings, Second Acta Medica Scandinav- or lack of adequate control groups. One gen- ica International Symposium: Physical Activ- eral difference between human and animal ity in Health and Disease. Almqvist and Wik- studies is human diversity in genetics, life- sell Trycheri, Uppsala,Sweden, 1986, p 149. styles, and implementation of the exercise 2. National Institutes of Health: Consensus De- program (intensity, attendance, etc.). An- velopment Conference on Osteoporosis. Vol other problem in interpretingexercise inter- 5, No 3. US Government Printing Office, pub vention studies is the wide range of training no 421-132:46, Washington, DC,1984. programs used. At this time, we are still at- 3. Matkovic V, Kostial K,Simonovic I, et al: Bone status and fracture rates in two regions of Yu- goslavia. Am J Clin Nutr 32:540, 1979.

Bone Concerns 99 4. Freudenheim JL, Johnson NE, and Smith EL: 18. Nelson ME, Fisher EC, Dilmanian FA, et al: A Relationships between usual nutrient intake 1-y walking program and increased dietary and bone-mineral content of women 35-65 calcium in postmenopausal women: Effects years of age: Longitudinal and cross-sec- on bone. Am J Clin Nutr 53:1304, 1991. tional analysis. Am J Clin Nutr 44:863,1986. 19. Dawson-Hughes B: Calcium supplementation 5. Mazess RB, and Barden HS: Bone density in and bone loss: A review of controlled clinical premenopausal women: Effects of age, di- trials. Am J Clin Nutr 54:2748, 1991. etary intake, physical activity, smoking, and birth-control pills. Am J Clin Nutr 53:132, 20. Heaney RP: Effect of calcium on skeletal de- 1991. velopment, bone loss, and risk of fractures. Am J Med 91:235,1991. 6. Smith EL, Reddan W, and Smith PE: Physical activity and calcium modalities for bone min- 21. Wolff J: Das Gesetz der Transformation eral increase in aged women. Med Sci Sports Knochen. A. Hirschwald, Berlin, 1892. Exerc 13:60, 1981. 22. Vogel JM, and Whittle MW: Bone mineral 7. Smith EL, Gilligan C, Smith PE, et al: Calcium changes: The second manned skylab mis- supplementation and bone loss in middle- sion. Aviat Space Environ Med 47:396,1976. aged women. Am J Clin Nutr 50:833, 1989. 23. Smith MC, Rambaut PC,Vogel JM, et al: Bone 8. Horsman A, Gallagher JC, Simpson M, et al: mineral measurement-experiment M078. In Prospective trial of oestrogen and calcium in Johnston RS,and Dietlein LF (eds): Biomed- postmenopausal women. Br Med J 2:789, ical Results from Skylab. National Aeronau- 1977. tics and Space Administration, Washington, DC, 1977, p 183. 9. Prince RL,Smith M, Dick IM,et al: Prevention of postmenopausal osteoporosis: A compar- 24. Bassett CA: Biophysical principles affecting ative study of exercise, calcium supplemen- bone structure. In Bourne GH (ed): The Bio- tation, and hormone-replacement therapy. N chemistry and Physiology of Bone, ed 2. Vol EnglJ Med 325:1189, 1991. III. Academic Press, New York, 1971, p 1. 10. Recker RR, Saville PD, and Heaney RP: Effects 25. Carter DR: Mechanical loading histories and of estrogen and calcium carbonate on bone cortical bone remodeling. Calcif Tissue Int loss in postmenopausal women. Ann Intern 368:19, 1984. Med 87:649, 1976. 26. Forwood MR, and Parker AW: Repetitive 11. Recker RR, and Heaney RP: The effect of milk loading, in vivo, of the tibiae and femora of supplements on calcium metabolism, bone rats: Effects of repeated bouts of treadmill- metabolism and calcium balance. Am J Clin running. Bone Min 13:35, 1991. Nutr 41:254,1985. 27. Lanyon LE, and Rubin CT:Static vs. dynamic 12. Policy KJ, Nordin EEC, Baghurst PA, et al: Ef- loads as a stimulus for bone remodeling. J fect of calcium supplementation on forearm Biomech 15:767, 1984. bone mineral content in postmenopausal women: A prospective, sequential controlled 28. Lanyon LE: Bone remodeling, mechanical trial. J Nutr 117:1929, 1987. stress and osteoporosis. In DeLuca HF, Frost HM, Jee WSS, et al (eds): Osteoporosis: Re- 13. Riis B, Thomasen K,and Christiansen C: Does cent Advances in Pathogenesis and Treat- calcium supplementation prevent postmeno- ment. University Park Press, Baltimore, 1981, pausal bone loss? A double-blind, controlled p!29. clinical study. N Engl J Med 316:173, 1987. 29. Rubin CT, and Lanyon LE: Regulationof bone 14. Ettinger B, Genant HK, and Cann CE: Post- formation by applied dynamic loads. J Bone menopausal bone loss is prevented by treat- Joint Surg66a:397,1984. ment with low-dosage estrogen with calcium. Ann Intern Med 106:40,1987. 30. Rubin, CT and Lanyon, LE: Regulation of bone mass by mechanical strain magnitude, 15. Baran D, Sorensen A, Grimes J, et al: Dietary Calcif Tissue Int 37:411,1985. modification with dairy products for prevent- ing vertebral bone loss in premenopausal 31. El Haj AJ, MinterSL, Rawlinson SCF, et al: Cel- women: A three-year prospective study. J lular responses to mechanical loading in Clin Endocrinol Metab 70:264,1989. vitro. J Bone Min Res 5:923,1990. 16. Dawson-Hughes B, Dallal GE, Krall EA, et al: 32. Pead MJ and Lanyon LE:Indomethacin mod- A controlled trial of the effect of calcium sup- ulation of load-related stimulation of new plementation on bone density in postmeno- bone formation in vivo. Calcif Tissue Int pausal women. N Engl J Med 3223:878, 1990. 45:44, 1989. 17. Elders PJM,Netelenbos JC, Lips P, et al: Cal- 33. Raab DM, Crenshaw TD, Kimmel DB, et al: A cium supplementation reduces perimeno- histomorphometric study of cortical bone ac- pausal bone loss. J Bone Min Res tivity during increased weight-bearing exer- 4(Suppl):1128, 1989 (abstr). cise. J Bone Min Res 6:741, 1991. 34. Jee WSS and Li XJ: Adaptation of cancellous bone to overloading in the adult rat: A single

