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Exercise promotes the uptake of glucose into muscle cells and increases insulin sensitivity. Other physical adaptations occur during exercise, as well, including increased muscle mass, decreased fat mass, and improved mitochondrial function. Previous research has demonstrated that training in the fasted state promotes greater glucose tolerance and insulin sensitivity and induces higher fatty acid oxidation compared to training in the fed state. A recent study bolsters these findings, demonstrating that exercising before eating breakfast may enhance some of the beneficial effects of exercise.
The six-week, single-blind, randomized, controlled trial involved 30 overweight or obese men who engaged in moderate-intensity cycling either before or after eating a high-carbohydrate, mixed-macronutrient breakfast. The men exercised for three, 30-minute sessions the first week and progressed to three, 50-minute sessions over the remaining weeks.
The men who exercised before eating had nearly 2-fold higher whole-body lipid utilization rates as well as decreased carbohydrate utilization compared to the men who exercised after eating. The effects were sustained throughout the entire six-week study period. These findings suggest that exercising before eating breakfast burns more fat, improves insulin sensitivity, and increases glucose uptake into muscle tissue compared to exercising after eating breakfast. Exercising after eating may blunt these effects, however.