Diet and endurance fitness3

Transcription

1 Diet and endurance fitness3 Clyde Williams Introduction To understand the link among diet, fitness, and endurance capacity it is helpful to briefly consider some of the main physiological concepts that support our understanding ofthis relationship. For example, as an individuab moves from rest to running, the oxygen uptake increases in an almost linear fashion. The individual will reach a point where there is no further increase in 2 consumption. This is the maximum 2 uptake (VO2 max) for the individual and the actual value is determined by the habitual level of physical activity and the size ofthe individual (1). When the work capacity of individuals during nonweight bearing activities, such as exercise on a cycle ergometer, are examined, the comparisons are normally made between the 2 uptake expressed in absolute terms, ie, liters per minute or indeed the absolute work load achieved in watts, whereas in weight bearing activities, such as walking and running, the values relative to body weight are reported in milliliters per kilogram per minute. Adults and children can have similar VO2max values when expressed relative to body weight and yet have quite different work capacities. Therefore when the influences of nutritional intervention on fitness are examined, the distinction among work capacity, endurance capacity, and endurance performance should be recognized. Work capacity is most commonly assessed in the laboratory by cycle ergometry, which allows the external work performed to be reported in absolute terms. Endurance capacity refers to the time taken to exercise to exhaustion irrespective of the mode ofexercise whereas enduranceperformance should be restricted to reporting the time taken to perform a set task or distance. There is an inverse relationship between VO2max and the time to run for a given distance (2, 3) and so it is not surprising that field tests of fitness have frequently used running times as the basis for assessment. Therefore abthough the prerequisite for success in distance running is a large VO2max and appropriate endurance training (4), it is often, unfortunately, assumed that a barge VO2max value is a prerequisite for endurance fitness per se and conversely the size ofan individual s VO2max value is a reflection of his or her endurance fitness. This is not the case because whereas nature dictates the potential size of an individual s VO2max value, it is nurture, through adequate nutrition and appropriate training, that beads to an improvement in endurance fitness. Furthermore the endurance performance and capacity of an individual can improve significantly without an accompanying improvement in VO2max (5, 6). Therefore simply reporting the VO2max value ofan individual does not necessariby provide an insight into his or her endurance fitness or training status. The endurance of an individual is determined by the capacity ofthe working muscles to cover their energy needs by aerobic metabolism of fatty acids and the limited carbohydrate stores ofthe body (7). Thus the greater the aerobic fitness of an individual the greater the contribution of fat metabolism to energy expenditure, which results in a more economical use of the limited muscle glycogen stores thus delaying the onset of fatigue (8). It has been suggested that a better indicator of aerobic fitness than VO2max can be obtained by determining the highest exercise intensity an individual could sustain while still maintaining a metabolic steady state (9). A blood lactic acid concentration < 2.2 mmol/l was proposed as reflection of a metabolic steady state because well-trained endurance runners appear to be able to complete marathon races without an accumulation of lactic acid above this level ( 1, 1 1). Furthermore this concentration isjust below the level at which there is an exponential rise in lactic acid concentration concomitant with an increase in exercise intensity. The concept of an anaerobic or aerobic threshold, which reflects the training status ofan individual, is an attractive one. Wasserman ( 12) proposed the term anaerobic threshold and provided evidence to suggest that it could be described by identifying a point at which there is a nonlinear increase in ventilation during exercise of increasing intensity. There has, however, been some discussion on how closely the ventilatory responses to exercise mirror the changes in blood lactic acid concentration ( 13, 14). Nevertheless the exercise intensity at which there is a significant increase in blood lactic acid may be regarded for practical purposes as the point where the aerobic provi- I From the Department of Physical Education and Sports Science, Loughborough University oftechnology, Loughborough, Leics, UK. 2 Supported by the Sports Council and Collett-Marwell Hauge a/s. Norway. 3 Address reprint requests to C Williams. Department of Physical Education and Sports Science, Loughborough University of Technology. Loughborough, Leics. LE1 1 3TU, UK...IrnJC/in Nuir 1989:49: Printed in USA American Society forclinical Nutrition 177

