Researchers recruited healthy, active young men and fed them a bad diet for six weeks. A group of them that exercised before breakfast gained almost no weight and showed no signs of insulin resistance. What’s more, they burned the fat they were taking in more efficiently.
According to the New York Times:
… Working out before breakfast directly combated the two most detrimental effects of eating a high-fat, high-calorie diet. It also helped the men avoid gaining weight.