In a groundbreaking 2010 study , researchers in Belgium persuaded young, healthy men to stuff themselves for six weeks with a diet consisting of 30 percent more calories and 50 percent more fat than the men had been eating. Some of the volunteers remained sedentary while gorging. Others began a strenuous, midmorning exercise routine after they had had breakfast. The third group followed the same workout regimen, but before they had eaten anything.
At the end of the six weeks, the sedentary group predictably was supersized and unhealthy, having gained about six pounds each. They had also developed insulin resistance and larded their muscles with new fat cells. The men who exercised after breakfast had also packed on pounds, about three pounds each, and developed insulin problems. But the men who had exercised first thing in the morning, before eating anything, had gained almost no weight and retained healthy insulin levels. Their bodies were also burning more fat throughout the day than were the other men.
Of course, the early-morning exercise prevented weight gain, which is not the same thing as inducing weight loss. But the results are encouraging for those who hope to shave off a few pounds, said Peter Hespel, a professor in the Research Center for Exercise and Health at Catholic University Leuven in Belgium and the study author.
“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 if you are cheating on the healthy and well-balanced diet part, “we demonstrated,” he said, “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.
