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Matthew Erlenbusch, Mark Haub, Kathy Munoz, Susan MacConnie and Belinda Stillwell

The purpose of this investigation was to clarify, via a meta-analysis, whether the literature favors a high-fat or a high-carbohydrate diet to yield superior endurance exercise performance. Twenty published trials were analyzed to compare exercise performance under different diets. The average effect size of −0.60 indicated that subjects following a high-carbohydrate diet exercised longer until exhaustion. The training status of subjects (trained vs. untrained) was significantly related to effect size (r = −0.576, P < 0.01) and effect sizes separated between trained and untrained subjects were −0.05 and −2.84 respectively. The test for homogeneity revealed significant heterogeneity among effect sizes (χ2 [19] = 43.30, P < 0.05), indicating all of the trials are not describing the same effect. Given this significant heterogeneity, a conclusive endorsement of a high-carbohydrate diet based on the literature is difficult to make. Highly dissimilar trial protocols are the primary reason for heterogeneity.

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Kevin Allen Jacobs and W. Michael Sherman

Carbohydrate (CHO) is the body's most limited fuel and the most heavily metabolized during moderate-intensity exercise. For this reason it is recommended that endurance athletes consume a high-CHO diet (8-10 g CHO ⋅ kg body weight−1 ⋅ day−1) to enhance training and performance. A review of the literature supports the benefits of CHO supplementation on endurance performance. The benefits of chronic high-CHO diets on endurance performance are not as clear. Recent evidence suggests that a high-CHO diet may be necessary for optimal adaptations to training. However, the paucity of data in this area precludes any concrete conclusions. The practicality of high-CHO diets is not well understood. The available evidence would indicate that a high-CHO diet is the best dietary recommendation for endurance athletes.

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Paul W. Macdermid and Stephen R. Stannard

This study compared a training diet recommended for endurance athletes (H-CHO) with an isoenergetic high protein (whey supplemented), moderate carbohydrate (H-Pro) diet on endurance cycling performance. Over two separate 7-d periods subjects (n = 7) ingested either H-CHO (7.9 ± 1.9 g · kg−1 · d−1 carbohydrate; 1.2 ± 0.3 g · kg−1 · d−1 fat; 1.3 ± 0.4 g · kg−1 · d−1 protein) or H-Pro (4.9 ± 1.8 g · kg−1 · d−1; 1.2 ± 0.3 g · kg−1 · d−1; 3.3 ± 0.4 g · kg−1 · d−1) diet in a randomized, balanced order. On day 8 subjects cycled (self-paced) for a body weight dependent (60 kJ/bm) amount of work. No differences occurred between energy intake (P = 0.422) or fat intake (P = 0.390) during the two dietary conditions. Performance was significantly (P = 0.010) impaired following H-Pro (153 ± 36) compared with H-CHO (127 ± 34 min). No differences between treatments were observed for physiological measures taken during the performance trials. These results indicate an ergolytic effect of a 7-d high protein diet on self-paced endurance cycling performance.

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Stavros A. Kavouras, John P. Troup and Jacqueline R. Berning

To examine the effects of a 3-day high carbohydrate (H-CHO) and low carbohydrate (L-CHO) diet on 45 min of cycling exercise, 12 endurance-trained cyclists performed a 45-min cycling exercise at 82 ± 2% VO2peak following an overnight fast, after a 6-day diet and exercise control. The 7-day protocol was repeated under 2 randomly assigned dietary trials H-CHO and L-CHO. On days 1–3, subjects consumed a mixed diet for both trials and for days 4–6 consumed isocaloric diets that contained either 600 g or 100 g of carbohydrates, for the HCHO and the L-CHO trials, respectively. Muscle biopsy samples, taken from the vastus lateralis prior to the beginning of the 45-min cycling test, indicated that muscle glycogen levels were significantly higher (p < .05) for the H-CHO trial (104.5 ± 9.4 mmol/kg wet wt) when compared to the L-CHO trial (72.2 ± 5.6 mmol/kg wet wt). Heart rate, ratings of perceived exertion, oxygen uptake, and respiratory quotient during exercise were not significantly different between the 2 trials. Serum glucose during exercise for the H-CHO trial significantly increased (p < .05) from 4.5 ± 0.1 mmol · L−1 (pre) to 6.7 ± 0.6 mmol · L−1 (post), while no changes were found for the L-CHO trial. In addition, post-exercise serum glucose was significantly greater (p < .05) for the H-CHO trial when compared to the L-CHO trial (H-CHO, 6.7 ± 0.6 mmol · L−1; L-CHO, 5.2 ± 0.2 mmol · L−1). No significant changes were observed in serum free fatty acid, triglycerides, or insulin concentration in either trial. The findings suggest that L-CHO had no major effect on 45-min cycling exercise that was not observed with H-CHO when the total energy intake was adequate.

