Abstract

The aim of this study was to compare the effect of preexercise breakfast containing high- and low-glycemic index (GI) carbohydrate (CHO) (2.5g CHO/kg body mass) on muscle glycogen metabolism. On two occasions, 14 days apart, seven trained men ran at 71% maximal oxygen uptake for 30 min on a treadmill. Three hours before exercise, in a randomized order, subjects consumed either isoenergetic high- (HGI) or low-GI (LGI) CHO breakfasts that provided (per 70 kg body mass) 3.43 MJ energy, 175 g CHO, 21 g protein, and 4 g fat. The incremental areas under the 3-h plasma glucose and serum insulin response curves after the HGI meal were 3.9- (P < 0.05) and 1.4-fold greater (P < 0.001), respectively, than those after the LGI meal. During the 3-h postprandial period, muscle glycogen concentration increased by 15% (P < 0.05) after the HGI meal but remained unchanged after the LGI meal. Muscle glycogen utilization during exercise was greater in the HGI (129.1 +/- 16.1 mmol/kg dry mass) compared with the LGI (87.9 +/- 15.1 mmol/kg dry mass; P < 0.01) trial. Although the LGI meal contributed less CHO to muscle glycogen synthesis in the 3-h postprandial period compared with the HGI meal, a sparing of muscle glycogen utilization during subsequent exercise was observed in the LGI trial, most likely as a result of better maintained fat oxidation.