Exercise on an empty stomach may actually hurt your chances.
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If your goal is to lose weight and burn fat, it can be tempting to try anything that will give you an advantage in reaching that goal. Exercising on an empty stomach has become common, suggesting that with no recent food source available for energy, the body would have to burn fat for fuel. While this sounds like an attractive option, it does not hold true and may actually be detrimental to your fat-loss goals.
One of the biggest problems with working out on an empty stomach is a lack of energy. If you felt tired, you would probably not be able to exercise as intensely as you would under normal circumstances. As a result, you would burn fewer total calories and, consequently, fewer fat calories. Consider this: Jogging at 5 mph for 30 minutes would burn approximately 303 calories in a 160-pound person. If that same person lowered the intensity and walked for 30 minutes at 3.5 mph, he would only burn 157 calories. Being able to exercise at a higher intensity would help that person burn an additional pound per month.
Percentage of Fat Calories
While the total number of calories burned might be reduced when you exercise on an empty stomach, the relative percentage of fat calories remains the same. In other words, whether you exercise on a full stomach, after a snack, or on an empty stomach, you will still burn the same number of your exercise calories from fat. Using the same example as above, if you are burning half of your calories from fat, you would burn 151 fat calories by jogging versus only 78 fat calories by walking. Again, higher intensity wins out for burning fat.
Preserving muscle mass should always be a goal during any weight or fat loss program. Muscle helps to raise the body's metabolic rate. The more muscle you have, the more calories you burn throughout the day. When you exercise on an empty stomach, your body will take protein from your muscles to use for fuel. Even if that percentage is relatively small, it can be detrimental to losing weight and burning fat. A small snack prior to exercise can alleviate the need to use protein for energy.
While exercising on an empty stomach may sound like a viable option for burning more fat, it clearly is not. Early morning exercisers should aim to eat something prior to working out to increase energy levels and reduce the risk of utilizing protein as fuel. Listen to your body. If you find that certain foods or too much food makes you feel nauseous or "heavy" while you exercise, experiment. Aim for a small meal containing protein and complex carbohydrates. Before starting any diet or exercise program, always consult with your physician and qualified nutrition and exercise professionals.