2 178 WILLIAMS sion of energy is complemented by an increased contribution from the nonoxidative degradation ofglycogen in the Embden-Meyerhoff pathway. It has been suggested that lactate reference concentrations of 2 mmol/l or 4 mmol/l provide a useful alternative to the determination of lactate thresholds during exercise of increasing intensity ( 1 5). The exercise intensity equivalent to 4 mmol/l has been shown to have a stronger correlation with endurance performance than VO2max ( 16). Endurance training increases the exercise intensity an individual can achieve for a given lactic acid concentration (17), which is the result of an increase in the aerobic capacity ofthe muscle cell (8). Thus aerobic or endurance fitness ofan individual is best described in terms ofthe proportion of the VO.,max value that can be used before there is a significant increase in blood lacticacid concentration irrespective of the absolute value of VO2max. When the exercise intensity is expressed as a % VO2max, it is called the relative exercise intensity. The relative exercise intensity reflects the physiological and psychological demands on an individual more accurately than, for example, the absolute values for work loads or running speeds. This is because the cardiovascular, thermoregulatory, and metabolic responses to exercise occur with respect to the rebative rather than the absolute exercise intensity ( 18). Therefore it is not surprising that the exercise tolerance ofan individual is also inversely proportional to the relative exercise intensity. In summary then whereas the absolute work capacity of an individual is directly related to the size of maximum 2 uptake, the aerobic or endurance fitness is reflected by the % VO2max that can be achieved before there is a significant increase in blood lactate concentration. Fatigue during prolonged exercise Fatigue during prolonged submaximab exercise is the result of glycogen depletion in active skeletal muscles ( 19, 2). Therefore it is not surprising that attention has been focused on ways ofincreasing the availability of carbohydrate before prolonged exercise and also on methods ofincreasing the contribution offatty acids to muscle metabolism. In exercise studies with laboratory rodents, an increase in fat metabolism has been shown to have a glycogen-sparing effect and an improved endurance capacity(21, 22). Caffeine, in amounts equivalent to cups of coffee, ie, 5-6 mg/kg body wt, rather than the fatty meal and heparin treatment has subsequently been favored as the prefered method of elevating plasma free fatty acids before exercise. This has been reported to increase plasma fatty acid concentrations and to improve exercise performance during cycle ergometry (23, 24). Whether or not the improvement in exercise tolerance is simply the result of elevated fatty acid concentrations remains to be established. The glycogen-sparing effect may result from a suppression of intramuscular glycogenolysis mediated through the influence of caffeine on the central nervous system or some other intracellular influence that remains to be established (25). Although caffeine ingestion has been shown to improve endurance capacity during cycle ergometry, it has not been shown to be effective during running (26). The general lack of influence on performance between the two modes ofexercise may be explained in terms of the differences in dosages of caffeine used or, as has been suggested recently, by differences in the preexercise intakes of carbohydrate (27). In the absence ofunrefutable evidence to show that increased plasma free fatty acid concentrations lead to an improvement in endurance capacity, the focus of attention must be on the contribution ofcarbohydrate metabolism to endurance fitness. In attempting to obtain a clear understanding of the link between depletion of muscle glycogen and fatigue, it is important to recognize that skeletal muscle is composed oftwo main populations ofmuscle fibers. The two populations are classified in terms oftheir speed of contraction and the metabolic source oftheir energy production. The type I fibers are slow contracting, slow fatiguing, oxidative fibers that have a high mitochondrial density and capillary supply whereas the type IIb fibers, also known as fast contracting, fast fatiguing, glycolytic fibers, have a low mitochondrial density and capacity for aerobic metabolism. The type ha fibers are fast contracting but have a significantly greater aerobic capacity than the type lib fibers and so are more resistant to fatigue (28). An examination ofskeletal muscle shows that during cycle ergometer exercise the postexercise glycogen concentration is lowest in the type I and type Ila fibers, suggesting their recruitment during exercise (29-3 1). Thus during prolonged submaximal exercise, fatigue occurs when the glycogen concentration in the type I and type ha fibers is significantly reduced; however at the point of fatigue the glycogen concentration of the type lib fibers may still be high and so it is important to be aware of the selective nature of muscle fiber recruitment when examining the link between muscle glycogen concentrations and the onset of fatigue. Carbohydrate-rich diets and endurance capacity The early studies ofchristensen and Hansen reported in 1939 (32) were the first to firmly establish the relationship between a high-carbohydrate diet and the improvement in endurance capacity. In these studies the exercise tolerance ofa group ofsubjects was examined on a cycle ergometer after 3-4 d on each of the following diets: a normal mixed diet, a diet of fat and protein, and also a diet rich in carbohydrate. After the high-carbohydrate diet the endurance capacity ofthese subjects almost doubled in comparison with their endurance performance after their normal mixed diet. In contrast the fat and protein diet reduced the exercise performance to almost half that achieved on a mixed diet (1). The classical single-beg exercise study of Bergstrom and Hubtman (33) showed quite clearly that the concen-

