CHO feeding before prolonged exercise: effect of glycemic index on muscle glycogenolysis and exercise performance

This study examined the effect of preexercise carbohydrate (CHO) ingestion on muscle CHO metabolism and performance during prolonged exercise. Six endurance-trained men performed three exercise trials on a cycle ergometer after ingesting either a high glycemic index (HGI), low glycemic index (LGI), or placebo (Con) meal 45 min before exercise. During each trial, subjects cycled at a workload corresponding to 70% peak oxygen uptake for 120 min, followed by a 15-min performance cycle where total work (kJ) was measured. Ingestion of HGI resulted in an elevated (P < 0.01) blood glucose concentration 15 min after ingestion, compared with LGI and Con, but there were no differences in the glycemic responses to the meals thereafter, despite plasma insulin concentration being higher (P < 0.01) throughout the rest period after ingestion of HGI compared with LGI and Con. Plasma free fatty acid concentrations were lower (P < 0.05) throughout exercise in HGI compared with LGI and Con. In addition, concentrations of this metabolite were lower (P < 0.05) during the first hour of exercise in LGI compared with Con. Despite these results, no differences were observed in either the rate of muscle glycogen utilization during submaximal exercise or work output during the performance cycle when the three trials were compared. These results demonstrate that preexercise CHO ingestion has no effect on muscle glycogen utilization or exercise performance, irrespective of the glycemic or insulinemic responses to the ingested meals.