Effects of 3 days of carbohydrate supplementation on muscle glycogen content and utilisation during a 1-h cycling performance

This study compared the effects of supplementing the normal diets of six trained cyclists [maximal oxygen uptake ((V) over dot O-2max) 4.5 (0.36) l.min(-1) values are mean (SD)] with additional carbohydrate (CHO) on muscle glycogen utilisation during a 1-h cycle time-trial (TT). Using a randomised crossover design, subjects consumed either their normal diet (NORM) for 3 days, which consisted of 426 (137) g.day(-1) CHO [5.9 (1.4) g.kg(-1) body mass (BM)], or additional CHO (SUPP) to increase their intake to 661 (76) g.day(-1) [9.3 (0.7) g.kg(-1) BM]. The SUPP diet elevated muscle glycogen content from 459 (83) to 565 (62) mmol.kg(-1) dry weight (d.w.) (P < 0.05). However, despite the increased pre-exercise muscle glycogen stores, there was no difference in the distance cycled during the TT [40.41 (1.44) vs 40.18 (1.76) km for NORM and SUPP, respectively]. With NORM, muscle glycogen declined from 459 (83) to 175 (64) mmol.kg(-1) d.w., whereas with SUPP the corresponding values were 565 (62) and 292 (113) mmol.kg(-1) d.w. Accordingly, both muscle glycogen utilisation [277 (64) vs 273 (114) mmol.kg(-1) d.w.] and total CHO oxidation [169 (20) vs 165 (30) g.h(-1) for NORM and SUPP, respectively] were similar. Neither were there any differences in plasma glucose or lactate concentrations during the two experimental trials. Plasma glucose concentration averaged 5.5 (0.5) and 5.6 (0.6) mmol.l(-1), while plasma lactate concentration averaged 4.4 (1.9) and 4.4 (2.3) mmol.l(-1) for NORM and SUPP, respectively. The results of this study show that when well-trained subjects increase the CHO content of their diet for 3 days from 6 to 9 g.kg(-1) BM there is only a modest increase in muscle glycogen content. Since supplementary CHO did not improve TT performance, we conclude that additional CHO provides no benefit to performance for athletes who compete in intense: continuous events lasting 1 h. Furthermore, the substantial muscle CHO reserves observed at the termination of exercise indicate that whole-muscle glycogen depletion does not determine fatigue at this exercise intensity and duration.