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Patricia Guimaraes Couto, Romulo Bertuzzi, Carla Caroline de Souza, Hessel Marani Lima, Maria Augusta Peduti Dal Molin Kiss, Fernando Roberto de-Oliveira and Adriano Eduardo Lima-Silva

This study analyzed the pacing employed by young runners in 10,000 m time-trials under 3 dietary regimens of different carbohydrate (CHO) intakes. Nineteen boys (13–18 years) ate either their normal CHO diet (56% CHO), high (70% CHO), or low (25% CHO) CHO diets for 48 hr; the boys then performed a 10,000 m run (crossover design). The high CHO diet led to faster final sprint (14.4 ± 2.2 km·h-1) and a better performance (50.0 ± 7.0 min) compared with the low CHO diet (13.3 ± 2.4 km·h-1 and 51.9 ± 8.3 min, respectively, p < .05). However, the final sprint and performance time in the high CHO or low CHO diets were statistically not significantly different from the normal CHO diet (13.8 ± 2.2 km·h-1 and 50.9 ± 7.4 min; p > .05). CHO oxidation rate during the constant load exercise at 65% of VO2max was elevated in high CHO diet (1.05 ± 0.38 g·min-1) compared with low CHO diet (0.63 ± 0.36 g·min-1). The rating of perceived exertion increased linearly throughout the trial, independently of the dietary regimen. In conclusion, the high CHO diet induced higher CHO oxidation rates, increased running speed in the final 400 m and enhanced overall running performance, compared with low CHO.

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Michael Gleeson, Andrew K. Blannin, Neil P. Walsh, Nicolette C. Bishop and Anya M. Clark

We examined the effects of a low-carbohydrate (CHO) diet on the plasma glutamine and circulating leukocyte responses to prolonged strenuous exercise. Twelve untrained male subjects cycled for 60 min at 70% of maximal oxygen uptake on two separate occasions, 3 days apart. All subjects performed the first exercise task after a normal diet: they completed the second exercise task after 3 days on either a high-CHO diet (75±8% CHO, n = 6) or a low-CHO diet (7±4% CHO, n = 6). The low-CHO diet was associated with a larger rise in plasma cortisol during exercise, a greater fall in the plasma glutamine concentration during recovery, and a larger neutrophilia during the postexercise period. Exercise on the high-CHO diet did not affect levels of plasma glutamine and circulating leukocytes. We conclude that CHO availability can influence the plasma glutamine andcirculaling leukocyte responses during recovery from intense prolonged exercise.

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Mark H. Roltsch, Judith A. Flohr and Patricia B. Brevard

The purpose of this study was to examine the metabolic consequences of a moderate variation in dietary fat content of male endurance athletes during submaximal exercise. Six males (age, 29.8 ± 11 years; weight, 72.3 ± 10 kg) · with an average maximum oxygen uptake (V̇O2max) of 66 ± 10 ml/kg/min were tested on their normal diet and 3 experimental diets. The energy contributions from protein, carbohydrates, and fats were 16/59/22 (3% alcohol), 14/53/33, 13/72/15, and 16/61/23% for the normal diet (N), fat supplemented diet (F), high carbohydrate diet (C), and adjusted normal diet (AN), respectively. The F diet was designed to significantly increase fat content compared to the normal diet and be easily maintained by the athletes. Caloric content of the F, C, and AN diets were adjusted to meet estimated total daily energy expenditure. The difference between the N and AN diets is that the AN has been adjusted to meet estimated total daily energy expenditure. The diets were randomly assigned after substrate utilization testing on the N diet and were consumed for 7 days prior to testing. Substrate utilization was recorded at steady state (73 ± 1.4% of V̇O2max) while running on a treadmill for 40 min. There were no significant differences in respiratory exchange ratio between any of the dietary manipulations. No significant differences were observed for lactate, V̇O2, or HR during submaximal testing on the N, F, C, and AN diets. These data indicate that a fat supplemented diet did not affect substrate utilization during 40 min of steady-state submaximal exercise when compared to a high carbohydrate diet or the participant’s normal and adjusted normal diets.