3 DIET AND ENDURANCE FITNESS 179 tration of muscle glycogen increases to supranormab vabues when a high-carbohydrate diet is consumed during the recovery from exercise that depletes the intramuscubar carbohydrate stores. This local phenomenon ofan increase in muscle glycogen concentration has been called glycogen supercompensation. The influence of different diets on the resynthesis ofglycogen during recovery from prolonged exhaustive exercise was also examined by Bergstrom et al (34). In their study it was found that fasting and or a fat and protein diet for 2-3 d produced a delayed muscle glycogen resynthesis whereas a high-carbohydrate diet for the same period of time produced rapid resynthesis leading to glycogen supercompensation (34). The positive relationship between the endurance performance during cycle ergometry and preexercise muscle glycogen concentration was also shown by Ahlborg et al (35). The correlation between initial muscle glycogen concentrations and endurance times for their nine subjects was.68 whereas the correlation between glycogen concentration corrected for body mass and totab work done during the experiment was a more impressive value of.87. An equally strong correlation was obtamed (r =.92) between the preexercise glycogen concentration and the exercise time to exhaustion when all three conditions were examined collectively, namely a mixed diet, a low-carbohydrate diet, and a high-carbohydrate diet (36). Further examination ofthe optimum conditions necessary to produce glycogen supercompensation led to what has become the traditional approach to carbohydrate loading. This involves exercise to exhaustion to bower muscle glycogen concentration followed by 3 d on a low-carbohydrate diet (mainly fat and protein) and then 3 d on a high-carbohydrate diet. The high-carbohydrate phase of the dietary and exercise manipulation should involve a daily carbohydrate intake of 5 g representing 7% ofthe daily energy intake (37). Exercisc time to exhaustion following this method of carbohydrate loading was increased by 57% when compared with the exercise time achieved when the subjects consumed their normal mixed diet. On the basis ofthe results ofthese experiments and the publicity they subsequently received (38), the exercise and dietary manipulation procedure for producing glycogen supercompensation has been widely adopted by athletes preparing for endurance competitions. Yet it has been apparently overlooked that almost all the results obtained have been from studies on cycling and not runfling. However an improvement of25% in endurance capacity was reported recently for subjects during treadmill running after 3 d on a high-carbohydrate diet (39). The subjects in this study increased their carbohydrate intake by complementing their normal diet with either simple or complex carbohydrates. There was no difference in the improvements in endurance capacity for the two different ways of carbohydrate loading. Another recent study showed that simple carbohydrates may produce an even greater increase in muscle glycogen concentrations during the carbohydrate-loading procedure than that produced by complex carbohydrates (4). Although the carbohydrate-loading procedure has been shown to improve exercise time to exhaustion, it is often overlooked that the increased carbohydrate intake also produces an increase in the concentration of liver glycogen to values significantly higher than normal (41). Blood glucose, derived from liver glycogen, does contribute to carbohydrate metabolism in working muscles especially towards the onset of fatigue (42). However the proportional contribution of liver glycogen to improved endurance performance is difficult to separate out from the influence ofan increased muscle glycogen store. Carbohydrate loading and endurance performance The question of whether or not an increased preexercise glycogen concentration will increase running speed as well as endurance capacity was addressed in a study conducted under race conditions (43). In this study the subjects were divided into two groups, one of which underwent carbohydrate loading before the race of 3 km while the other group remained on their normal mixed diet. In the second part ofthe study the race conditions were recreated some 3 wk later when the dietary preparation of each group was reversed. They found that the time to complete the 3-km race was improved by 8 mm (5.6%) when the subjects