| To examine the acute effects of exercise timing and meal glycemic index (GI) on fat oxidation and glycemic response, five apparently healthy young men participated in four randomly ordered morning trials during which measurements were made at rest, during exercise, and for two hours post-exercise. A factorial design [exercise timing (pre-prandial, post-prandial) x meal GI (low-GI, high-GI)] was used for repeated measures of energy expenditure and whole-body fat oxidation, as well as of plasma glucose and insulin levels after an overnight fast. Subjects were required to perform 400 kcal of moderate treadmill exercise either before consuming a 400 kcal low-GI (ELG) or high-GI (EHG) oatmeal breakfast, or after consuming the low-GI (LGE) or high-GI (HGE) meal. Exercising fat oxidation was significantly greater during ELG and EHG (14.7 +/- 1.4 and 14.8 +/- 3.2g, respectively) than during LGE and HGE (8.9 +/- 3.1 and 9.8 +/- 2.7g, respectively) (p < .001), as was total fat oxidation beyond rest and (ELG: 21.3 +/- 3.7g; EHG: 20.2 +/- 5.9g; LGE: 18.1 +/- 6.0g; HGE: 17.1 +/- 3.4g) (p < .05), although energy expenditure was unaffected by experimental conditions. No significant effect of meal GI on fat oxidation was observed and, unexpectedly, the glycemic response was not significantly different across experimental conditions. Total whole-body fat oxidation for the entire morning period is therefore greatest when exercise is performed in the post-absorptive state, a strategy that could help maximize acute exercise-induced fat oxidation. |
