Preexercise carbohydrate ingestion, glucose kinetics, and muscle glycogen use: effect of the glycemic index

Eight trained men cycled at 70% peak oxygen uptake for 120 min followed by a 30-min performance cycle after ingesting either a high-glycemic index (HG I), low-glycemic index (LGI), or placebo (Con) meal 30 min before exercise. Ingestion of HGI resulted in an elevated (P < 0.01) blood glucose concentr ation compared with LGI and Con. At the onset of exercise, blood glucose fe ll (P < 0.05) such that it was lower (P < 0.05) in HGI compared with LGI an d Con at 15 and 30 min during exercise. Plasma insulin concentration was hi gher (P < 0.01) throughout the rest period after ingestion of HGI compared with LGI and Con. Plasma free fatty acid concentrations were lower (P < 0.0 5) throughout exercise in HGI compared with LGI and Con. The rates of [6,6- H-2]glucose appearance and disappearance were higher (P < 0.05) at rest aft er ingestion and throughout exercise in HGI compared with LGI and Con. Carb ohydrate oxidation was higher (P < 0.05) throughout exercise, whereas glyco gen use tended (P = 0.07) to be higher in HGI compared with LGI and Con. No differences were observed in work output during the performance cycle when comparing the three trials. These results demonstrate that preexercise car bohydrate feeding with a HGI, but not a LGI, meal augments carbohydrate uti lization during exercise but does not effect exercise performance.