Good question! Many dieters think that they’ll burn more calories if they work out on an empty stomach. But the reality is, eating or not eating before exercising doesn’t affect the number of calories you burn -- but it may decide which kind of calories you burn, says Roger E. Adams, PhD, CISSN. Aerobic exercise (read: cardio) burns glucose (blood sugar) first and fat second, Adams says. Translation: Your body will work to burn off your preworkout snack before any fat on your body will be used to fuel your exercise. So if you go for a run on an empty stomach, your total calorie expenditure will be the same as if you had a snack before, but your percentage of calories burned from fat may increase.
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But working out on an empty stomach won’t whittle away love handles. That’s because the fat calories burned during workouts are from the muscles, not from fat cells, so you won’t see a difference in your overall body fat. And if you’re doing anaerobic exercises, like resistance training, your body relies heavily on glucose (think: carbs) to fuel those types of activities, Adams says. Another reason to grab a snack before you hit the gym: If you’re running on empty, you won’t work as hard or burn as many calories during your workout. To get the most out of your sweat sessions, fuel your workouts with a 400- to- 800-calorie mix of protein and carbs (like whole-grain cereal with 2 percent or soy milk or a banana with peanut butter) at least two to four hours before you hit the gym.
See More: Getting in Shape