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Kevin A. Jacobs, David R. Paul, Ray J. Geor, Kenneth W. Hinchcliff and W. Michael Sherman

The purpose of the current study was to examine the influence of dietary composition on short-term endurance training–induced adaptations of substrate partitioning and time trial exercise performance. Eight untrained males cycled for 90 min at ~54% aerobic capacity while being infused with [6,62H]glucose before and after two 10-d experimental phases separated by a 2-week washout period. Time trial performance was measured after the 90-min exercise trials before and after the 2nd experimental phase. During the first 10-d phase, subjects were randomly assigned to consume either a high carbohydrate or high fat diet while remaining inactive (CHO or FAT). During the second 10-d phase, subjects consumed the opposite diet, and both groups performed identical daily supervised endurance training (CHO+T or FAT+T). CHO and CHO+T did not affect exercise metabolism. FAT reduced glucose flux at the end of exercise, while FAT+T substantially increased whole body lipid oxidation during exercise and reduced glucose flux at the end of exercise. Despite these differences in adaptation of substrate use, training resulted in similar improvements in time trial performance for both groups. We conclude that (a) 10-d high fat diets result in substantial increases in whole body lipid oxidation during exercise when combined with daily aerobic training, and (b) diet does not affect short-term training-induced improvements in high-intensity time trial performance.

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D.R. Paul, S.M. Mulroy, J.A. Horner, K.A. Jacobs and D.R. Lamb

The effects of employing a high-carbohydrate diet (carbohydrate-loading) to increase glycogen storage in skeletal muscle are not well established in female athletes. On 4 occasions—2 familiarization trials and 2 experimental trials—6 well-trained female subjects completed 6 × 15-min continuous intervals of cycling (12 min at 72% V̇O2max, 1 min at maximal effort, and 2 min at 50% V̇O2max), followed by a time trial 15 min later. The women consumed their habitual diets (HD; 6–7 g carbohydrate/kg lean body mass) for 3 days after the second familiarization trial and before the first experimental trial. During the 3 days following the first experimental trial, the subjects consumed a high-carbohydrate diet (CD; 9–10 g carbohydrate/kg lean body mass) prior to the second experimental trial. Mean (±SEM) pre-exercise muscle glycogen concentrations were greater after CD versus HD (171.9 ± 8.7 vs. 131.4 ± 10.3 mmol/kg wet weight, P < 0.003). Although 4 of the 6 subjects improved their time-trial performance after CD, mean performance for the time trial was not significantly different between diets (HD: 763.9 ± 35.6 s; CD: 752.9 ± 30.1 s). Thus, female cyclists can increase their muscle glycogen stores after a carbohydrate-loading diet during the follicular phase of the menstrual cycle, but we found no compelling evidence of a dietary effect on performance of a cycling time trial performed after 90 min of moderate-intensity exercise.

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Janice L. Thompson, Melinda M. Manore and Jerry R. Thomas

Studies examining the effects of diet (D) and diet-plus-exercise (DE) programs on resting metabolic rate (RMR) report equivocal results. The purpose of this study was to use meta-analysis to determine if exercise prevents the decrease in RMR observed with dieting. Results from the 22 studies included in this analysis revealed that the majority of studies used female subjects ages 31-45 years, who were fed a relatively low-fat, high-carbohydrate diet of less than 5,023 kJ · day1. The predominant prescribed exercise was aerobic in nature, 31-60 min in duration, performed 4-5 days per week, and of moderate intensity (51-70% of VO2max). Contrary to what is reported in narrative reviews, RMR decreased significantly with both D and DE programs, and the drop with D was significantly greater than that with DE. In conclusion, the addition of exercise to dietary restriction appears to prevent some of the decrease in RMR observed in premenopausal women.