The Effects of Carbohydrate Loading on Muscle Glycogen Content and Cycling Performance

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1 International Journal of Sport Nutrition, 1995, 5, O 1995 Human Kinetics Publishers, Inc. The Effects of Carbohydrate Loading on Muscle Glycogen Content and Cycling Performance Laurie H.G. Rauch, Ian Rodger, Gary R. Wilson, Judy D. Belonje, Steven C. Dennis, Timothy D. Noakes, and John A. Hawley This study compared the effects of supplementing the normal diets of 8 endurance-trained cyclists with additional carbohydrate (CHO), in the form of potato starch, for 3 days on muscle glycogen utilization and performance during a 3-hr cycle ride. On two occasions prior to the trial, the subjects ingested in random order either their normal CHO intake of 6.15 f 0.23 gl kg body masslday or a high-cho diet of f 0.57 g/kg body masslday. The trial consisted of 2 hr of cycling at -75% of V02peak with five 60-s sprints at 100% VOzpeak at 20-min intervals, followed by a60-min performance ride. Increasing CHO intake by % for 3 days prior to the trial elevated preexercise muscle glycogen contents, improved power output, and extended the distance covered in 1 hr. Muscle glycogen contents were similar at the end of the 3-hr trial, indicating a greater utilization of glycogen when subjects were CHO loaded, which may have been responsible for their improved cycling performance. Key Words: diet, potato starch Three to four days prior to marathon or ultramarathon races many endurance athletes reduce their training volume and ingest a high (>7 g/kg body masslday) carbohydrate (CHO) diet. The "glycogen-loading" regimen typically increases the resting muscle glycogen content from -100 to >I40 mmol/kg wet wt and extends the time to exhaustion during prolonged (>3 hr) submaximal exercise (3). Conversely, dietary and exercise manipulations that reduce preexercise glycogen stores are associated with an impaired ability to continue exercise at a given percentage of peak oxygen uptake (V0,peak) (3). More recent studies have shown that while CHO loading may increase endurance at a given work rate, it does not improve the speed that can be maintained during high-intensity (>70% of V02peak) exercise lasting either 2 hr (9) or 80 min (25). However, athletic performance 52 hr is unlikely to be limited The authors are with the Bioenergetics of Exercise Research Unit of the Medical Research Council and the University of Cape Town, Department of Physiology, University of Cape Town Medical School, Observatory 7925, South Africa.

2 26 / Rauch, Rodger, Wilson, et a/. by the availability of muscle glycogen; starting muscle glycogen stores are more likely to influence athletic performance in more prolonged (23 hr) exercise. Bosch et al. (5) showed that muscle glycogen content after 2 hr of cycling at 70% of V0,peak declined to 89 mmolkg wet wt in glycogen-loaded subjects compared to 44 mmolkg in subjects who were nonloaded. Accordingly, the primary aim of this study was to compare the effects of initial muscle glycogen concentration on the speed attained during the last hour of a 3-hr high-intensity cycle ride involving a series of sprints, to mimic the way cyclists would typically race (20). A further purpose of this investigation was to determine whether a single CHO supplement, potato starch, could be successfully employed for glycogen loading without producing the gastrointestinal discomfort that athletes often experience after consuming the large quantities of pasta, rice, and bread required to achieve sufficient CHO intake (17). Subjects and Preliminary Testing Materials and Methods Twelve well-trained male endurance cyclists were recruited to participate in this investigation after being fully informed of the nature and risks and having given their written consent. All had been involved in endurance training for 3-5 years and competed regularly in local and national cycle races. Unfortunately, 2 of the 12 subjects experienced such severe gastrointestinal distress with CHO loading that they failed to complete the trial, while another 2, who also complained of diarrhea, had to be excluded because their starting muscle glycogen contents were not different before and after CHO loading (114 vs. 100 and 133 vs. 138 