INFLUENCE OF HIGH AND LOW GLYCEMIC INDEX CARBOHYDRATE MEALS ON ENDURANCE CAPACITY

Abstract

1404 The purpose of this study was to examine the effect of high and low glycemic index (GI) carbohydrate (CHO) meals on endurance running capacity. Eight active subjects (5 males, 3 females) ran on a treadmill at 68.0 ± 1.0% VO2max to exhaustion on two occasions separated by 7 days. Three hours before the run, after an overnight fast, each subject was given in a single-blind, random order isoenergetic meal (850 ± 21 kcal; 67% carbohydrate, 30% protein and 3% fat) containing either high (HGI) or low(LGI) GI foods providing 2.0g CHO/kg body weight. Our preliminary findings indicated that ingestion of the HGI meal resulted in a 290% and 243% greater incremental area under the 1-hour blood glucose response curve and 3-hour serum insulin response curve respectively. Performance times were not different between the HGI and LGI trials (111±6 min and 113±5 min respectively). During the first 80 minutes of exercise in the LGI trial, CHO oxidation was 12% lower and fat oxidation 118% higher than HGI trial. Although serum insulin concentrations did not differ at the onset of and during exercise, blood glucose at 20 min into exercise in the HGI trial was 3.9 ± 0.3 mmol.l-1 compared to 4.4 ± 0.3 mmol.l-1 during LGI (p<0.05). There was also a trend towards decreased FFA concentrations during exercise in the HGI trial. Blood lactate and serum cortisol concentrations were not different between the two trials. It is likely that the enhanced CHO oxidation during the high GI trial is a consequence of increased muscle and liver glycogen stored before exercise. However, the low GI meal may have provided less CHO to the muscle for subsequent utilisation because of delayed digestion and absorption. Thus, despite the CHO sparing effect of the low GI meal, endurance running capacity did not differ.