100 Basic Concepts of Exercise Physiology photon absorptiometry and histomorphom- 52. Dalsky GP, Stocke KS, Ehsani AA, et al: etry study. Anat Rec227:418,1990. Weight-bearing exercise trainingand lumbar 35. Frost, HM: A new direction for osteoporosis bone mineral content in postmenopausal research: A review and proposal. Bone women. Ann Intern Med 108:824,1988. 12:429,1991. 36. Donaldson CL, Hulley SB, Vogel JM, et al: Ef- 53. Krolner B, Toft B, Nielson SP, et al: Physical fect of prolonged bed rest on bone mineral. exercise as prophylaxis against involutional Metabolism 19:1071, 1970. vertebral bone loss: A controlled trial. Clin 37. Hulley SB, Vogel JM, and Donaldson CL:Ef- Sci 64:541,1983. fect of supplemental calcium and phospho- rus on bone mineral changes in bed rest. J 54. Rikli RE, and McManis BG:Effects of exercise Clin Invest 50:2506, 1971. on bone mineral content in postmenopausal 38. KrolnerB,and Toft B: Vertebral bone loss:An women. Res Q Exerc Sport 61:243, 1990. unheeded side effect of therapeutic bed rest. Clin Sci 64:537,1983. 55. Simkin A, Ayalon J, and Leichter I: Increased 39. LeBlanc AD, Schneider VS, Evans HFJ,et al: trabecular bone density due to bone-loading Bone mineral loss and recovery after 17 exercises in postmenopausal osteoporotic weeks of bed rest. J Bone Min Res 8:843,1990. women. Calcif Tissue Int 40:59, 1986. 40. KazarianLE,and Von Gierke HE:Bone loss as a result of immobilization and chelation: Pre- 56. Smith EL,Smith PE, Ensign CJ, et al: Bone in- liminary results in macaca mulatta. Clin Or- volution decrease in exercising middle-aged thop 65:57, 1969. women. Calcif Tissue Int 36:S129,1984. 41. Young DR, Niklowitz WJ, and Steele CR:Tib- ial changes in experimental disuse osteopo- 57. Aloia JF, Cohn SH, Ostuni JA, et al: Preven- rosis in the monkey. Calcif Tissue Int 35:304, tion of involutional bone loss by exercise. 1983. Ann Intern Med 89:356, 1978. 42. Niklowitz WJ, Bunch TE, and Young DR: The effects of immobilization on cortical bone in 58. Sandier RB, Cauley JA, Horn DL,et al: The ef- monkeys (m nemestrina). Physiologist fects of walking on the cross-sectional di- 26(Suppl):S115, 1983. mensions of the radius in postmenopausal 43. Huddleston AL, Rockwell D, Kulund DN, et al: women. Calcif Tissue Int 41:65,1987. Bone mass in lifetime tennis athletes. JAMA 44:1107,1980. 59. Cavanaugh DJ, and Cann CE: Brisk walking 44. Jones HH, Priest JS, Hayes WC, et al: Humeral does not stop bone loss in postmenopausal hypertrophy in response to exercise. J Bone women. Bone 9:201,1988. Joint Surg 59A.-204, 1977. 45. Montoye HJ, Smith EL,Pardon DF,et al: Bone 60. Chow RK, Harrison JE, and NotariusC: Effect mineral in senior tennis players. Scandina- of two randomised exercise programmes on vian Journal of Sports Science 2:26,1980. bone mass of healthy postmenopausal 46. Block J, Genant HK, Black D, et al: Greater women. Br Med J 292:607,1987. vertebral bone mineral in exercising young men. Western J Med 145:39, 1986. 61. Gleeson PB, Protas EJ, LeBlanc AD, et al: Ef- 47. Davee AM, Rosen CJ, and Adler RA:Exercise fects of weight lifting on bone mineral density patterns and trabecular bone density in col- in premenopausal women. J Bone Min Res lege women. J Bone Min Res 5:245, 1990. 5:153, 1990. 48. Heinrich CH, Going SB, Pamenter RW, et al: Bone mineral content of cyclically menstru- 62. Pruitt LA, Jackson RD, Bartels RL, et al: ating female resistance and endurance Weight-training effects on bone mineral den- trained athletes. Med Sci Sports Exerc 22:558, sity in early postmenopausal women. J Bone 1990. Min Res 7:179,1992. 49. Nilsson BE, and Westlin NE: Bone density in athletes. Clin Orthop Rel Res 77:179, 1971. 63. Notelovitz M, Martin D, Tesar R, et al: Estro- 50. Jacobson PC,Beaver W, Grubb SA,et al: Bone gen therapy and variable-resistance weight density in women: Collegeathletes and older training increase bone mineral in surgically athletic women. J Orthop Res 2:328,1984. menopausal women. J Bone Min Res 6:583, 51. Orwoll ES, Ferrant J, and Owatt SK: The re- 1991. lationship of swimming exercise to bone mass in men and women. Arch Intern Med 64. Rockwell JC, Sorenson AM, Baker S, et al: 149:2197, 1989. Weight training decreases vertebral bone density in premenopausal women: A pro- spective study. J Clin Endocrinol Metab 71:988, 1990. 65. Sinaki M, Wahner HW, Offord KP, et al: Effi- cacy of nonloadingexercises in preventionof vertebral bone loss in postmenopausal women: A controlled trial. Clin Proc 64:762, 1989. 66. Cann CE, Martin MC, Genant HK, et al: De- creased spinal mineral content in amenor- rheic women. JAMA 251:626,1984. 67. Drinkwater BL, Nilson K, Chesnut CH, et al:

Bone Concerns 101 Bone mineral content of amenorrheic and eu- 77. Cann CE, Cavanaugh DJ, Schnurpfiel K, et al: monorrheic athletes. N Engl J Med 311:277, Menstrual history is the primary determinant 1984. of trabecular bone density in women. Med Sci 68. Drinkwater BL, Bruemner B, and Chesnut, Sports Exerc 20:S59,1988 (abstr). CH: Menstrual history as a determinant of current bone density in young athletes. 78. Lloyd T, Buchanan JR, Bitzer S, et al: Inter- JAMA 263:545,1990. relationships of diet, athletic activity, men- 69. Fisher EC, Nelson ME, Frontera WR, et al: strual status and bone density in collegiate Bone mineral content and levels of gonado- women. Am J Clin Nutr 46:681,1987. tropins and estrogens in amenorrheic run- ning women. J Clin Endocrinol Metab 79. Louis O, DemeirlierK, Kalender W, et al: Low 62:1232, 1986. vertebral bone density values in young non- 70. Jones KP, Ravnikar VA, Tulchinsky D, et al: elite female runners. Int J Sports Med 12:214, Comparison of bone density in amenorrheic 1991. women due to athletics, weight loss, and pre- mature menopause. Obstet Gynecol 66:5, 80. Myburgh KH, Hutchins J, Fataar AB, et al: 1985. Low bone density is an etiologic factor for 71. Linnell SL, Stagger JM, Blue PW, et al: Bone stress fractures in athletes. Ann Intern Med mineral content and menstrual regularity in 113:754, 1990. female runners. Med Sci Sports Exerc 16:343, 1984. 81. Lloyd T, Triantafyllou J, Baker ER, et al: 72. Marcus R, Cann C, Madvig P, et al: Menstrual Women athletes with menstrual irregularity function and bone mass in elite women dis- have increased musculoskeletal injuries. tance runners. Ann Intern Med 102:156,1985. Med Sci Sports Exerc 18:374,1986. 73. Nelson ME, Fisher EC, Catsos PD, et al: Diet and bone status in amenorrheic runners.Am 82. Warren MP, Brooks-GunnJ, Hamilton LH, et J Clin Nutr 43:910, 1986. al: Scoliosis and fractures in young ballet 74. Snyder AC, Wenderoth MP, Johnston CC, et dancers. N Engl J Med 314:1348,1986. al: Bone mineral content of elite lightweight amenorrheic oarswomen. Hum Biol 58:863, 83. Prior JC, Vigna YM, Schechter MT, et al: Spi- 1986. nal bone loss and ovulatory disturbances. N 75. Cook SD, HardingAF, Thomas KA,et al: Tra- Engl J Med 323:1221,1990. becular bone density and menstrual function in women runners. Am J Sports Med 15:503, 84. Cann CE, Martin MC, and Jaffe RB:Duration 1987. of amenorrhea affects rate of bone loss in 76. Buchanan JR, Myers C, Lloyd T, et al: Deter- women runners: Implications for therapy. minants of peak trabecular bone density in Med Sci Sports Exerc 17:214,1985 (abstr). women: The role of androgens, estrogen and exercise. J Bone Min Res 3:673, 1988. 85. Wolman RL, Clark P, McNally E, et al: Men- strual state and exercise as determinants of spinal trabecular bone density in female ath- letes. Br Med J 301:516,1990. 86. Drinkwater BL, Nilson K, Ott S, et al: Bone mineral density after resumption of menses in amenorrheic athletes. JAMA 256:380,1986.

CHAPTER 6 Nutrition for Sports GABE MIRKIN, M.D. NUTRIENTS DRINKING DURING COMPETITION Carbohydrates Dehydration and \"Heat Cramps\" Proteins Women May Need Less Fluid Than Fats ENERGY STORAGE Men Comparing Women and Men When to Drink ENDURANCE What to Drink \"Hitting the Wall\": Depletion of Cold or Warm? EATING AND DRINKING AFTER Muscle Glycogen \"Bonking\": Depletion of Liver COMPETITION Glycogen PROTEIN REQUIREMENTS INCREASING ENDURANCE VITAMINS Training to Increase Endurance Mechanism of Function UTILIZING FAT INSTEAD OF Vitamin Needs of Female Athletes Vitamin C and Colds GLYCOGEN Vitamins and Birth Control Pills DIET AND ENDURANCE Vitamins and Premenstrual Food Intake during the Week before Syndrome Competition MINERALS Eating the Night before Competition Iron Eating the Meal before Competition Calcium Eating before Exercising Sodium Eating during Competition Potassium DRINKING BEFORE COMPETITION Trace Minerals THE ATHLETE'S DIET w,ith the exception of iron and calcium, nutrient requirements for female ath- letes are the same as those for their male counterparts. Women suffer far more frequently than men from deficiencies of iron and calcium. Ten percent of healthy, white, middle-class female adolescents are iron deficient, while 5% have iron-deficiency anemia.1 Athletes are at greater risk than nonathletes for devel- oping iron deficiency,2 which, even in the absence of anemia, can limit athletic endurance. Hypoestrogenic female athletes are at increased risk of developing osteo- porotic bone fractures.3 In addition to hormone replacement, the prevention and 1O2