mmolkg, respectively). The characteristics of the 8 subjects who successfully completed all phases of the investigation were as follows: age = 22.4 k 0.6 years; mass = 71.3 k 1.4 kg; V0,peak = 4.72 f 0.15 Llmin, or 66.3 f 1.3 ml/kg/min; peak sustained power output (PPO) = 376 rt 12 W; the ratio of PPO to mass = 5.28 f 0.11 Wkg; and peak heart rate (HR,,,,) = 191 k 2 beatslmin. The PPO, V02peak, and HR, values were obtained from maximal exercise tests performed on an electronically braked cycle ergometer (Lode, Gronigen, Holland), modified to the configuration of a racing bicycle with adjustable saddle heights and handlebar positions. In this test, the subjects started cycling at an exercise intensity equivalent to 3.33 Wkg body mass for 150 s; thereafter, the work rate was increased by 50 W for the next two 150-s workloads and then by 25 W every 150 s until the subjects were exhausted (12). PPO was defined as the highest exercise intensity the subject completed plus the proportion of the final exercise intensity that he sustained, as described by Kuipers et al. (16). V0,peak data were calculated from each subject's PPO (12) and were used to determine the work rate corresponding to -75% of V02peak, for use in the experimental trials. HR,, values were recorded with a Polar heart rate monitor (Polar Electro OY, Kempele, Finland). In addition to completing the maximal exercise test, all subjects completed a familiarization ride on a Kingcycle cycle simulator (Kingcycle Ltd., High Wycombe, Bucks, U.K.). This ergometer allows subjects to ride their own racing bicycles. Bicycles were attached by the front fork to the frame of the simulator,

3 Carbohydrate Loading / 27 with the bottom bracket resting on an adjustable support arm. By raising or lowering the bottom bracket support arm, we could adjust the rolling resistance of the rear tire on an air-braked flywheel to match the resistance experienced by a 70-kg cyclist on a level road. From the rolling resistance and the output of a photo-optic sensor monitoring the revolutions of the flywheel, an ~~~-com~atible computer calculated the power output (W) that would be generated by a 70-kg cyclist riding at that speed (kmhr) on level terrain and the distance (km) that would be covered. Power outputs at given speeds were calculated using the following equation: W = RPS RPS, where W is watts and RPS is the revolutions per second on the flywheel. ~djustments of the tension on the bottom bracket to produce a rolling resistance of a 70-kg cyclist were achieved by a series of "rundown" calibrations before each trial, during which the subject accelerated to a work rate of -300 W and then immediately stopped pedalling, while remaining seated on the bike in the time-trial position. These calibrations were repeated until the computer display indicated that the slowing of the flywheel matched a reference power decay curve for a 70-kg cyclist. Preliminary data on the test-to-test reliability of the Kingcycle cycle simulator have shown that the mean and standard deviation of the coefficient of variation of the time taken for 5 subjects who each completed three 40-km time trials were % (G.S. Palmer, unpublished observations). Dietary Analysis Prior to the experimental trials, a 3-day dietary record, including 1 day of a weekend, was obtained from each subject. Subjects were given precise written and verbal instructions on how to record all fluid and food consumed and were requested to note the frequency of feedings and any ingestion of extra vitamin or mineral supplements. The nutritional composition of each subject's diet was determined by a commercial computer program (Food Finder Diet Analysis, Medtech, Tygerberg, Cape Town, South Africa) to estimate the subject's normal dietary CHO intake and the quantity of additional CHO ingestion required to supercompensate the muscle glycogen stores (9, 24). Dietary Manipulation and Experimental Trials All subjects completed a random order of two experimental trials, separated by a minimum of 4 days, during which they either ingested their normal diets (Norm trial) or CHO loaded (Load trial) for 3 days prior to exercise testing. CHO loading was achieved by having subjects supplement their habitual diets with potato starch (Roquette