increased their prerace muscle glycogen concentrations through the carbohydrate-loading procedure. Although the running speed of each subject was not increased during the early part of the race as a result ofcarbohydrate-boading, runners were able to sustain their optimum pace during the batter part of the race. A closer examination of the results of this study shows that the subjects were a mixture of experienced and recreational runners. The experienced runners abready had higher prerace muscle glycogen concentrations than did the recreational runners before the start of the experiment and so it is not surprising then that the recreational runners had the largest increase in their prerace glycogen concentrations and recorded the greatest improvements in running times ( 12 mm vs 5 mm). Trained individuals, irrespective oftheir sport, appear to have higher resting muscle glycogen concentrations than do untrained individuals (44, 45). There is little doubt about the positive relationship between the initial glycogen concentration and endurance performance, which is exploited by increasing muscle glycogen concentration before competition. There are doubts, however, about the advisability of including the low-carbohydrate phase in the dietary and exercise manipulation procedure. Not only is it a very unpleasant period, which undermines the confidence of the athlete at a time when he is preparing for competition, but there is the real fear of exercise-induced hypoglycemia during the 3 d on the low-carbohydrate diet. Therefore the current practice is one in which the athlete is encouraged to decreases his training during the 3 d before competition

4 18 WILLIAMS Cl) 4.O a- U) ) C - C Time (mm) FIG I. Self-selected speeds during 2 h of treadmill running when subjects ingested a glucose polymer solution during one trial and a placebo during the other trial. while at the same time increasing the carbohydrate content of his diet. The aim is to achieving 55-6 g carbohydrate/d, representing 7% ofdaiby energy intake of male athletes, which has been shown to be effective in producing supercompensation ofmuscle glycogen (45). It is important to remember that in all these studies male subjects have been used and a recommended intake of 5 g carbohydrate would in energy terms be equivabent to the whole ofthe daily energy intake offemale endurance athletes (46). Therefore it is much more helpful to think in terms ofa carbohydrate intake of 6-8 g/kg rather than a total amount ofcarbohydrate. When recovery from prolonged heavy exercise has to be achieved on a daily basis, which is not uncommon for sportsmen involved in tournaments ofdifferent kinds, then the calculated amount ofcarbohydrate required to restore muscle glycogen concentrations to normal is ofthe order of 55-6 g/d (47). Carbohydrate supplementation before and during exercise When a glucose solution is ingested before exercise, the concomitant increase in insulin concentration depresses the mobilization of fatty acids and so denies the working muscles of its other main substrate. As a result there is of necessity an increase in the contribution of glycogen to muscle metabolism, which leads to a somewhat paradoxical increase in the rate of glycogen depletion and so to an earlier onset offatigue. Costill et ab (23) showed that when a group of runners exercised for 3 mm on a treadmill after ingesting 75 g glucose in 3 ml ofwaterjust 45 mm before exercise, they developed persistent hypoglycemia and their muscle glycogen concentrations were lower than the values recorded when water alone was consumed. In a follow-up study they confirmed the predictions ofthe earlier study in that the endurance capacity ofthe subjects was 19% bess after they consumed the glucose solution as compared with the results obtained after either the same volume of milk or water was consumed (48). In an attempt to provide additional carbohydrate shortly before the start ofexercise without producing an earlier onset of fatigue, fructose has been used instead of glucose. The insulin response to fructose ingestion is less pronounced. than it is to glucose and so theoretically it would not be expected to depress fatty acid mobilization to the same extent as a glucose solution. Furthermore the pronounced hypoglycemia that occurs during exercise after ingestion ofa glucose solution is avoided when fructose rather than glucose is the preexercise carbohydrate as is the increased rate of glycogen metabolism during subsequent exercise (49, 5). Even though the preexercise ingestion of a fructose solution maintains more stable blood glucose and insulin concentrations during exercise than does the ingestion of a glucose solution, it does not appear to produce any greater improvements in fat metabolism nor in endurance capacity and tends to be metabolized at a slower rate than an equivalent amount ofglucose (5 1-53). Ingesting carbohydrate sobutions immediately before exercise does not appear to proyoke the same degree ofhyperinsubinemia or hypoglycemia as occurs when carbohydrate is consumed within the hour before exercise. The explanation appears to be that at the onset of exercise the increase in circulating catechobamines suppresses the insulin response to carbohydrate ingestion (54). The provision ofadditional carbohydrate before exercise has also been explored using solid rather than liquid supplementation in an attempt to improve exercise toler-

