In the study, researchers convinced healthy, young men to gorge themselves for six weeks on a diet consisting of 20 percent more calories and 50 percent more fat than what they were eating before. Some volunteers were asked to remain sedentary; another portion began a strenuous, midmorning exercise routine after eating breakfast; and the third group performed the same workout regimen, only they completed the exercise before eating in the morning.
In the end, the sedentary group surprised no one and became ‘supersized and unhealthy,’ having gained about six pounds each, in addition to developing insulin resistance. The men who exercised after breakfast also developed insulin problems, but only packed on about three pounds each. And the men who exercised first thing in the morning on an empty stomach gained almost no weight and retained healthy insulin levels. They were also burning more fat throughout the day.
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,' Peter Hespel, a professor in the Research Center for Exercise and Health at Catholic University Leuven in Belgium and the study author told the New York Times.