Preexercise Carbohydrate Meals: Application of Glycemic Index

Abstract

LEARNING OUTCOME: To list physiologic and performance differences following consumption of two preexercise meals of varying glycemic indices. The lowering of plasma glucose can contribute to fatigue in subjects performing strenuous endurance exercise. Plasma glucose may be maintained by preexercise carbohydrate meals. A low glycemic index meal (LGI) may offer an advantage by providing a source of glucose to the blood for a long period of time accompanied by a minimal insulin surge. The purpose of this study was to compare postprandial metabolic, physiologic, and performance responses to a LGI and a moderately high GI meal (HGI), each providing three foods totaling 1.5 g carbohydrate per kg body weight. After an overnight fast, 10 male endurance trained cyclists consumed one of the test meals or water 30 min prior to cycling 2 h at 70% of maximum oxygen uptake (VO2 max) followed by cycling to exhaustion at 100% of VO2 max. Data were analyzed using two-way ANOVA for repeated measures (P < 0.05). Plasma insulin levels were significantly lower after LGI than after HGI from 15 min after ingestion through 20 min of exercise. Significantly higher respiratory exchange ratios (RERs) were observed after HGI than after LGI until 2 h of exercise, at which time there was a significant decline in the RER after HGI. At 2 h of exercise, plasma glucose levels were significantly higher and ratings of perceived exertion lower after LGI as compared to after HGI. Time to exhaustion was 59% longer after LGI (206.5 ± 43.5 s) than after HGI (129.5 ± 22.8 s). These results suggest that maximal performance following endurance exercise may be affected by the glycemic index of preexercise meals. The LGI induced a lower rise in plasma insulin after ingestion and then maintained plasma glucose at higher levels at the end of a period of strenuous exercise, which may have better supported subsequent maximal effort.