Moore, L. J., Midgley, A. W., Thomas, G. J., Keatley, S., McNaughton, L. R., & Siegler, J. (2009). Effect of the glycemic index of a pre-exercise meal on metabolism and cycling time trial performance. ACSM 56th Annual Meeting, Seattle, Washington. Presentation number 1464.

The aim of this study was to investigate the effects of low or high glycemic index (GI) foods consumed before a 40-km time trial on metabolism and subsequent endurance performance. Male cyclists (N = 10) consumed either a high-GI or low-GI meal, providing 1g/kg of body mass of carbohydrate, 45 minutes before a 40 km time trial performed on a stationary cycle.

The time-trial performance was significantly improved and whole blood glucose concentration at the point of exhaustion was significantly higher in the low-GI trial compared to the high-GI trial. The low-GI carbohydrate oxidation rate was significantly higher than in the high-GI treatment. Fat oxidation rate was significantly higher for the high-GI trial than in the low-GI trial. Insulin rose significantly following the high-GI meal compared to the low-GI meal but dropped significantly to similar values throughout the time trial.

Implication. Greater carbohydrate oxidation and glucose concentration in a low-GI pre-performance meal may produce a glucose-sparing effect, maintain energy production towards the end of an extended exercise, and improve performance.

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