Someone who is less than 30 percent above ideal weight will burn more calories if he exercises after a meal. If more than 30 percent above his ideal weight, he will do better to exercise before he eats.
After-meal exercise takes advantage of a process called diet-induced thermogenesis. This means that metabolism increases after the ingestion of food, swallowing up seven to 10 percent of the calories eaten. Several studies suggest that this process increases if exercise is done after eating.