Freres, Lestrem, France). This starch was derived from raw, spray-dried, whole potatoes and was 78% CHO by weight. In an attempt to induce a range of elevated preexercise muscle glycogen contents, the CHO intakes of the subjects were adjusted to between 8 and 12 g of CHO/kg body masslday for the 3 days prior to the Load trial. This was achieved by randomly supplementing half the subjects' habitual diets with -3 g starchkglday and the other half with -5 g starch/kg/day. The potato starch was made more palatable by adding 20 g starch/100 ml chocolate-flavored milkshake mix. In order to reduce gastrointestinal disturbances, subjects were instructed to ingest the starch mix in small amounts (not

4 28 / Rauch, Rodger, Wilson, et a/. more than 250 m1/2 hr) throughout the day. Apart from the addition of starch supplement, subjects did not change the composition or quantity of food they normally ate. This was done to minimize the dietary intervention that subjects had to undergo. During both trials dietary records were recorded to aid compliance. On the morning of the trials, subjects came to the laboratory 3 hr after a standardized breakfast that was similar in content and composition to that which they would normally ingest before competition (7040 g of CHO). While the subjects rested, a percutaneous muscle biopsy was taken from the vastus lateralis of the right leg, according to the technique of Bergstrom (2) as modified by Evans et al. (8). At this time, an incision was also made in the left leg for the immediate (within 3 min) postexercise muscle biopsy, and a Jelco l&gauge cannula (Critikon, Halfway House, TVL, South Africa) was inserted into a forearm vein for blood sampling. The subject's bicycle was then mounted onto the Kingcycle and a rundown calibration was conducted, as described previously, for the final 1-hr performance ride. After the calibration of the Kingcycle, the subjects began a 5-min self-paced warm-up on an electronically braked ergometer and then proceeded to cycle for 2 hr at an exercise intensity equal to 65% of PPO (244 f 8 W, -75% of V02peak). After 20, 40, 60, 80, and 100 min of the 2-hr submaximal ride, subjects performed a 60-s sprint at the maximal work rate (376 f 12 W) they attained during the maximal test, and then the workload was reduced to 0 W with subjects still turning the pedals for 60 s before continuing submaximal exercise. These sprints were designed to mimic the way cyclists would typically race (20) and to promote muscle glycogen depletion prior to the subsequent performance ride. For the performance ride, the subjects transferred (within 60 s) from the ergometer to the Kingcycle and maintained as high an exercise intensity as possible for 1 hr. In this ride, the only feedback to the subjects was the elapsed time; that is, they were kept blind as to distance covered, speed attained, and heart rate achieved during the ride. In order to offset the potential negative effects of dehydration and to prevent possible hypoglycemia, subjects ingested 750 ml/hr of an 8 g/100 ml short-chain glucose polymer solution (Energade Marathon, Bromor Foods, Salt River, Cape Town) during the first 2 hr of exercise. However, only water was available to drink, ad libitum, in the 1-hr performance ride. CHO-containing solutions were not provided during the performance rides because cyclists voluntarily ingest variable volumes of fluid during exercise. It was felt that making all subjects drink the same volume might interfere with their ability to concentrate on cycling as hard as possible. Analytical Techniques Venous blood samples (3 ml) were collected into ice-cold tubes containing potassium oxalate and sodium fluoride at rest, at 30-min intervals during the 2-hr ride, and at the end of the performance ride. After the trial, the blood samples were centrifuged for 10 min at 2,500 revlmin in a refrigerated centrifuge (0 "C), and the supernatant was stored at -20 "C for subsequent analyses of plasma glucose and lactate concentration. Plasma glucose concentration was measured with an automated glucose analyzer (Beckman Glucose Analyser 2, Fullerton, CA, U.S.A.), which uses the glucose oxidase assay (14). Plasma lactate concentrations were measured with a standard enzymatic spectrophotometric technique (1 1).

