Harris, M., Black, S., Giusti, J., & Cordain, L. (1997). Effect of differing glycemic index pre-exercise meals on substrate utilization and performance. Medicine and Science in Sports and Exercise, 29(5), Supplement abstract 727.

The purpose of this investigation was to assess the effect of the glycemic index (GI) of carbohydrates (CHO) ingested in the pre-exercise period on glucose utilization, free fatty acid availability, maintenance of serum glucose, and performance.

Well-trained males (N = 10) and females (N = 8) completed three trials presented in random order: fasted (control) and high and moderate GI pre-exercise meals providing 80 gm for females and 100 gm for males and 7 kcal/kg body weight. Meals were consumed one hour before exercising at 70% VO2max on a cycle ergometer for 60 min followed by a maximum trial to exhaustion. Variables were assessed before and during the initial exercise bout.

Serum glucose in both meal trials was significantly lower at the start of the exercise but rose steadily during exercise and was significantly higher by minute 45 when compared to the fasting condition. Both meal trials also demonstrated significantly higher RER which indicated increased CHO utilization. No differences occurred in serum lactate, free fatty acid release, or maximal exercise performance.

The glycemic index may be of limited utility in predicting metabolic response in well-trained individuals with enhanced insulin sensitivity.

Implication. The glycemic index is not a very important criterion for food selection for CHO feeding in well-trained athletes.

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