The time of day you choose to exercise and whether or not you have eaten a meal before your session affects fat burning during exercise. Studies have shown that exercising following an overnight fast stimulates significantly greater fat burning compared with exercising soon after a meal.
One such study also investigated the effect of exercise on glucose intolerance and insulin resistance in the fasted and fed states. It found that fasted training was more effective than fed training for improving glucose tolerance and insulin resistance: fasted subjects displayed greater insulin sensitivity levels than fed subjects did. The fasted subjects also significantly increased fat burning, which resulted in a much smaller weight increase in the fasted group. Thus, even though they consumed a high-calorie diet, fasted subjects gained much less weight after the six-week program. This and other studies have shown that the body burns up much more fat during and after exercise if no food is eaten beforehand. This means the best time of day to exercise in order to lose belly fat is before breakfast or your first meal.