5 Carbohydrate Loading / 29 Muscle glycogen content in the frozen muscle samples was determined in duplicate by the method of Passoneau and Lauderdale (21). The coefficient of variation was 6% for duplicate glycogen assays of a single piece of muscle and 4% for assays of the glycogen content of three separate pieces of the same muscle biopsy. Statistical Analysis All results are expressed as mean f SEM. The statistical significance of the effects of dietary CHO on muscle glycogen content, power output, and the distance attained during the performance ride was assessed with a paired Student's t test. A p value of <.05 was regarded as significant. Plasma glucose and lactate concentrations were analyzed with a two-way analysis of variance. Where significant differences were found, a Sheffe's post hoc test was performed to determine where this difference occurred. Results Table 1 shows that the CHO-loading diet increased the subjects' intake of CHO (p <.0001) and overall energy <.0005) without significantly altering their intake of protein and fat. CHO consumption was increased by 72 f 9% and energy intake was raised by 42 f 7% (Table 1). Muscle Glycogen Utilization The percent increases in the subjects' dietary CHO intakes were not related to their percent increases in preexercise muscle glycogen contents (Table 2). Irrespective of the amount of additional CHO ingested, the preexercise muscle Table 1 The Composition of the Subjects' Diets Under Normal and Carbohydrate- Loading Conditions Normal Loaded M SEM M SEM CHO (8) (g/kg) (% total energy) Protein (8) Fat Total (% total energy) (g) tglkg) (% total energy) Note. CHO = carbohydrate. *CHO intake significantly greater during the CHO-loading trial than during the normal trial, p < **Total caloric intake significantly greater during the CHO-loading $rial, p <.0005.

6 Table 2 Carbohydrate Intake and Muscle Glycogen Contents of Individual Subjects on Their Normal Diets and During the 3-Day % -3 Carbohydrate-Loading Regimen io Normal Loaded f G- Muscle Muscle % increase in -2 CHO CHO glycogen CHO CHO glycogen % increase in muscle 2 Subject (glday) (gkglday) (mmolkg) &/day) (gflcg/da~) (mmolkg) CHO intake glycogen lii 7 p.- 1" a 6a 7 8" Mean SEM Note. CHO = carbohydrate. Subjects who supplemented their normal diets with 3 g starchkg body masslday.

7 Carbohydrate Loading / 31 glycogen contents were 47 f 4% higher at the start of the Load trial than at the start of the Norm trial ( vs. 104 f 8 mmolkg, p <.0001). However, at the end of the 3-hr Load and Norm trials, muscle glycogen contents were not significantly different (26 f 7 vs. 18 f 2 mmolkg), indicating that more muscle glycogen was utilized after CHO loading than after a normal diet ( vs mmolkg, p <.0005). There were no significant alterations in plasma glucose and lactate concentrations during the two trials. Average plasma glucose and lactate concentrations in the Norm and Load trials were 5.30 f 0.8 vs mmol/l and 3.61 f 0.32 vs f 0.30 mmol/l, respectively (Table 3). Exercise Performance Figures 1 and 2 display the mean power outputs and heart rates of the subjects during the first 2 hr of exercise (Figure 1) and during the 60-min performance ride (Figure 2). The average power output during the 60-min performance ride was higher in the Load ride than in the Norm ride (233 f 15 vs W, p <.05; Figure 2), as too was the average speed (38.02 f 1.10 vs km/hr, p <.05; Figure 3). All except 1 of the subjects went faster during the last hour of the Load trial compared to the last hour of the Norm trial. There was no significant correlation between the preexercise muscle glycogen concentration and cycling performance for either the Norm or the Load trial. The 1 subject who rode more slowly after CHO loading than after his normal diet utilized only 9 mmolkg of his additional 57 mmolkg starting muscle glycogen concentration during the Load trial. He finished the Load trial with a higher muscle glycogen concentration than after the Norm trial (68 vs. 20 mmoll kg). In contrast, the other 7 subjects who rode faster after CHO loading utilized all their additional muscle glycogen and finished the Load trial with muscle glycogen contents similar to those at the end of the Norm trial (19 f 3 vs mmolkg). Table 3 Plasma Glucose and Lactate Concentrations During the 2-hr Steady-State Rides and Immediately Postexercise Glucose Lactate Norm CHO-load Norm CHO-load M SEM M SEM M SEM M SEM Rest min min min min min (postexercise) Note. Norm = normal diet; CHO = carbohydrate.