Nutrition for Sports 103 treatment of this condition should include Table 6-1. ESSENTIALNUTRIENTS ingestion of adequate amounts of calcium. Water Linoleic acid A proper diet can help female athletes to 8 or 9 amino acids maximize performance. However, many ath- 13 vitamins letes have nutritional misconceptions that Approximately 21 minerals hinder performancerather than help it. For Glucose (for energy) example, many athletes incorrectly believe that a high-protein diet improves perfor- in competition by following sound scientific mance and increases muscle size and nutritional practices. A brief discussion of strength, that vitamin requirements are sig- basic principles of nutrition will precede the nificantly greater for athletes, that fluid re- sections on the application of such princi- quirements during exercise should be dic- ples to athletic competition. tated by thirst, and that salt tablets should be taken in hot weather.4,5 All of these myths Carbohydrates will be refuted in this chapter. Carbohydrates are composed of sugars. In 1967,the women's world record for the They can be monosaccharides, such as glu- marathon was 3:15:22, set by Maureen Wil- cose and fructose in fruit; disaccharides, ton of Toronto, Canada. By 1985, the world such as lactose in milk or sucrose in candy; record was lowered to 2:21:06 by Ingrid and polysaccharides, such as starch in a po- Kristiansen of Norway. The fantastic im- tato or fiber in celery. provement in world records in all sporting events is due primarily to superior training Before carbohydrates can be absorbed, methods, but it is also due to improved they must first be hydrolyzed into one or knowledge about nutrition.In the late 1960s, more of the following four sugars: glucose, it was common for athletes to eat high-pro- fructose, galactose, and mannose. Of these tein diets, to reduce their intake of food on sugars, only glucose circulates beyond the the days before competition, to ingest no portal system. The other three are con- food or liquids during competition, and to verted to glucose by hepatocytes before eat only a limited amount of food after com- they can re-enter the circulation (Fig. 6-1). petition. Today knowledgeable athletes fol- low none of these old regimens.6 Circulating glucose can be used by all cells as a source of energy. Glucose that is This chapter reviews some of the basic not used immediately can be stored as gly- physiologic principles that serve as the cogen only in the liver and muscles. When foundation for advising athletes how to use these tissues are saturated with glycogen, nutrition to improve sports performance. excess glucose is then converted to fat. Liver glycogen can yield glucose to the circula- NUTRIENTS tion, where it subsequently can be used by other tissues. On the other hand, the glu- Humans require approximately 46 nutri- cose from muscle glycogen can be utilized ents to be healthy. An essential nutrient is only by that particular muscle. one that cannot be produced by the body in adequate amounts and, therefore, must be Proteins supplied by the diet (Table 6-1). Lack of an Fifteen percent of ingested protein is hy- essential nutrient can impairperformance, but taking large amounts of any specific nu- drolyzed to amino acids and polypeptides in trient has not been shown to improve per- the stomach, while the remaining protein formance. undergoes hydrolysis in the small intestine. These metabolites are actively transported Athletes can improve their performances

104 Basic Concepts of Exercise Physiology Figure 6-1. Sugar circulation.All carbohydrates are sugars bound together. They can be single sug- ars, as in fruit and honey; two sugars bound together, as in milk and table sugar; and hundreds and thousands of sugars bound together, as in corn and beans.

Nutrition for Sports 105 into intestinal epithelial cells. Once there, creatic lipase into monoglycerides, free fatty most of the polypeptides are hydrolyzed to acids, and glycerol (Table 6-2), which enter form amino acids, which are then absorbed the epithelial cells lining the intestines. into the general circulation. Once there, the monoglycerides are hydro- lyzed to form glycerol and fatty acids. Then, The main functions of proteins are to triglycerides are formed again, arecom- form structural components, enzymes, bined with cholesterol and phospholipids, hormones, neurotransmitters, antibodies, and are covered with a lipoprotein coating transport molecules, and clotting factors. to form chylomicron particles, which pass Protein also can be a source of energy. As through the lymphatic system into the gen- much as 10%of energy during exercise can eral circulation. Short-chain fatty acids can come from protein, with more than half be absorbed directly into the circulation. coming from one amino acid, leucine. Since Excess fat is stored primarily in fat cells and leucine represents only a small fraction of muscles. the amino acids in ingested protein, the leu- cine that is used for energy must come from ENERGY STORAGE a source other than ingested protein. It also Only fats and carbohydrates are stored does not come from muscle sources of leu- cine. Most of the leucine that is used for en- for future use as an energy source. The ergy is formed de novo. The nitrogen for the human body cannot store extra protein. Fat newly formed leucine comes from other stores energy in the most economic way, as branched-chain amino acids (isoleucine it provides 14 times as much energy per and valine), and most of the carbon comes given weight as stored liver glycogen, which from glucose and other amino acids. must be stored with other liver tissue. One pound of stored fat will yield 3500 kcal, Before amino acids can be used for en- whereas 1 Ib of liver contains only enough ergy, deamination or transamination must glycogen to yield 250kcal. This great dispar- occur to remove the nitrogen. Athletictrain- ity in energy storage is explained by the fact ing can double the levels of important trans- that fat occupies 85%of the space in fat cells, aminases, such as SCOT and SGPT, and this while liver glycogen is diluted by other cel- increases significantly the body's ability to lular elements and occupies less than 15% of utilize leucine and other amino acids for en- the space in liver cells. ergy. The body of the average athlete contains Fats only enough stored fat to support exercise More than 95% of the fat in foods is in the for 119 hours, enough stored muscle glyco- gen for11/2hours, and enough stored liver form of triglycerides. Fat is separated from glycogen for 6 minutes. Table 6-3 shows other foodstuffs in the stomach, but it is not how limited the stores of carbohydrates are degraded until it is emulsified (dispersed in and how extensive the fat stores are. water) by bile salts in the small intestine. The fat globules are then hydrolyzed by pan- Table 6-2. GLYCEROL AND FATTY ACIDS C-C-C-C • • • C-C-COOH C-OH C-C-C • • • C-C-COOH | | + C-C-C • • • C-C-COOH C-C-C-C • • • C-C-COOH *C-OH 1i i C-C-C • • • C-C-COOH C-C-C-C • • • C-C-COOH + FATTY ACIDS TRIGLYCERIDE C-OH , GLYCEROL

106 Basic Concepts of Exercise Physiology Table 6-3. MAXIMAL BODY STORAGE swimming performances. It is likely that the CAPACITY FOR CARBOHYDRATES AND insulating properties of fat, rather than the FATS7 glycogen-sparing effect, gave them an ad- vantage. Loss of body heat is a major prob- Storage Site Weight of Available lem in distance swimming. Furthermore, Tissue (g) Energy (kcal) having extra fat raises a swimmer higher out of the water and reduces drag (see Chapter Muscle glycogen 125-300 500-1200 4). Liver glycogen 50-100 200-400 Body fat 6000-15,000 50,000-140,000 Comparing Women and Men ENDURANCE At the same level of fitness, the average Endurance is the ability to continue exer- woman has 7% to 10% more body fat than the cising muscles for an extended period of average man. For example, top female mar- time. To continue exercising, muscles re- athon runners have 12% to 20% body fat, quire energy, the major sources of which are compared with 5% to 10% for their male triglycerides and glycogen in muscles and counterparts. triglycerides and glucose in blood. The main advantage of fats is that the Muscles use primarily fats and carbohy- body can store vast amounts. The main ad- drates as their energy sources. At rest, mus- vantage of carbohydrates is that they can be cles use mostly fats for energy. During ex- utilized under anaerobic conditions. Fatme- ercise, muscles use more carbohydrates, tabolism always requires oxygen. As exer- with a higher percentage of carbohydrates cise intensity increases, the percentage of and a lower percentage of fat being used as energy derived from muscle glycogen also the intensity of the exercise is increased. In increases. Much of the exercise during most spite of their increased percentage of body competition events is done at maximum or fat, women use the same percentage of fat as near-maximum intensity. The limiting factor men through all intensities of exercise. For for exercising at an intensity greater than example, at race pace for the marathon, top 70% mofusVcole2ms acxanissttohree.1a2mount of glycogen male and female runners have been shown that to derive the same 50%of their energy from fat,8 and top female athletes have not dem- \"Hitting the Wall\":Depletion of onstrated greater endurance than male ath- Muscle Glycogen letes. Muscle endurance depends on the ade- In running events from 100to 1500 meters, quacy of muscle glycogen stores. Depletion world records for women are 7% to 10% of muscle glycogen causes pain and fatigue slower than those for men.9,10 In running and causes an athlete to lose much of her events from 1500 meters to the marathon, strength and to have difficulty coordinating world records for women are 13% to 15% muscle movements. Athletes refer to this as slower than those for men.11 The extra fat hitting the wall, a common occurrence in that most women carry slows them down marathon runners after they have raced during running. However, having extra fat is more than 18miles. The more glycogen that an advantage during swimming. Penny Dean can be stored in a muscle, the longer it can of California set the world's record for men be exercised. Recent research has called and women for a single crossing of the En- into question this explanation. Since bicycle glish Channel in 7 hours and 40 minutes (in racers run out of their muscle glycogen after 1978), and CynthiaNicholsof Canada set the 2 hours of racing and do not \"hit the wall,\" record for a double crossing at 19hours and another explanation would be more feasible. 12 minutes (in 1977). Their extra fat may well be the reason for their great endurance

