Effects of elevated and exercise-reduced muscle glycogen levels on insulin sensitivity

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Abstract
The effects of an exercise-induced muscle glycogen reduction and an elevated muscle glycogen concentration on glucose tolerance and the insulin response to an oral glucose tolerance test (GTT) were examined. GTTs were administered to seven male subjects after 3 days on a mixed diet (C), after exhaustive exercise and 1 day on a high-fat protein diet (L-FP), after exhaustive exercise and 1 day on a mixed diet (L-M), and after exhaustive exercise and 3 days on a high-carbohydrate diet (H-CHO). The L-M treatment resulted in a significant reduction in muscle glycogen (C, 79.6 +/- 4.2 mmol/kg wet wt vs. L-M, 53.9 +/- 1.2 mmol/kg wet wt) and a 31.7% reduction in the insulin-glucose (IG) index, a measure of insulin sensitivity in vivo. Muscle glycogen was also significantly reduced by the L-FP treatment (49.1 +/- 2.4 mmol/kg wet wt), but there was no change in the IG index. Preventing a decrease in the IG index during the L-FP treatment may have been a result of elevated free fatty acids (67%) and ketones (552%) prior to the GTT. Muscle glycogen was significantly increased by the H-CHO treatment (124.8 +/- 11.1 mmol/kg wet wt); however, the IG index was not different from that of the C treatment. The results suggest that an exercise-induced reduction in muscle glycogen can improve insulin sensitivity in vivo but that this effect is diet dependent.