8 32 / Rauch, Rodger, Wilson, et al. J..... l..... l EXERCISE DURATION (min) Figure 1 - Power output (top panel) and the percentage of peak heart rate for subjects during the first 2 hr of exercise. Discussion The first finding of the present study was that 3 days of CHO loading with potato starch, in concert with a reduction in training, elevated muscle glycogen concentrations to those previously reported for a variety of solid (6, 22, 25), liquid (17), or combined solid and liquid (5, 17) supplements (Table 1). The increase in preexercise muscle glycogen concentration observed in the present study after the CHO-loading regimen (104 f 8 to 153 f 9 mmolkg, p <.0001) was greater than that previously observed when trained runners consumed a 90% CHO diet (809 gtday for days) comprised mainly of pasta and rice (17). In that study, the mean percentage increase in muscle glycogen concentration with a high pasta diet was only 34 f 15% (103 f 13 to 130 f 13 mmolkg), which was significantly lower than when subjects consumed an isoenergetic diet from which 82% (654 g) of CHO energy came from a concentrated 17 g1100 ml) maltodextrin solution (107 f 9 to 150 f 12 mmolkg, 45 f 12%, p <.05). Although more severe exercise-dietary regimens, involving CHO depletion before CHO loading, have been shown to elevate muscle glycogen stores to ~200 mmol/kg (1, 4, 13, 23, 25), the values found in the present investigation are consistent with those reported by others for trained cyclists (26). In agreement with other investigations (10, 15, 25, 26), the present study found that elevated preexercise glycogen levels after CHO loading increased the

9 Carbohydrate Loading / 33 0 NORMAL: 161 f 2 bpm PERFORMANCE RIDE (min) Figure 2 - Average power output (top panel) and the percentage of peak heart rate for subjects during the 1-hr performance rides DISTANCE IN f hr (km) Figure 3 - The distance covered during the 1-hr performance rides. utilization of muscle glycogen during exercise so that the amount of muscle glycogen remaining at the end of the 3-hr Load and Norm trials was similar (26 rt 7 vs. 18 k 2 mmol/kg). These postexercise muscle glycogen contents are considerably lower than some (7) but not all (5, 15, 24) of the values found at exhaustion in prolonged, submaximal exercise. However, the subjects in the

10 34 1 Rauch, Rodger, Wilson, et al. current study performed a series of maximal sprints throughout the first 120 min of exercise (Figure 1). High-intensity intermittent exercise has been shown to reduce muscle glycogen content by up to 72% in less than 10 min (18). Muscle glycogen was not different in subjects who consumed between 581 and 906 g CHOIday over a 72-hr period. Thus, there was no relationship between the amount of CHO subjects ingested during the glycogen-loading regimen and their preexercise muscle glycogen concentrations (Table 2). This result appears to contradict the earlier finding of Costill et al. (6), who reported a significant correlation (r = 34, p <.05) between the amount of CHO consumed in the first 24 hr following exhaustive exercise and the muscle glycogen concentration in well-trained runners. However, the range of CHO intake by the subjects in that study was large ( gl24 hr), and muscle glycogen contents were sampled only 24 hr after exhaustive exercise (6). The CHO intake of the subjects in the current investigation, which was undertaken over 72 hr, averaged -750 glday (range gl24 hr). This finding, taken together with Costill et al.'s (6) finding that muscle glycogen content was not significantly different when subjects consumed either 525 or 648 g of CHO over 24 hr, suggests that the extent of muscle glycogen storage is not increased by the ingestion of very large (>700 g/day) quantities of dietary CHO. More to the point, an intake of such large quantities of CHO over a 2- to 3-day period resulted in 33% of our subjects experiencing gastrointestinal disturbances. The second and most