Nutrition for Sports 107 The most likely cause is that hard running After the athlete eats, her musclesfillwith damages the fibrous connective tissue in glycogen, and this reduces production of muscles, while the smooth rotary motionof glycogen synthetase. Therefore, the effects pedalling doesnot. of depletion trainingare short-lived, and de- pletion training should be repeated at fre- \"Bonking\": Depletion of Liver quent intervals. However, athletes usually Glycogen do not perform depletion training more fre- quently than once a week, because depletion Brain endurance depends on circulating of muscle glycogen leads to increased utili- glucose. More than 98%of the energy for the zation of muscle protein for energy. This brain is derived from blood glucose, which damages the muscle, delays recovery, and depends on hepatic glycogen stores for limits the amount of intense training theath- maintenance. When the blood concentra- lete can accomplish. tion of glucose falls to low levels, the athlete may feel very tired and can suffer from a syn- Many recreational athletes do not appre- copal episode or seizures or both. Athletes ciate the importance of depletion training refer to this as \"bonking.\" Bicyclists who do and enter marathons before they have put not eat during endurance races may experi- this training technique to adequate use.Asa ence this after 4 or more hours of cycling. result, they have inadequate muscle glyco- gen stores to enable them to run the neces- sary distance. INCREASING ENDURANCE UTILIZING FAT INSTEAD OF GLYCOGEN An athlete can improve endurance by using training methods and dietary manip- In addition to depletion training, other ulations that increase muscle glycogen stor- techniques that have been promoted to de- age and decrease muscle glycogen utiliza- crease glycogen utilization by muscles dur- tion by increasing fat utilization.13 ing exercise includeeating a high-fat diet for several days prior to competition, takingnu- Training to Increase Endurance tritional supplements, and takingsympatho- To improve the ability of muscles to store mimetic agents. increased amounts of glycogen13 and utilize At least one study showed that eating a increased amounts of fat (and less glyco- high-fat diet for several days prior tocom- gen),14 athletes use a training technique petition will increase muscle utilization of called depletion. They exercise until muscle fat. However, endurance was not improved glycogen has been nearly depleted (Table by this technique.16,17 It is not unusual for 6-4). This causes muscle cells to increase blood glucose concentrations to fall as low production of glycogen synthetase, which as 30 mg/dL during vigorous exercise. Eat- increases glycogen synthesis and, in turn, ing a high-fat diet does not reduce muscle glycogen storage.15 glycogen utilization or prevent the devel- opment of hypoglycemia (with or without Table 6-4. AVERAGE TIMES FOR MUSCLE symptoms) during exercise.17 GLYCOGEN DEPLETION IN ELITE. ATHLETES There is no evidence that taking large amounts of any vitamin,mineral,protein, or Marathon runner lA-2Vih carbohydrate will cause muscles to increase 4-6 h their utilization of fat.18 Bicycle racer Cross-country skier 10-12 h Claims have been made that carnitine supplements enhance endurance. Carnitine is a protein that transports fat into mito-

108 Basic Concepts of Exercise Physiology chondria, where fat is catabolized for en- bodies process foods in the same ways. An ergy. However, there is no evidence that any athlete can increase her endurance by eat- supplement will increase mitochondria! fat ing the right meals 3 days before, the night content enough to increase fat utilization. before, or several hours before competition. Myocytes and hepatocytes synthesize large amounts of carnitine from lysine and methi- Food Intake during the Week onine, and human myocytes contain enough before Competition carnitine to support fat metabolism even under extreme exercise conditions.19 The In 1939, Scandinavian researchers fact that most athletes include meat, fish, or showed that eating a high-carbohydrate diet chicken—rich sources of carnitine—in their for several days before a competitive event diets provides another reason why athletes increases muscle glycogen stores and en- do not need carnitine supplements. durance, while a low-carbohydrate diet de- creases muscle glycogen stores and endur- Caffeine raises blood triglyceride levels ance.22 In the mid-1960s,other investigators by increasing catecholamine production proposed a method of \"carbohydrate load- and sensitivity. Catecholamines increase ing\" that was practiced by many endurance triglyceride utilization by promoting free athletes throughout the world.23,24 fatty acid release from adipocytes and up- take by myocytes. Taking caffeine prior to 1 Seven days prior to competition, the workouts has been shown to increase en- athlete performs a long depletion work- durance in training sessions by increasing out. muscle utilizationof fat,20 but it has not been shown to increase endurance in competi- 2 For the next 3 days, she keeps the gly- tion. A possible explanation for this differ- cogen content of her exercised muscles ence in responses is that caffeine may be ef- low by eating a low-carbohydrate diet. fective in prolonging endurance only when endogenous catecholamine levels are low. 3 For the next 3 days, she eats her regular In a laboratory setting, athletes may be re- diet plus extra carbohydrate-rich foods. laxed and have low circulating levels of cat- echolamines. Raising catecholamines in this Athletes should not ingest extra carbohy- situation may enhance performance. How- drate for more than 3 consecutive days. In ever, prior to competition most athletes that time, muscles and liver will be at their have very high levels of catecholamines. maximum capacity for storing glycogen, so Raising their levels further may not help no additional glycogen can be stored. In ad- them and,indeed, may harm them. Large dition, carbohydrate packing should not be amounts of catecholamines can cause trem- used in events lasting less than 60minutes, ors and irritability. because it will not be helpful and may even be harmful.25 The muscles of trained athletes Seven days of supplementation of a high- are not depleted of glycogen in so short a carbohydrate diet with dihydroxyacetone time. Carbohydrate packing may reduce per- and pyruvate has been shown in one study formance in events requiring great speed to increase endurance.21 Further studies are over short distances, since each gram of gly- needed before this practice can be accepted cogen is stored with almost three additional as an effective means of increasing endur- grams of water, making the muscles much ance. heavier than usual. DIET AND ENDURANCE Few top athletes practice this 7-day regi- Female athletes should follow the same men today because it can hinder perfor- mance. During the depletion phase, the ath- nutritional principles as men, since their lete cannot train properly and usually is irritable and unable to perform mental tasks effectively. During the high-carbohydrate phase, the ingestion of vast amounts of car-

Nutrition for Sports 109 bohydrates has been reported to cause evening prior to competition does not seem chest pain,26 myoglobinuria, andnephritis.27 to hinder performance and may actually However, these side effects are rare. Marked help it. However, more research is needed overeating raises blood lipid levels, and this to resolve this question. can lead to occlusion of the coronary arter- ies in exercisers who already have signifi- Eating the Meal before cant arteriosclerosis. Furthermore,this reg- Competition imen has not been shown to be more effective than simply reducing the workload The major function of the precompetition and ingesting some extra carbohydrates.28 meal is to maximize hepatic glycogen (see Table 6-5). Serum glucose is sufficient to As a result of all of these concerns, most support brain function for only 3 minutes. top athletes in endurance sports avoid the To prevent hypoglycemia, hepatocytes must low-carbohydrate phase and modify the release glucose constantly. However, there high-carbohydrate phase. The runner can is enough glycogen in hepatocytes to last maximize muscle glycogen by a combina- only 12 hours when the athlete is at rest.32 tion of reducing her workload and eating a Obviously, during exercise, liver glycogen is regular diet that contains at least 55% of its depleted much faster than that. calories from carbohydrates.29 The 7-day carbohydrate-packing regimen thus is Timing of Meal changed to eating a high-carbohydrate diet and stopping intense exercise 4 days prior To maximizehepatic glycogen stores, the to competition. precompetition meal should be ingested 5 or fewer hours before competition. If the Eating the Night before meal is eaten more than 5 hours before com- Competition petition, the hepatocytes will be depleted of a considerable amount of stored glycogen On the night before a competitive event, and will have less than maximal glycogen many athletes eat a high-carbohydrate stores when the athlete starts competition. meal. The primary purpose of this meal is to Several previous studies showed that eating increase muscle glycogen stores (Table 6- sugared food just before competition in- 5). The pregame meal cannot serve this creased an athlete's chances of developing function, since it takes at least 10 hours to postprandial hyperinsulinemia,which can replenish muscle glycogen stores.30 cause hypoglycemia.33 However, the vast majority of recent reports concludethat pre- It is controversial whether muscle glyco- event glucose consumption can cause re- gen storage is promoted more by ingestion duced blood glucose levels during exercise, of starch or monosaccharides and disaccha- but it has no effect on endurance.34,35 The rides. One recent study showed that a high- brains of well-conditioned athletes can con- monosaccharide and high-disaccharidediet tinue to function at lower blood sugar levels caused more muscle glycogen to be stored than those of unfit individuals. than did a high-starch diet.31 Based on these findings, ingestion of simple sugars on the At rest, blood glucose levels as low as 25 mg/dL usuallycause a deterioration inbrain Table 6-5. PRIMARY FUNCTION OF function and loss of consciousness. How- MEALS BEFORE AN AFTERNOON ever, physically fit individuals can usually COMPETITION tolerate such levels during exercise without Supper (the day before): To increase muscle glycogen developing any symptoms at all,36 even though they are using up their muscle gly- stores cogen stores at an accelerated rate37 and will Breakfast: To increase hepatic glycogen stores feel fatigue sooner than usual.38

