Fat-burning tip: Exercise will burn fat stores at any time of day or night. There are several good reasons to exercise early in the morning, if you can manage it. You’ll get it done before other things have a chance to get in the way of your workout. And for many people, an a.m. fitness routine offers a psychological boost that can make the rest of the day much easier.
Fat-burning claim: Exercising on an empty stomach will burn fat as fuel. Perpetrators of this theory refer to the body's insulin response, which goes like this:
Fat-burning fact: It's true that insulin inhibits release of fat from fat cells, but not true that exercising on an empty stomach makes you burn more fat overall. Anytime you are burning more fat during a workout, you are almost always burning fewer total calories, which basically defeats the purpose if your goals are weight loss and improved fitness. What really determines whether your body will turn to stored fat or carbohydrates (glucose) for fuel? The intensity and duration of your exercise. In a nutshell, you’ll burn more fat and more total calories when you exercise at high intensity levels. When you exercise at 70% to 90% of your max heart rate, your body can’t turn stored fat into fuel fast enough to meet the demands of your active muscles. But the more intense your exercise, the more total calories you'll burn during your workout and during the recovery phase afterward. When it comes to weight loss, total calorie expenditure is more important than where the calories are coming from (glucose or stored fat).
Fat-burning tip: Real fat-burning takes place during low-intensity, daily activities—not during exercise itself. The more calories (glycogen) you burn during exercise and recovery, the more your body will rely on fat stores to fuel the rest of your day. So instead of trying to figure out whether (or what) to eat before you exercise, choose whatever will help you put the most effort and time into your workout session. Everyone is different. Don't eat if it upsets your stomach to exercise soon afterward, but if you feel lightheaded or fatigued when exercising on an empty stomach, then you should eat a pre-workout snack or meal.