5 DIET AND ENDURANCE FITNESS Cl) 5.5 C., C, CHO -.- Placebo Time (mm) FIG 2. Blood glucose concentrations during 2 h oftreadmill running at self-selected speeds. ance. In general the provision ofsolid or liquid carbohydrate immediately before and during exercise has had beneficial effects on exercise capacity (for review, see 55). However these improvements have been modest with one exception compared with the improvements in endurance capacity obtained by dietary carbohydrate loading over a period of days. Although many studies have attempted to determine the optimum composition ofsolutions for carbohydrate supplementation during exercise with respect to the rate ofgastric emptying, others have simply explored ways of providing as much carbohydrate as possible. In one such study the subjects walked on treadmill to the point of fatigue either ingesting a glucose polymer solution or water alone. The concentration ofthe glucose polymer solution was 2% and the total amount consumed, equivalent to - 12 g carbohydrate, was administered during the first of 3.5 h of exercise. Endurance time was increased by 1 1.5% during the glucose polymer trial cornpared with the water trial but after the prolonged walk there were no differences in either running time to exhaustion nor performance during a series of psychomotor ability tests (56). A similar improvement in work capacity was reported for a group of trained cyclists who exercised for 3 h at an intensity equivalent to 74% VO2max while consuming a glucose polymer solution. An analysis of the results showed that a significant improvement in work capacity occurred only in the subjects for whom a decrease in blood glucose concentration had been recorded during the control experiment (57). In this experiment the subjects consumed 14 ml of a 5% glucose polymer solution after the first 2 mm of exercise; thereafter they consumed 3 ml ofa 6% solution ofthe glucose polymer every 3 mm. A more dramatic improvement in endurance capacity was achieved during a similar experiment using trained cyclists who ingested a glucose polymer solution during exercise at 7 1% VO2max to fatigue. In this experiment the subjects ingested 3 ml ofa 5% carbohydrate solution that provided 2 g/kg body wt after the first 2 mm of exercise and 3 ml ofa 1% solution that provided.4 g/kg body wt every 2 mm for the remainder of the exercise period. Fatigue occurred after 3 h of exercise during the control experiment and after 4 h during the carbohydrate trial. Interestingly the rate of muscle glycogen metabolism was the same for the first 3 h ofexercise during both trials. The authors concluded that the trained cyclists were able to utilize the carbohydrate supplied during the extra hour ofexercise and so the improvement in endurance was not due to glycogen sparing per se (58). Therefore the results of this particular study clearly demonstrate a beneficial effect from carbohydrate supplementation during prolonged exercise that is ofthe same order ofrnagnitude as achieved after increases in dietary carbohydrate. During running, fluid ingestion is less well tolerated than during cycling and it appears that no more than about a liter offluid can be consumed over 1-2 h without causing extreme discomfort (C Williams, unpublished observations). However when a liter of a carbohydrate solution was consumed over a 2-h period of treadmill running, there was some evidence to suggest a beneficial effect (59). Fifteen recreational runners ( 1 men and 5 women) completed two treadmill races lasting 2 h on two occasions separated by 7 d. Immediately before the first trial each subject drank either 25 ml ofa carbohydrate solution (32.5 g maltodextrin, g sucrose, 5.7 g fructose) or a sweetened placebo and during the run the subjects consumed the remaining 75 ml in 125-mL aliquots at 2-mm intervals. After the fit-st 15 mm, which was run at a speed equivalent to 7% VO2max, the subjects were free to choose their own running speeds by use ofa computer-linked hand-held microswitch that also albowed the distance covered and the chosen speeds to be

6 182 WILLIAMS recorded. The aim was to complete as great a distance as possible during the 2 h on both trials to which the subjects were randomly assigned. The results of the study showed that although there was no significant difference in the distance covered during the 2 h, the running speeds after 9 mm were significantly faster during the carbohydrate trial (Fig 1). Blood glucose concentrations were significantly higher (Fig 2) at the beginning and at the end of the carbohydrate trial than were the results obtained during the placebo trial. The absence of greater benefit may be explained in terms ofthe relatively short though realistic duration of this endurance performance test and the limited amount ofcarbohydrate that could be provided without causing gastrointestinal distress. In summary it is clear from the available evidence that an increased dietary carbohydrate intake produces a significant improvement in endurance capacity. However carbohydrate in liquid or solid form consumed immediately before and during exercise may also have a positive influence on the endurance capacity of active individuals. It is worth recognizing however that the quantities of carbohydrate consumed in the form of solid and liquids immediately before and during exercise may only be effective in activities in which the body weight is supported. 