Influence of high and low glycemic index meals on endurance running capacity

Purpose: The purpose of this study was to examine the effect of high and lo w glycemic index (GI) carbohydrate (CHO) pre-exercise meals on endurance ru nning capacity. Methods: Eight active subjects (five male and three female) ran on a treadmill at similar to 70% (V) over dot O-2max to exhaustion on two occasions separated by 7 d. Three hours before the run after an overnig ht fast, each subject was given in a single-blind, random order, isoenerget ic meal of 850 +/- 21 kcal (mean +/- SEM; 67% carbohydrate, 30% protein, an d 3% fat) containing either high (HGI) or low (LGI) GI carbohydrate foods p roviding 2.0 g CHO.kg(-1) body weight. Results: Ingestion of the HGI meal r esulted in a 580% and 330% greater incremental area under the 3-h blood glu cose and serum insulin response curves, respectively. Performance times wer e not different between the HGI and LGI trials (113 +/- 4 min and 111 +/- 5 min, respectively). During the first 80 min of exercise in the LGI trial, CHO oxidation was 12% lower and fat oxidation was 118% higher than in the H GI trial. Although serum insulin concentrations did not differ between tria ls, blood glucose at 20 min into exercise in the HGI trial was lower than t hat during the LGI trial at the same time (3.6 +/- 0.3 mmol.L-1 vs 4.3 +/- 0.3 mmol.L-1; P < 0.05). During exercise, plasma glycerol and serum free fa tty acid concentrations were lower in the HGI trial than in the LGI trial. Conclusions: This results demonstrate that although there is a relative shi ft in substrate utilization from CHO to fat when a low GI meal is ingested before exercise compared with that for a high GI meal, there is no differen ce in endurance running capacity.