important finding of this study was that CHO-loaded subjects exercised at a significantly higher power output ( vs. 219 f 17 W, p <.05; Figure 2) and completed a greater distance in the 60-min performance ride ( vs f 1.29 km, p <.05; Figure 3) after 2 hr of strenuous cycling than when they consumed their normal diets. The exception was 1 subject who rode more slowly after CHO loading and failed to utilize his additional muscle glycogen stores. Previous laboratory investigations of the ergogenic effects of CHO loading during prolonged exercise have usually relied upon changes (or lack of change) in physiological function or on the exercise time to exhaustion at a fixed, submaxima1 work rate as a measure of performance (5, 7, 19). While this is a valid laboratory measure of performance, it does not equate to the race situation, in which the workload is likely to fluctuate with variations in pace (20), and in which competitors aim to cover a predetermined distance as fast as possible (26). Few studies have examined the effects of a high-cho diet (>SO0 glday for 48 hr) on prolonged exercise performance. Flynn et al. (9) failed to show a significant increase in the total work output achieved during 2 hr of isokinetic cycling in trained subjects who started exercise with elevated (>I60 mmolkg) muscle glycogen contents and who were fed CHO throughout exercise. Williams et al. (27) also found no difference in the performance of trained runners who ingested a high-cho diet for 3 days prior to a 30-km treadmill time trial. Likewise, Sherman et al. (25) were unable to demonstrate improvements in 21- km running time when subjects started the exercise tests with markedly different muscle glycogen contents. Taken collectively, these findings suggest that CHO loading may be of little benefit to performance in endurance events lasting 1-2 hr, where normal (i.e., 120 mmolkg) muscle glycogen stores are probably sufficient to sustain the desired exercise intensities.

11 Carbohydrate Loading / 35 The exceptions are the studies Widrick et al. (26) and Karlsson and Saltin (15). Widrick et al. found that 6-hr fasted subjects with low (110 mmolkg) preexercise muscle glycogen concentrations, who did not ingest CHO, exercised at significantly lower power outputs (5.2%, p <.05) and completed a 65-km cycling time trial in a significantly longer time (3.4%, p <.05) than when preexercise muscle glycogen concentrations were elevated (>I70 mmolkg). Karlsson and Saltin (15) showed improved running time for a 30-km race in subjects who had elevated preexercise muscle glycogen concentrations (194 k 68 mmolkg wet wt), compared to when they had "normal" values ( mmolkg wet wt). Our investigation confirms these studies and strongly suggests that glycogen loading only becomes of benefit to sustaining a predetermined pace Bfter -150 min bf exercise. In conclusion, the results of the current investigation show that in trained cyclists, increasing CHO intake by 72% via the ingestion of a potato starch supplement elevated muscle glycogen stores to supercompensated values (i.e., >I50 mmolkg). In addition, cyclists covered a greater distance in a 60-min performance ride, which followed 2 hr of strenuous cycling, when preexercise muscle glycogen contents were elevated. Finally, ingesting more than g of CHOJday in the form of potato starch may not be necessary to attain supramaximal muscle glycogen stores, and, further, may lead to gastrointestinal disturbances in some individuals. References 1. Astrand, P.O. Diet and athletic performance. Fed. Proc. 26: , Bergstrom, J. Muscle electrolytes in man. Scand. J. Lab. Invest. Suppl. 68, Bergstrom, J., L. Hermansen, E. Hultman, and B. Saltin. Diet, muscle glycogen and physical performance. Acta Physiol. Scand. 71: , Bergstrom, J., E. Hultman, and A.E. Roch-Norlund. Muscle glycogen synthetase in normal subjects. Basal values, effect of glycogen depletion by exercise and of a carbohydrate-rich diet following exercise. Scan. J. Clin. Lab. Invest. 