In a groundbreaking study, Belgian researchers tracked weight loss in young, healthy men who engaged in exercise at various times of the day while following a diet with 30 percent more calories and 50 percent more fat than usual.
 

One group of participants underwent a strenuous, midmorning exercise routine after eating breakfast. A second group followed the same workout regimen, but before they had eaten anything.
 

After six weeks, the men who exercised after breakfast packed on additional pounds, about three pounds each, and developed insulin problems. But the men who had exercised first thing in the morning, before eating anything, gained almost no weight and retained healthy insulin levels. Their bodies also burned more fat throughout the day than were the other men.
 

The early-morning exercise prevented weight gain, which is not the same thing as inducing weight loss. But the results suggest pre-breakfast workouts are the best, said lead researcher Peter Hespel, a professor in the Research Center for Exercise and Health at Catholic University Leuven in Belgium.
 

“The optimal strategy to prevent increases in body weight is obviously to combine a healthy, well-balanced diet with a physically active lifestyle,” he said. But “we demonstrated that early-morning exercise in the fasted state is more potent than an identical amount of exercise in the fed state” for maintaining healthy waistlines.