The depletion or reduction of bodily carbohydrate reserves is associated with fatigue during endurance exercise. Various carbohydrate supplementation and exercise regimens have been used experimentally to increase carbohydrate reserves before exercise or to maintain the availability of carbohydrate for oxidation during exercise. On the other hand, the improved endurance capability observed after aerobic training has been attributed to an increased oxidation of fat relative to carbohydrate; this carbohydrate sparing presumably delays the point at which reduced carbohydrate reserves cause fatigue. This effect has led to the suggestion that a greater availability of fat during exercise can improve performance via the carbohydrate-sparing effect of “fat loading.” Although this is a plausible hypothesis, it is not supported by a sufficient number of valid, credible, and replicated studies. Thus, it appears prudent to advise endurance athletes to consume a diet that is largely carbohydrate to optimize training and competitive performance and, more importantly, to promote optimal health.
W. Michael Sherman and Nicole Leenders
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.
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.
Nicole Leenders, W. Michael Sherman, David R. Lamb and Timothy E. Nelson
The purpose of this study was to determine if oral creatine (CR) ingestion, compared to a placebo (PL), would enable swimmers to maintain a higher swimming velocity across repeated interval sets over 2 weeks of supplementation. Fourteen female and 18 male university swimmers consumed a PL during a 2-week baseline period. Using a randomized, double-blind design, during the next 2 weeks subjects consumed either CR or PL. Swimming velocity was assessed twice weekly during 6 × 50-m swims and once weekly during 10 × 25-yd swims. There was no effect ofCR on the 10 × 25-yd interval sets for men and women and no effect on the 6 × 50-m interval sets for women. In contrast, for men, CR significantly improved mean overall swimming velocity in the 6 × 50-m interval after 2 weeks of supplementation, whereas PL had no effect. Although ineffective in women, CR supplementation apparently enables men to maintain a faster mean overall swimming velocity during repeated swims each lasting about 30 s; however, CR was not effective for men in repeated swims each lasting about 10-15 s.