Two bouts of exercise before meals, but not after meals, lower fasting blood glucose

Reduced counterregulatory responses to a next-day hypoglycemic challenge and hypoglycemia result from two spaced episodes of moderate-intensity exercise and have been characterized as exercise-associated autonomic failure. We hypothesized that this phenomenon is caused by postabsorptive state at the time of exercise rather than by autonomic failure.

Participants were nine healthy postmenopausal women in a crossover study. Two hours of treadmill exercise at 43% of maximal effort were performed twice a day, separated by 5 h, either 1 h before (Before-Meals trial) or 1 h after a meal (After-Meals trial). Plasma insulin, counterregulatory hormones (glucagon, growth hormone, cortisol), and metabolites (glucose, free fatty acids, ketones) were measured to evaluate the effects of nutritional timing. Analyses of HR and vagal tone were measured to assess autonomic function.

Before-Meals exercise, but not After-Meals exercise, reduced postabsorptive plasma glucose by 20.2% during a 16-h period, without a change in counterregulatory response, and elicited postexercise ketosis. A 49% increase in insulin-glucagon ratio during meals, a 1 mM decline in glucagon glycemic threshold, and a reduced vagal tone during exercise were associated with Before-Meals but not with After-Meals trials.

These results demonstrate that exercise performed in postabsorptive, but not in postprandial state, lowers glucoregulatory set point and glucagon glycemic threshold and is accompanied by reduced vagal tone, counterregulatory responses, and glucagon glycemic threshold and by increased insulin-glucagon ratio. Reduced counterregulatory response, altered neuroendocrine function, and sustained lowering of blood glucose are most likely the consequences of reduced carbohydrate availability during exercise.