Pre-exercise carbohydrate meals : application of glycemic index

The purpose of this study was to compare postprandial glycemic, insulinemic, and physiologic responses to a pre-exercise meal calculated to have a low glycemic index (LGI) with one calculated to have a moderately high glycemic index (HGI); each meal provided three foods totaling 1.5 g carbohydrate/kg body weight. After an overnight fast, 10 trained cyclists consumed one of the test meals or water 30 min before cycling 2 h at 70% of maximum oxygen uptake (VO2max), followed by cycling to exhaustion at 100% of VO2max. Plasma insulin levels were significantly lower (P < 0.05) after LGI than after HGI through 20 min of exercise. Significantly higher (P < 0.05) respiratory exchange ratios were observed after HGI than after LGI until 2 h of exercise. At that time plasma glucose levels were significantly higher and ratings of perceived exertion lower (P < 0.05) after LGI compared with 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 a pre-exercise LGI may positively affect maximal performance following sustained exercise. The LGI maintained higher plasma glucose levels at the end of 2 h of strenuous exercise than the HGI, which may have better supported subsequent maximal effort.