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.

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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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