110 Basic Concepts of Exercise Physiology Composition of Meal just prior to exercising. It is speculated that, when pain does occur, it is due to stomach Precompetition meals should be high in muscle spasms, which result from ischemia carbohydrates. It does not make any differ- caused by the shunting of blood from the ence whether the meal is also high in fat.A stomach muscles to the exercising mus- combination of a high-carbohydrate meal 4 cles.40 During exercise, gastric motility in- hours before exercise and around 50 g of creases,41-43 and splanchnic blood flow de- carbohydrate 5 minutes before exercise can creases.44 increase glycogen stores and maximize en- durance.39 The athlete can eat any foods she A drug company has advertised that tak- likes, as long as she suffers no discomfort ing fructose before exercise, compared to and has an empty stomach by the time she glucose, results in a much lower rate of mus- starts to exercise (Table 6-6). cle glycogen depletion, because fructose does not cause a rapid rise in either blood Theoretically, fat and protein are poor sugar or insulin.45 However, there is no evi- choices for the precompetition meal. Fat de- dence that eating fructose prior to exercis- lays stomach emptying, and the urea and ke- ing offers any advantage over eating nothing tones released by the catabolism of protein at all, and there is evidence that eating fruc- can promote diuresis. However, no con- tose is less advantageous than eating noth- trolled studies have demonstrated adverse ing at all. It is true that fructose ingestion effects from fat or protein in precompetition may cause a lower rise than glucose in blood meals, and many athletes can tolerate high- glucose and insulin levels,46,47 but eating fat and high-protein precompetition meals fructose does cause an increase in circulat- without having their performances hin- ing glucose and insulin levels, whereas eat- dered. ing nothing does not. Fructose ingestion also causes a greater rate of muscle glycogen utilization, compared with eating noth- Eating Before Exercising ing.48,49 The fact that fructose costs 15 times Provided that the exercise is not too in- as much as glucose offers an added disad- tense and the amount of food eaten is not too vantage. great, most exercisers will not suffer from abdominal pain or discomfortwhen they eat Eating During Competition Table 6-6. EXAMPLESOF It is not necessary for most conditioned PRECOMPETITION MEALS THAT PASS athletes to eat during events that last less RAPIDLY FROM THE STOMACH than 2 hours. However, athletes can benefit from eating during events lasting longer Break fast #1 than that. The abilitityof exercising muscles Breakfast cereal with milk to utilize ingested carbohydrates in place of A few small pieces of fruit muscle glycogen is dependent oncondition- Toast with butter ing. The higher the level of fitness, the better 1 cup of coffee able the athlete is to utilize ingested carbo- 1 glass of milk hydrates during exercise.50 No more than 1A glass of orange juice In contrast to ingestion of food before ex- Break fast #2 ercising, ingestion of food during exercise Pancakes does not cause significant pancreatic output A small pat of jelly of insulin. At rest, eating causes hypergly- Breakfast cereal cemia, which promotes insulin release. Milk or coffee However, during exercise, muscles remove Glass of water glucose so rapidly from the circulation that 1/2 glass of fruit juice or a small piece of fruit

Nutrition for Sports Ill blood levels of glucose rarely rise high the week prior to competition by doubling enough to induce significant insulin release her daily intake of fluid for 1 week. An in- from the pancreas.51 Insulin-induced hypo- crease from 1Lto 2 Lcan increase blood vol- glycemia caused by eating during intense ume by 10%.60 The extra fluid is not lost com- exercise does not occur.52 pletely as urine. Almost any food can be used for energy. She can also increase her intake of fluids When taken duringexercise, glucose has not during exercise by distending her stomach been shown to be more effective than table with a large amount of fluid just before she food in prolonging endurance. Studies com- competes. If she drinks 600 mL of water just paring glucose with fructoseoffer conflicting before competition, almost 400 mLwill pass results. One study showed that neither glu- into the intestines in 20 minutes.61 Then she cose nor fructose is better than placebo in should try to ingest 3 oz (about 90 mL) of reducing muscle glycogen utilization.43 An- water every 10 minutes. other study showed that fructose has a greater muscle-glycogen-sparing effect,53 DRINKING DURING while a third study showed that glucose has COMPETITION a greater glycogen-sparing effect.54 Although most fit athletes do not gain any Any maneuver that causes muscles to in- advantage from eating during competition in crease the rate at which they utilize fat for events lasting less than 2 hours, they can al- energy theoretically should help to con- ways benefit from keeping themselvesade- serve muscle glycogen and prolong endur- quately hydrated. Competitive runners and ance. A high-fat diet has been shown to in- swimmers can lose approximately \\1A L of crease endurance in rats,55 but neither a fatty fluid during an intense 1-hour workout. Al- meal nor glycerol has been shown to pro- though athletes exercising in warm, humid long endurance in humans.56,57 environments can see their sweat and ap- preciate their obvious fluid loss, those ex- Ten years ago, maltodextrin glucose poly- ercising in water sports may not be able to mer solutions, Exceed (Ross Laboratories, perceive that this loss has occurred. Columbus, Ohio) and MAX (Coca-Cola), ap- peared to enhance endurance in events last- Dehydration and \"Heat ing longer than 2 hours.58 The polymers in Cramps\" these drinks are composed of five glucose molecules and seemed to supply calories at As the athlete becomes progressively a low osmotic pressure, thereby not delay- more dehydrated, her blood volume de- ing absorption and resultant glucose utili- creases. There may not be an adequate vol- zation. However, recent data show that they ume of circulating blood to carry heat from are not superior to free glucose for maintain- exercising muscles to the skin, where the ing hydration and blood glucose levels, and heat can be dissipated, and, at the same they have not been shown to increase en- time, to carry oxygen to heavily exercising durance.59 muscles. Reduced cutaneous blood flow will raise body temperature, and this will impair DRINKING BEFORE performance. The decreased blood volume COMPETITION can also limit the amount of blood that flows to the most heavily exercising muscles. The The maximal amount of fluid that can be resultant hypoxia can cause sustained pain- absorbed during exercise is 600 to 800 ful muscle contractions, known as heat mL/h. No matter how much fluid an athlete cramps.62 ingests during competition, she will not be able to absorb enough to keep up with her losses. She can increase hydration during

112 Basic Concepts of Exercise Physiology Women May Need Less Fluid What to Drink than Men Adequate hydration will usually prevent Earlier studies showed that men have bet- heat cramps and hyperthermia. Water is the ter tolerance than women for exercising in preferred drink to be taken during exercise the heat. However, these studies did not lasting less than 1 hour. Extra calories64 and compare men and women exercising at com- minerals65 are usually not needed. With ad- parable percentages of their Vo2max. More equate dietary intake, the athlete will store recent studies have shown that women are enough hepatic and muscle glycogen to last able to tolerate exercise in the heat as well 1 hour.64,65Athletes who exercise longer than as men, provided that they both have the that need energy sources and minerals also. same Vo2max.63 The rules for energy-containing fluids have changed dramatically in the last few years. During exercise, women perspire less In 1968, studies showed that 2.5% was the than men of the same fitness level,63 but highest concentration of sugar that could be there is no evidence that women tolerate ex- contained in an exercise drink and still be ercise in the heat better than men. There- absorbed.66,67 This posed a problem because fore, female athletes should take the same drinks taste best when they contain a 7%to precautions as men to ensure that they are 10% concentration of sugar. Soft drinks and adequately hydrated duringhot-weather ex- fruit juices contain 7% to 10% sugar. Soon ercise. after these studies, many exercise drinks containing 2.5%sugar appeared on the mar- When to Drink ket. They did not taste good because the The athlete should drink before she feels concentration of sugar was too low, so some of the manufacturers added saccharin to thirsty. Bythe timethat she perceives thirst, sweeten the taste. she already will have lost 1to 2 Lof fluid and will not be able to replace that deficit while Twenty years later, new studies refuted she exercises. During intense exercise, it is the 1968 report. The 1968data were col- impossible to absorb fluids as fast as they lected on resting subjects. Exercise in- are lost. The maximum rate of gastric emp- creases gastric emptying for both solid tying is about 800 mL/h. It is common for meals and liquids.68 When the same studies competing athletes to perspire as much as were repeated using people who were exer- 2000 mL/h. cising, 7% to 10% sugared drinks were ab- sorbed rapidly. Based on the most recent Thirst is a late sign of dehydration during evidence, special exercise drinks are not exercise because osmoreceptors in the necessary, although many athletes prefer brain will not signal a thirst sensation until them. All 10%drinks are equally effective in the blood sodium concentration rises con- supplying energy. A basic 10% sugared drink siderably. The primary mode of fluid loss may be prepared by dissolving 8 table- during exercise is sweating. Sweat contains spoons of sugar in 1 L of water. Each table- some sodium, although it is hypotonic in spoon of sugar contains 12 g of sucrose. comparison to blood. As sodium is secreted into sweat, the serum sodium level rises Drinks with low levels of minerals are ab- more slowly than if water alone were lost. As sorbed slightly more quickly than pure a result, significant amounts of fluid are lost water, but the difference is not significant. before hypernatremia develops enough to Mineral loss through sweat occurs so slowly cause thirst. Therefore, on a warm day, the that conditioned athletes rarely develop hy- athlete should drink a cup of cool water just ponatremia, hypokalemia, or hypocalcemia before she starts to exercise and every 15 during exercise.65 In fact, the opposite is minutes during exercise. more likely to occur. Serum sodium and po- tassium levels rise during exercise and do

