Researchers at the University of Bath in the U.K. had a group of overweight men take a moderate treadmill test for an hour on an empty stomach, and then, on another occasion, two hours after eating a high-carbohydrate breakfast.

They found that the expression of two genes in the adipose tissue linked to burning stored fat and energy increased when the men fasted and exercised, and decreased when the men ate before exercising.

Moral of the story: Hold out until after your workout. Your waistband will thank you.