13 References I. Astrand P, Rodahl K. Textbook of work physiology. London: McGraw-Hill BookCompany, Davies CTM, Thompson MW. Aerobic performance of female marathon and male ultramarathon athletes. Eur J Appl Physiol I979:41: Ramsbottom R. Nute MLG, Williams C. Determinants of 5km running performance in active men and women. Br J Sports Med 1987:21: Costill DL. Inside running: basics of sports physiology. Indianapolis: Benchmark Press Inc Daniels J, Yarborough RA, Foster C. Changes in VO2max and running performance with training. Eur J AppI Physiol 1978:39: Williams C, Nute MG. Training-induced changes in endurance capacity offemale games players. In: Watkins J. Reilly T, Burwitz L. eds. Sports science. London: E & F Spon Ltd. 1986: Davies KJA. Packer L, Brooks GA. Biochemical adaptations of mitochondria. muscle and whole animal respiration to endurance training. Arch Biochem Biophys 198 1:29: Gollnick PD, Saltin B. Significance of skeletal muscle oxidative enzyme enhancement with endurance training. Clin Physiol 1982:2: Londeree BR, Ames SA. Maximum steady state versus state of conditioning. Eur J AppI Physiol 1975: 34: Costill DL. The metabolic responses during distance running. J AppI Physiol 197:28: I I. Williams C. Brewer J, Patton A. The metabolic challenge of the marathon. BrJ Sports Med 1984: 18: Wasserman K. Whipp BJ. Koyal SN. Beaver ML. Anaerobic threshold and respiratory gas exchange during exercise. J AppI Physiol 1973:35: Davis JA. Anaerobic threshold: review of the concept and directions for future research. Med Sci Sports Exerc 1985; 17: Brooks GA. Anaerobic threshold: review of the concept and direction for future research. Med Sci Sports Exerc 1985; 17: Kindermann W, Simon G, Keul J. The significance ofthe aerobicanaerobic transition for the determination ofwork load intensities during endurance training. Eur J Appl Physiol l979;42: Jacobs J. Blood lactate: implications for training and research. Sports Med 1986;3: Hurley BF, Hagberg JM, Allen WK, et al. Effect of training on blood lactate level during submaximal exercise. J Appl Physiol l984;56: Rowell LB. Human cardiovascular adjustments to exercise and thermal stress. Physiol Rev 1974;54: Hermansen L, Hultrnan E, Saltin B. Muscle glycogen during prolonged severe exercise. Acta Physiol Scand l967;7 1: Gollnick PD, lanuzzo CD, Williams C, Hill R. Effect of prolonged severe exercise on the ultra structure ofhuman skeletal muscle. Int z Angew Physiol 1969;27: Rennie MJ, Holloszy JO. Inhibition of glucose uptake and glycogenolysis by availability ofoleate in well-oxygenated perfused skeletal muscle. Biochem J 1977; 168: Rennie MJ, Winder WW, Holloszy JO. A sparing effect of increased plasma fatty acids on muscle and liver glycogen content in exercising rat. Biochem J 1976; 156: Costill DL, Coyle ED, Dalsky G, Evans W, Fink W, Hoopes D. Effects of elevated plasma FFA and insulin on muscle glycogen usage during exercise. J AppI Physiol 1977;43: Ivy J, Costill DL, Fink WJ, Lower RW. Influence ofcaffeine and carbohydrate feedings on endurance performance. Med Sci Sports Exerc l979;ll:6-li. 25. Powers 5K, Dodd S. Caffeine and endurance performance. Sports Med 1985;2: Casal D, Leon A. Failure of caffeine to affect substrate utilization during prolonged running. Med Sci Sports Exerc 1985; 17: Weir J, Noakes TD, Myburgh K, Adams B. A high carbohydrate diet negates the metabolic effects ofcaffeine during exercise. Med Sci Sports Exerc 1987; 19:1-S. 28. Essen B, Jansson E, Henrrikson J, Taylor AW, Saltin B. Metabolic characteristics of fibre types in human skeletal muscle. Acta Physiol Scand 1975:95: Gollnick PD, Piehl K, Armstrong RB, Saubert CW, Saltin B. Diet, exercise and glycogen changes in human muscle fibres. J AppI Physiol 1972; 33: Gollnick PD, Armstrong RB, Saubert CW, Sembrowich WL, Shepherd RE, Saltin B. Glycogen depletion patterns in human skeletal muscle fibres during prolonged work. Pfluegers Arch 1973;344:l-l Vollestad NK, Vaage, Hermansen L. Muscle glycogen depletion patterns in type I and subgroups oftype II fibres during prolonged severe exercise in man. Acta Physiol Scand 1984; 122: Christensen EH, Hansen. Arbeitsfahigkeit und Ehrnahrung. Scand Arch Physiol 1939:81: Bergstrom J, Hultman E. Muscle glycogen synthesis after exercise: an enhancing factor localized to the muscle cells in man. Nature 1966;2: Bergstrom J, Hultman E. Muscle glycogen synthesis in relation to diet studied in normal subjects. Acta Med Scand 1967; 182: Ahlborg B, Bergstrom J, Ekelund L-G, Hultman E. Muscle glycogen and muscle electrolytes during prolonged physical exercise. Acta Physiol Scand l967;7: Bergstrom J, Hermansen L, 1-lultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta Physiol Scand 1967;71: 14-5.