Nutrition for Sports 113 not fall unless the exercise is intense and Table 6-7. IMMEDIATE prolonged. Increased serum sodium levels POSTCOMPETITION MEALTO PREPARE are due to the loss of sweat, which is hypo- FOR ANOTHERCOMPETITION WITHIN A tonic in relation to blood. Increased serum FEW HOURS* potassium levels are due to release of potas- sium from myocytes, preventing overheat- Food Carbohydrate ing of muscles during exercise. Blood cal- cium levels usually are not altered during 1 orange 10 g exercise. Magnesium levels in blood de- 1 slice of bread 13 g crease slightly during exercise, but this is 2 chocolate chip cookies 12 g due primarily to cellular uptake of magne- 1 banana 30 g sium and not to a significant loss of magne- 1 12-ounce soft drink 35 g sium from the body.69,70 No fluid restriction *A 50-kg woman needs 75 to lOOgofCHO. Cold or Warm? In events such as gymnastics, track and In the 1960s, studies showed that cold field, wrestling, and swimming, athletes may be scheduled to compete in several events drinks (4°C) are absorbed faster and are less on the same day. It is very important for likely to cause abdominal cramps than warm them to drinkand eat immediatelyafter they ones.67 The theory was that cold water finish each event. Even if they rehydrate causes the stomach to contract and pushflu- completely (as evidenced by a return to nor- ids rapidly into the intestines. However, mal weight), it will still take 4 to 5 hours for more recent studies show that temperature the water to redistribute among the body does not make much difference.71,72 Further- fluid compartments.79 Delaying carbohy- more, carbonated drinks are absorbed as drate ingestion % hour markedly delays rapidly as noncarbonated ones.73 muscle glycogen replenishment.80 The rec- ommended amount of carbohydrate inges- EATING AND DRINKING AFTER tion for immediate maximal rate of replen- COMPETITION ishment is 1.5 g/kg of body weight81 (Table 6-7). Doubling that amount does not in- Much of postcompetition tiredness is due crease glycogen replenishment further. to depletion of muscle glycogen stores and dehydration. Recovery from vigorous exer- PROTEIN REQUIREMENTS cise depends on muscle glycogenreplenish- ment and rehydration.74 It makes no differ- When adjusted for weight, protein re- ence whether such replacement is quirements are the same for men and accomplished by eating simple sugars or women. The protein requirement of 0.8g/kg complex carbohydrates.75,76 Fructose offers body weight per day is based on body mass. no advantage over other carbohydrates, as It is increased significantly by reduced ca- glucose causes more rapid muscle glycogen loric intake, but had previously been felt to restoration than fructose does.77 Carbohy- be increased only slightly during exer- drate intake in athletes averages around 250 cise.82-84 However, several recent studies g/d. This is far too little to afford maximal using leucine turnover measurements seem glycogen replacement. It takes at least 600 to show an increase of up to 20% in protein g/d of carbohydrate for maximum compen- turnover during aerobic exercise.85,86 The sation. Therefore, it is important for athletes case for increased protein needs duringex- to eat carbohydrate-rich meals after com- ercise is supported further by other studies petition.78 showing increased excretion of 3-methyl

114 Basic Concepts of Exercise Physiology histidine,87 increased urea nitrogen losses,87 tein (1.3 X 100) or 160 g of lower-quality and depression of protein synthesis.88 protein in a week. This is accomplished by eating the equivalent of only 2 cups of corn Further research is necessary before pro- and beans per day. tein can be considered a significant source of energy during exercise.89 Studies show an Since the body cannot store extra protein, increased utilization of only the branched- the excess is catabolized into ammonia and chain amino acids, leucine, isoleucine, and organic acids, much of which is excreted in valine. This does not make a strong case for the urine. These compounds act as diuretics increased protein utilization during exer- and, during exercise in hot weather, can cise. The branched-chain amino acids are cause dehydration and increase the risk of degraded by active skeletal muscles to re- heat stroke.94 Ingesting excessive amounts lease nitrogen, which is combined with py- of protein can also increase calcium require- ruvate in muscles to form alanine. The liver ments by increasing urinary loss.95 While removes nitrogen from alanine to form glu- this is probably of little significanceto most cose, as a source of energy. However, turn- women, it may accelerate bone loss in hy- over rates for amino acids that are not poestrogenic female athletes. Taking more branched chain, such as lysine, are unaf- than 4 g of extra protein per kilogram per fected by exercise,90 nitrogen losses are not day can also cause loss of appetite and di- consistently elevated during and after exer- arrhea. cise,90 and no loss of muscle mass can be de- tected during exercise.90 VITAMINS Taking extra protein does not increase Sixty million Americans, or 37% of the protein turnover rates in exercisers,91 but adult population, take vitamin supple- when combined with a heavy resistance ments.96 More women (42%) than men program, it was shown to increase protein (31%) take vitamins, presumably because retention slightly.92In that study, an extra 2 they are more health conscious than men. g of protein supplements per kilogram per Three out of four Americans think that tak- day was added to the subjects' usual intake ing extra vitamins will give them more en- of 1.3 g. The vast majority of the extra pro- ergy.97 One out of five believes that lack of vi- tein was oxidized for energy, with only a tamins causes arthritis and cancer,98 and small amount retained in lean tissue. one out of 10 does not know that vitamin re- quirements can be met without taking sup- The sole stimulus to make a muscle plements.99 Although 10% may seem like a stronger is to exercise that muscle against small part of the population, this figure sig- resistance. This stimulus is so strong that nifies that 25 million Americans believe that muscles can be enlarged and strengthened they have to take vitamin supplements to be by proper resistance training,even if a sub- healthy. ject is fasting or losing weight and if all of her other muscles are becoming smaller.93 Mechanism of Function A vitamin is an organic compound that the It does not take much extraprotein to sup- ply amino acids for enlarging muscles. An body requires in small amounts for health. athlete with an excellent strength-training While the exact mechanisms of function for program may gain 1 Ib of muscle in a week. several vitamins are not completely under- Since muscle is 72% water, 1 Ib of muscle stood, much is known about the function of contains only about 100 g of protein. How- the B vitamins, which are parts of enzymes. ever, the loss of efficiency in high-quality Because the enzymes containing these vita- protein utilization is around 30% and in mins are required in only small amounts, poorer-quality protein, around 60%. There- fore, to build 100g of extra protein, the ath- lete must consume 130g of high-quality pro-

Nutrition for Sports 115 they catalyze reactions without being de- These four vitamins catalyze the reactions pleted. that convert carbohydrates and protein to energy.105 For example, heavy exercise can The B vitamins enter the cells that are to increase riboflavin requirements by as use them. Such cells produce apoenzymes, much as 17%,105 but the total daily needs for which combine with the vitamins to form riboflavin can be met by drinking three holoenzymes. Cells produce only limited glasses of milk. The total needs for all four amounts of apoenzymes, leaving unbound B \"energy\" vitamins can be met by eating a vitamins in excess. The Recommended Di- varied diet that contains more than 2000 cal- etary Allowances (RDAs) for Bvitamins, de- ories per day, because all four of these vita- termined by the Food and Nutrition Board mins are found in meat, fish, chicken, milk, (FNB) of the National Research Council of and whole grains. the NationalAcademy of Sciences, \"are the levels of intake ... adequate to meet the Although the refining process removes known nutritional needs of practically all thiamine, niacin, riboflavin, and panto- healthy persons.\"100 It also represents the thenic acid from flour, most manufacturers amount of B vitamins that will saturate the routinely add these vitamins in order to apoenzymes of the target cells.100 Ingesting comply with interstate shipping laws. Thus, more vitamins does not increase the rate of athletes who eat breads made from refined reactions, because cellular apoenzymes are flour rarely need supplements containing the limiting factor. these \"energy vitamins.\" To help your patients understand why ex- Vitamin Cand Colds cess dosages of B vitamins are not needed, Some athletes take large doses ofvitamin you can use the following analogy offered by Herbert and Barrett.101 Consider the human C in the hope that it will help to protect them body to be like a traffic intersection. Many from developing upper respiratory infec- cars (chemical reactions) pass through the tions. However, virtually all double-blind intersection, but only one police officer (vi- studies on the subject show that vitamin C tamin) is necessary to direct traffic. Bringing does not prevent colds.106 in many police officers (excess vitamins) will not cause more cars (chemical reac- Vitamins and Birth Control Pills tions) to pass through the intersection. Whether women who take oral contracep- Vitamin Needs of Female tives require vitamin supplementation re- Athletes mains controversial.107-109A review of the lit- erature shows that, on the average, women The diets of athletes who take in more who take birth control pills have lower than 2000calories per day usually supply vi- serum levels of riboflavin, pyridoxine, fola- tamins in amounts greater than their cin, cyanocobalamin, and ascorbic acid and RDAs.102 People who try to control their higher body levels of vitamin K.109 However, weight usually restrict their intake of food, their tissue levels110 and blood levels111 are and this can lead to an intake of vitamins still within the normal range. There is no ev- that is less than the RDA. However, the idence that such women are more likely to RDAs are set so far above minimum require- develop clinical symptoms of vitamin defi- ments that dieters rarely develop signs or ciency. Since birth control pills increase the symptoms of vitamindeficiency, even if they need for these vitamins only a smallper- do not meet the RDAs.103 centage, if at all, it seems unlikely that vita- min requirements change appreciably be- Prolonged exercise can increase require- cause of oral contraceptive use. ments for thiamine, niacin, riboflavin, and pantothenic acid beyond their RDAs.104

