Huang, W. Y., Wong, S. H., Sun, F. H., & Tsang, K. F. (2012). Effect of glycemic index of snack bars on substrate utilization during subsequent moderate intensity exercise. Presentation 2338 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.

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This study investigated whether the glycemic index of pre-exercise snack bars will affect substrate utilization during subsequent moderate-intensity exercise. Volunteer young physically active males (N = 7) completed two main trials in a counterbalanced crossover design with seven days between trials. In the trials, Ss reported after an overnight fast. Ss ate one of two snack bars: a low-GI snack bar or a moderate-GI snack bar. The estimated glycemic index values for the two snack bars were 28 and 68, respectively. All snack bars provided similar energy and carbohydrate content (1 g CHO/kg body mass). However, the energy percentages of fat and protein were different (low- vs. moderate-GI: fat, 30% vs. 24%; protein, 9% vs. 16%). After resting for 90 minutes, Ss completed 45 minutes of cycling at 60% VO2max. Substrate utilization was measured every 30 minutes during the post-prandial period and every 15 minutes during exercise. Blood glucose and lactate concentrations were measured every 15 minutes.

Blood glucose concentrations peaked at 30 minutes and returned to baseline level at the end of post-prandial period in both trials. During exercise, blood glucose concentrations were suppressed in the moderate-GI trial compared to that which existed at the onset of exercise, but not in the low-GI trial. During the post-prandial period, blood glucose was higher at 60 minutes in the moderate-GI trial compared to that in the low-GI trial. The incremental area under the blood response curve of glucose value was also higher in the moderate-GI trial than that in the low-GI trial. No differences were observed in blood lactate concentrations, fat, and carbohydrate oxidation amounts between the two trials both during the post-prandial and exercise periods.

Implication. Substrate utilization during a 45-minute moderate intensity exercise was not affected by the pre-exercise low- or moderate-GI snack bar consumption.

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