29: , Bosch, A.N, S.C. Dennis, and T.D. Noakes. Influence of carbohydrate loading on fuel substrate turnover and oxidation during prolonged exercise. J. Appl. Physiol. 74: , Costill, D.L., W.M. Sherman, W.J. Fink, C. Maresh, M. Witten, and J.M. Miller. The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. Am. J. Clin. Nutr. 34: , Coyle, E.F., A.R. Coggan, M.K. Hemmert, and J.L. Ivy. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrates. J. Appl. Physiol. 61 : , Evans, W.J., S.D. Phinney, and V.R. Young. Suction applied to muscle biopsy maximizes sample size. Med. Sci. Sports Exerc. 14: , Flynn, M.G., D.L. Costill, J.A. Hawley, W.J. Fink, P.D. Neufer, R.A. Fielding, and M.D. Sleeper. Influence of selected carbohydrate drinks on cycling performance and glycogen use. Med. Sci. Sports Exerc. 19:37-40, Galbo, H., J.J. Holst, and N.J. Cristensen. The effect of different diets and insulin on the hormonal response to prolonged exercise. Acta Physiol. Scand. 107: 19-32, Gutmann, I., and A.W. Wahlefeld. L-(+)-Lactate determination with lactate dehydrogenase and NAD. In Methods of Enzymatic Analysis, H.U. Bergmeyer (Ed.), New York: Academic Press, 1984, pp

12 36 / Rauch, Rodger, Wilson, eta/. 12. Hawley, J.A., and T.D. Noakes. Peak sustained power output predicts maximal oxygen uptake and performance time in trained cyclists. Eur. J. Appl. Physiol. 65:79-83, Hultman, E. Studies on muscle metabolism of glycogen and active phosphate in man with special reference to exercise and diet. Scand. J. Clin. Lab. Invest. 19:l-64, Hyvarinen, A., and E. Nikkila. Specific determination of blood glucose with o- Toluidine. Clin. Chem. Acta 7:140, Karlsson, J., and B. Saltin. Diet, muscle glycogen, and endurance performance. J. Appl. Physiol. 3 1 : , Kuipers, H., F.T.J. Verstappen, H.A. Keizer, P. Guerten, and G. Van Kraneburg. Variability of aerobic performance in the laboratory and its physiological correlates. Int. J. Sports Med. 6: , Lamb, D.R., A.C. Snyder, and T.S. Baur. Muscle glycogen loading with a liquid carbohydrate supplement. Int. J. Sports Nutr , MacDougall, J.D., G. Ward, D. Sale, and J. Sutton. Muscle glycogen repletion after high-intensity, intermittent exercise. J. Appl. Physiol. 42: , Madsen, K., P.K. Pedersen, P. Rose, and E.A. Richter. Carbohydrate supercompensation and muscle glycogen utilization during exhaustive running in highly trained athletes. Eur. J. Appl. Physiol. 61: , Palmer, G.S., J.A. Hawley, S.C. Dennis, and T.D. Noakes. Heart rate responses during a four day cycle stage race. Med. Sci. Sports Exerc. 26: , Passoneau, J.V., and V.R. Lauderdale. A comparison of three methods of glycogen measurement in tissues. Anal. Biochem. 60: , Roberts, K.M., E.G. Noble, D.B. Hayden, and A.W. Taylor. Simple and complex carbohydrate-rich diets and muscle glycogen content of marathon runners. Eur. J. Appl. Physiol. Occup. Physiol. 75:70-74, Saltin, B., and L. Hermansen. Glycogen stores and prolonged severe exercise. In Nutrition and Physical Activity (Vol. 5). B. Blix (Ed.), Stockholm, Almquist & Wiksell, 1967, pp Sherman, W.M., D.L. Costill, W.J. Fink, F.C. Hagerman, L.E. Armstrong, and T.F. Murray. Effect of a 42.2-km footrace and subsequent rest or exercise on muscle glycogen and enzymes. J. Appl. Physiol.: Respirat. Environ. Exerc. Physiol. 55(4): , Sherman, W.M., D.L. Costill, W.J. Fink, and J.M. Miller. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int. J. Sports Med. 2: , Widrick, J.J., D.L. Costill, W.J. Fink, M.S. Hickey, G.K. McConell, and H. Tanaka. Carbohydrate feedings and exercise performance: Effect of initial muscle glycogen concentration. J. Appl. Physiol. 74: , Williams, C., J. Brewer, and M. Walker. The effect of a high carbohydrate diet on running performance during a 30-km treadmill time trial. Eur. J. Appl. Physiol. 65: Acknowledgment This study was supported by a grant from the South African Potato Producers' Organisation..