116 Basic Concepts of Exercise Physiology Vitamins and Premenstrual Table 6-8. Minerals Trace Syndrome Fluorine Major Silicon Strength, speed, endurance, and coordi- Calcium Vanadium nation have not been shown to vary consis- Phosphorus Chromium tently throughout the menstrual cycle. Fe- Chlorine Manganese male athletes report greater perceived Potassium Iron exertion premenstrually. Premenstrual syn- Sulfur Cobalt drome (PMS) is discussed more thoroughly Sodium Nickel in Chapter 13. Magnesium Copper Zinc Several investigators have suggested that Selenium nutritional factors play a role in PMS and Molybdenum have proposed dietary therapy for this syn- Tin drome. Pyridoxine has been touted as a Iodine treatment for PMS,because it is claimed to raise serotonin levels in the brain. Pyridox- ments which contain iron. Most healthy ine is a coenzyme for 5-hydroxy-tryptophan people can take iron supplements without decarboxylase, which catalyzes trypto- developing obvious toxicity.124 However, a phan's conversion to serotonin. High levels recent study from Finland124a showed that of serotonin are associated with mood ele- high stored iron levels may increase a per- vation; low levels are associated with de- son's chances of developing a myocardial pression. There is no evidence, however, infarction. It is proposed that free iron cat- that PMS sufferers have low brain levels of alyzes free radical production which con- serotonin or that giving extra pyridoxine verts LDL cholesterol to oxidized LDL to will raise brain levels. Two studies showed form arteriosclerotic plaques in arteries. that taking pyridoxine improves PMSsymp- Iron supplements can harm people who toms,112,113 while another showed no im- have hereditary disorders of iron metabo- provement.114 Although many women con- lism, such as hemochromatosis and por- sider pyridoxine, in any dosage, to be phyria. harmless, large doses of pyridoxine have been reported to cause neural toxicity.115,116 As many as one out of every four female athletes is iron deficient.117 Men and non- MINERALS menstruating women need about 12 mg of iron per day. The average man ingests ade- The major minerals are listed in Table 6- quate iron from dietary sources alone. The 8. Iron and calcium are the only supple- average woman ingests around 12mg of iron ments that healthy female athletes may per day, but menstruating women need 18 need to take. An adequate diet can provide mg of iron per day, the extra 6 mg needed to adequate amounts of all minerals, but many replace the iron that is lost through men- diets are deficient in these two. strual bleeding. Birth control pills reduce iron requirements by decreasing menstrual Iron blood loss and increasing iron absorption.118 Because of the high prevalence of iron de- Iron deficiency, even in the absence of ficiencyamong female athletes and because anemia, can impair endurance.119Approxi- of its detrimental effect upon performance, I mately 40% of the iron in the body is in the recommend that female athletes who have iron reserves, such as the liver, bone mar- ferritin levels below 25 take daily supple- row, and spleen. The rest is contained in he- ments containing 30 to 60 mg of elemental moglobin. Iron-deficiency anemia does not iron. All others should avoid all supple- occur until almost all of the iron reserves are

Nutrition for Sports 117 depleted. Iron deficiency reduces the con- and vitamin D all help to prevent osteopo- centration of a-glycerophosphate oxidase in rosis. Estrogen appears to be the most im- muscle, and this impairs glycolysis and portant. With adequate calcium intake, es- leads to lactic acid accumulation in muscle trogen replacement, and exercise, even and blood.120 An increase in lactate causes a osteoporotic bones can increase in den- lowering of pH, and this reduces muscular sity.125 Low bone density of any cause in- endurance.121 People who have iron defi- creases a woman's chances of developing ciency, even without anemia, have a re- stress fractures during exercise.126 duced rate of lactic acid clearance from the blood, and they tire earlier during exercise. Exercise can enlarge bones and increase Restoring their iron reserves to normal in- bone density.127,128The bones in the racquet- creases theirendurance.122 holding arm of a tennis player are larger and denser than those in the other arm.Runners The most accurate test for detecting iron have denser femoral shafts than rowers, deficiency is a microscopic examination of dancers, and sedentary controls.129 How- bone marrow for stained iron. However, ob- ever, exercise will not maintain bone den- taining marrow is painful, invasive, and ex- sity effectively in women who lack estro- pensive. A simple, noninvasive screening gen.130 For example, exercise-associated test for iron deficiency is the measurement amenorrhea is associated with decreased of serum ferritin. Caution must be used in in- bone density,131 and estrogen replacement terpreting the results, since inflammation helps to maintain bone density in hypoes- anywhere in the body can raise ferritin lev- trogenic women.132 Birth control pills do not els. A person who has an inflammatory pro- affect bone density in women whose bodies cess may have normal serum ferritin levels produce estrogen.133 Women who have despite having iron deficiency. Further- higher-than-normal levels of androgens more, exercise raises serum ferritin levels. have denser bones.134 Patients who have a microcytic, hypochro- mic anemia with normal ferritin levels with- Nevertheless, adequate calcium intake is out elevated fetal hemoglobin should have essential for maintenance of bone density. their serum ferritin levels repeated after Children who do not ingest adequate they stop exercising for aweek.123 amounts of calcium during growth have smaller bones with reduced amounts of cal- Up to 30% of heme iron, found in meat, cium, and develop osteoporosis at an in- fish, and chicken, is absorbed, while less creased rate as adults.135,136 Increasing di- than 10% is absorbed from nonheme iron etary calcium can improve calcium balance sources. Acidity enhances iron absorption in women who lack estrogen.137 Hypoestro- from nonheme sources but not from heme genic women require 1500mg of calcium per sources. Thus, eating an orange with spin- day to maintain zero calcium balance, ach enhances iron absorption from the spin- whereas euestrogenic women require 1000 ach, but taking vitamin Cwith meat does not mg to do so.138 However, estrogen is far more increase absorption of iron from meat. On effective than dietary calciumin maintaining the other hand, alkalinity, fiber, and tannins bone density.139 Hypoestrogenic, amenor- reduce iron absorption from both heme and rheic women who do not have acontraindi- nonheme sources. For example, taking ant- cation to estrogen replacement therapy acids, eating fibrous vegetables, or drinking should be treated with estrogen and, if di- tea or coffee decreases iron absorption from etary calciumis inadequate, calcium supple- all sources. ments. Calcium The best dietary sources of calcium are Estrogen, androgenic hormones, exer- dairy products and soft-boned fish, such as canned salmon and sardines (Table 6-9). cise, dietary calcium,etidronate,calcitonin, Dairy products provide 72% of dietary cal- cium for the average American.140 Those who

