Glycogen stores in the muscles and liver decrease in the night as you sleep. Conventional wisdom would suggest that exercising on an empty stomach would promote fat burning and weight loss. A study from the University of Padua in Italy found the opposite: physical activity after a light meal was best for losing body fat. Researchers studied fat metabolism in eight college-aged males who ran on a treadmill at 65 percent of maximum heart rate for 36 minutes on two different occasions— either fasted or after a light breakfast. Metabolism was higher in the fed condition during and 12 hours after exercise. Fat use was also higher after exercise and the fed condition. Consume a light meal about 30 to 60 minutes before exercise if your goal is to lose weight.
Source: International Journal Sports Nutrition Exercise Metabolism, 21: 48-54