7 DIET AND ENDURANCE FITNESS Ahlborg B, Bergstrom JB, Brohult J, Ekelung L-G, Hultman E, Maschio G. Human muscle glycogen content and capacity for prolonged exercise after different diets. Forsvarmedicin 1967:3: Astrand P. Diet and athletic performance. Fed Proc 1967:26: Brewer J, Williams C, Patton A. The influence of high carbohydrate diets on endurance running performance. Eur J Appl Physiol 1988:57: Roberts KM. Noble EG, Hayden DB, Taylor AW. Simple and complex carbohydrate-rich diets and muscle glycogen content of marathon runners. EurJ Appl Physiol l987;57: Nilsson LH, Hultman E. Liverglycogen in man-the effect of total starvation or a carbohydrate-poor diet followed by carbohydrate refeeding. Scand J Clin Lab Invest 1973; 32: Wahren J. Substrate utilization by exercising muscle in man. Prog Cardiol l973;2: Karlsson J, Saltin B. Diet, muscle glycogen and endurance performance. J Appl Physiol 197l;3l: Gollnick PD, Armstrong RB, Saltin B, et al. Effect oftraining on enzyme activity and fiber composition of human skeletal muscle. J Appl Physiol l973;34:l7-l Sherman WM, Costill DL, Fink W, Miller J. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int J Sports Med I 98 1;2: Walker M. Dietary planning for performance. In: Shrimpton DH, Ottaway PB, eds. Nutrition in sport. Cambridge, UK: Resource Publications, 1986: Costill DL, Miller J. Nutrition for endurance sport: carbohydrate and fluid balance. Int J Sports Med I 98: 1: Foster C, Costill DL, Fink WJ. Effect of pre-exercise feeding on endurance performance. Med Sci Sports 1979; 1 1: Koivisto VA, Karonen S-L, Nikkila EA. Carbohydrate ingestion before exercise: comparison ofglucose, fructose and sweet placebo. J AppI Physiol 1981:51: Hargreaves M, Costill DL, Katz A, Fink WJ. Effect of fructose ingestion on muscle glycogen usage during exercise. Med Sci Sports Exerc 1985; 17: Massicotte D, Peronnet F, Allah C, Hillaire-Marcel C, Ledoux M, Brisson G. Metabolic response to (13C) glucose and ( 3C) fructose ingestion during exercise. J AppI Physiol l986;61: McMurray RG, Wilson JR. Kitchell BS. The effects of fructose and glucose on high intensity endurance performance. Res Q Exerc Sport l983;54:l Hargreaves M, Costill DL. Fink WJ, King DS, Fielding RA. Effect of pre-exercise carbohydrate feedings on endurance cycling performance. Med Sci Sports Exerc 1987; 19: Galbo H. Hormonal and metabolic adaptations to exercise. New York: Geoge Thieme Verlag, Costill DL. Carbohydrates for exercise, dietary demands and optimum performance. IntJ Sports Med l988;9:l-l Ivy JL, Miller W, Dover V. et al. Endurance improved by ingestion ofa glucose polymer supplement. Med Sci Sports Exerc 1983; 15: Coyle ED, Hagberg SA, Hurley WH, Eshani JM, Holloszy JO. Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. J AppI Physiol l983;55: Coyle ED, Coggan AR, Hemmert ME, Ivy JJ. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J AppI Physiol 1986:61: Williams C, Nute MG, Walker M. Influence of carbohydrate supplementation on running performance. Proc Nutr Soc 1987:46: 1 l9a(abstr).