118 Basic Concepts of Exercise Physiology Table 6-9. FOODS THATCONTAIN Sodium APPROXIMATELY 250 MG CALCIUM Most people do not need to consume extra 1 glass milk 1 cup yogurt sodium when they exercise. The require- 1% cups cottage cheese ment for sodium for people at rest is 0.2 g/d. IHcups icecream With prolonged exercise in very hot IHozhard cheese weather, the maximal requirement for so- 2 oz sardines with bones dium is approximately 3 g/d. The average 4 oz canned salmon with bones American diet contains between 6 and 18 g of sodium chloride per day, of which 40% is do not meet their calcium requirements sodium (2.4 to 7.2g). Manufacturers add so- from diet alone should take calcium supple- dium chloride to foods as a preservative, ments (Table 6-10), unless they are predis- and some people add sodium chloride to posed to nephrolithiasis. foods to improve the taste. Athletes who try to limit sodium intake by avoiding salty-tast- The Food and Drug Administration has ing foods and by adding no sodium to foods found significant amounts of lead in some still take in about 3 g of sodium each day. samples of bone meal and dolomite.141Do- lomite is most frequently harvested from the Sodium chloride tablets should not be shells of shellfish at the bottom of harbors. given routinely to exercising athletes. Be- Dolomite taken from polluted harbors can sides being unnecessary, they can cause contain toxic amounts of lead, mercury, ar- gastric irritation, nausea, and, in very large senic, and other heavy metals. Bone meal doses, potassium deficiency. also may contain significant amounts of toxic metals, since it usually comes from the Sodium deficiency can occur in healthy bones of older animals.142 With aging, toxic people because of an inadequate intake of metals accumulate in the bones of all ani- sodium or excessive use of diuretics. It can mals, including humans. Because dolomite also occur in people with hormonal or renal and bone meal are usually sold as food sup- defects. Any exerciser who feels tired and plements rather than drugs, manufacturers weak or develops painful muscle cramps are not required by the government to list should have serum levels of sodium mea- the heavy metal content of their products. sured. If present, hyponatremia requires a Therefore, labels on packages containing thorough evaluation to determine the cause these products do not list their heavy metal (e.g., diabetes insipidus, diabetes mellitus, content. water intoxication, and so on). Table 6-10. CALCIUM CONTENT IN 600- Many women who experience premen- MGSUPPLEMENT strual fluid retention as part of PMS may benefit from dietary sodium restriction at Number of Pills the times of symptoms during each cycle. % Required to Despite anecdotal reports of the success of Content of Pill Mg Calcium Ingest 1 G this regimen, no scientific studies have as- sessed its effectiveness. Calcium 240 40 4 carbonate 12 Potassium Calcium lactate 78 13 18.5 Potassium deficiencyis an extremely rare Calcium 54 9 6 gluconate condition in trained athletes. The kidney Calcium 171 28 and sweat glands are highly efficient in con- phosphate serving potassium in response to low body (dibasic) levels. Even with prolonged exercise in very hot weather, potassium needs can be met by an intake of only 3 to 4 g/d.143 However, po-

Nutrition for Sports 119 tassium deficiency can occur as the resultof They argue that repeated harvesting of potassium restriction and sodium loading.144 crops depletes the soil of essential minerals. When the soil in a certain region is deficient The only way that one researcher could in a mineral, the plants and animals that create a low-potassium diet for athletes and grow in that region will suffer from a defi- still provide enough calories for exercise ciency of that mineral also. That may have was to feed them candy and little else been possible in the past, but it is extremely throughout the day. Even then, the athletes unlikely to occur now. Although it is possi- did not develop potassium deficiency.145 Al- ble that some soils lack certain minerals,our most all foods are rich in potassium. Since transportation system is so extensive and ef- potassium is found primarily within cells, ficient that very few Americans eat foods any food that contains cells also contains grown only in a single locality. It is impos- potassium. sible for all soils to be deficient in the same single mineral. Hypokalemia always requires a thorough evaluation to determine the cause. Potas- Oral contraceptive agents may reduce re- sium deficiency can be caused by drugs, quirements for copper slightly and raise such as diuretics and corticosteroids, and those for zinc, but there is no evidence that certain foods, such as licorice. Prolonged di- the latter is enough to require supplemen- arrhea and vomiting also can cause potas- tation. Women who take birth control pills sium deficiency(Table 6-11). With diarrhea, have higher serum levels of copper and potassium is lost in the stool. With vomiting, lower levels of zinc than those who do not loss of hydrogen ions causes a metabolic al- take such pills.146,147 Estrogen is thought to kalosis, which increases potassium loss in raise serum copper levels by increasing the urine to conserve renal hydrogen ions. serum ceruloplasmin levels.148 The mecha- nism by which oral contraceptives lower Bulimia can present in athletes as weak- serum zinc levels is not known.149 ness and tiredness with laboratory evidence of potassium deficiency. If blood samples THE ATHLETE'S DIET show reduced potassium levels, and24-hour urine collections contain increased amounts Of course, your patients cannot become of potassium, suspect vomiting as the cause. great athletes just by altering their diets. They have to choose their parents wisely Trace Minerals and train harder than their competitors. Humans require approximately 14 trace From the foregoing discussion, it is obvious that they can get all the nutrients their bod- minerals in small amounts. There is no evi- ies need from the foods they eat. With the dence that athletes need trace mineral sup- possible exceptions of iron and calcium, a plements, with the exception of iron, be- female athlete's requirement for nutrients is cause trace mineral deficiencies are the same as it is for male athletes. The only extremely rare in healthy athletes. supplements that are required commonly are iron and calcium.Taking large doses of Some lay publications for athletes claim vitamin and mineral supplements can be incorrectly that trace mineral deficiencies toxic. Adverse side effects have been re- are common causes of fatigue in athletes. ported from large doses of even the rela- tively harmless water-soluble vitamins, Table 6-11. MECHANISM BY WHICH such as niacin, pyridoxine,and folic acid. To VOMITING CAUSES HYPOKALEMIA help your patients perform sports more ef- Loss of hydrogen ions fectively, you should recommend that they Raised blood pH eat a varied diet that is rich is carbohydrates Renal hydrogen retention Renal potassium loss

120 Basic Concepts of Exercise Physiology and that they follow the rules for eating and meal on the night before competition, and timing foods and drinks that are outlined in by eating an easily absorbed meal 5 or fewer this chapter. hours prior to competition. Maintaining ad- Several lay books claim that a high-fiber, equate hydration, even before experiencing low-fat diet will improve athletic perfor- thirst, will also improve endurance. The rate mance. There is no evidence to support this. of recovery following intense exercise can In fact, one study showed that exercisers be hastened by eating extra carbohydrates who ate a diet that contained 10% fat had the and drinking large amounts of fluids soon same improvement in Vo2max as those who after exercising. obtained 45% of their calories from fat.150 Nevertheless, you may want to recommend Vitamin supplementation is not neces- restricting dietary fat, saturated fat, and sary, since requirements can be met through cholesterol because it may help to reduce a diet. Healthy athletes do not need to in- woman's chances of developing coronary crease their intake of sodium, potassium, or artery disease and certain types of cancers trace minerals because the body can usually in the future. compensate for increased loss or decreased Taking into account that foods have nutri- intake by increasing retention. ents in different combinations and that foods in similar groups have similarnutrient REFERENCES content, the Department of Agriculturede- veloped a simple plan for eating a varied diet 1. Cook JD, Clement AF, and Smith NJ: Evalu- that will supply all nutrients. The four food ation of the iron status of a population. groups are Blood 48:449,1976. 1 Fruits and vegetables 2. Smith NJ, Stanitski CL,Dyment PC, et al: De- 2 Grains and cereals creased iron stores in high school female 3 Milk and milk products runners. Am J Dis Child 139:115,1985. 4 High-protein foods, which include 3. Lloyd T, Triantaflou SJ, Baker ER, et al: meat, fish, fowl, and beans Women athletes with menstrual irregularity have increased musculoskeletal injuries. The athlete should make sure that she Med Sci Sports Exerc 18:374,1986. eats a wide variety of foods from all four groups each day. 4. Wolf EMB, Wirth JC, and Lohman TG: Nutri- tional practices of coaches in the Big Ten. SUMMARY The Physician and Sportsmedicine 7:113, 1979. With the exception of iron and calcium, nutrient requirements for female athletes 5. Grandjean AC, Hursh LM, Majure WC, et al: are the same as those for their male coun- Nutrition knowledge and practices of col- terparts and can be met by consumingfoods lege athletes. Med Sci Sports Exerc 13:82, that contain energy sources that are ade- 1981. quate to maintain exercise. Iron deficiency, even in the absence of anemia, can impair 6. Mirkin GB, and Shangold MM: Sports Medi- endurance. Amenorrhea can be associated cine. JAMA 254:2340, 1985. with exercise and can increase calcium re- quirements. 7. Davison AJ, Banister E, and Tauton J: Rate limiting processes in energy metabolism. In Endurance can be enhanced bymaximiz- Taylor AW (ed): Application of Science and ing muscle and liver glycogen stores by re- Medicine to Sport. Charles A Thomas, ducing the volume of training4 days before Springfield, IL, 1975, p 105. competition, by eating a high-carbohydrate 8. Costill DL, Fink WJ, Getchell LH, et al: Lipid metabolism in skeletal muscle of endur- ance-trained males and females. J Appl Physiol 47:787, 1971. 9. Dyer KF:Making up the difference: Some ex- planations for recent improvements in women's athletic performance. Search 16:264, 1985. 10. Dyer KF: The trend of the male-female dif- ferential in various speed sports 1936-84. J BiosocScil8:169, 1986.

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