The Benefits of Exercising Before Breakfast - The New York Times
At the end, the nonexercising group was, to no one’s surprise, super-sized, having packed on an average of more than six pounds. They had also developed insulin resistance — their muscles were no longer responding well to insulin and weren’t pulling sugar (or, more technically, glucose) out of the bloodstream efficiently — and they had begun storing extra fat within and between their muscle cells. Both insulin resistance and fat-marbled muscles are metabolically unhealthy conditions that can be precursors of diabetes.
The men who ate breakfast before exercising gained weight, too, although only about half as much as the control group. Like those sedentary big eaters, however, they had become more insulin-resistant and were storing a greater amount of fat in their muscles.
Only the group that exercised before breakfast gained almost no weight and showed no signs of insulin resistance. They also burned the fat they were taking in more efficiently. “Our current data,” the study’s authors wrote, “indicate that exercise training in the fasted state is more effective than exercise in the carbohydrate-fed state to stimulate glucose tolerance despite a hypercaloric high-fat diet.”