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question

Will doing cardio on an empty stomach maximize fat burning?

Neal Spruce
Neal Spruce on behalf of dotFIT
Fitness
answer

First off, it's important to note that at least 8 to12 hours may pass between dinner or an evening snack until waking. This means energy levels are at their lowest upon waking or after a long break between meals. Secondly, exercise itself does not burn a great amount of fat no matter how long the activity. It is the contribution of exercise to a person’s total daily energy expenditure (TDEE), including the intensity, that affects fat loss. In other words, exercise simply adds to your daily calorie burn, and as long as you don’t consume more to compensate (keeping your intake below your burn) the body must draw on its fat stores and you’ll lose fat throughout the entire day. So if you properly eat before exercise, you have the potential to perform better, enhance recovery and burn more calories, including fat calories. The higher the intensity of your workout (which you can now perform thanks to having filled your energy stores with a pre-workout snack), the more calories from fat you will use throughout the day. The energy or calorie deficit, not the workout or when you eat, determines how much weight/fat you lose. We’re not suggesting you add calories to your daily intake. Simply adjust the way you distribute your total allowed calories (based on your goal) throughout the day. Spacing meals properly has added benefits, such as using more calories to digest each meal, a steady stream of nutrition (enhancing recovery and energy) as well as controlling hunger.

Dr. Michael Roizen, MD
Dr. Michael Roizen, MD
Internal Medicine
answer
It will maximize fat burning in your muscles, but not all over your body.  When your body doesn’t have a ready energy store, it turns to the fat and sugars in your muscles.  When that’s used up, the next thing to go is your muscle, which is not good.  However, if the early morning is the time you have allotted to exercise, and love to exercise, get up a little earlier, make sure you drink two glasses of water, and have a light meal that contains both carbs and protein.  Liquids, like a fresh fruit smoothie with non-fat yogurt, will be less likely to cause cramps during your work out. But any exercise is better than none and I like exercising before breakfast.
Mike Fantigrassi
Mike Fantigrassi on behalf of National Academy of Sports Medicine
Fitness
answer

You will burn a higher percentage of fat by exercising on an empty stomach, but only if using a low intensity exercise (walking, light cardio).  However, by exercising at a low intensity you will not burn many calories, which is ultimately what causes weight loss.

As exercise intensity increases (running, weight training) you use more and more carbohydrates as a fuel source.  If exercising at a higher intensity your body will turn to proteins (muscle) as a fuel source if you are running low on carbohydrates, which will happen if exercising for an extended period on an empty stomach.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.