There's a controversy running around the internet. Inspired by several publications in reputable journals, various websites appear to suggest that you can lose more weight by running on an empty stomach. The theory goes that when you run in the morning after fasting all night, you will have depleted your carbohydrate stores, and your muscles burn fat instead. Indeed, research substantiates this. For example, a study published in the January 2011 "Journal of Applied Physiology" found that training with limited carbohydrate availability may stimulate adaptations in muscle cells so they more efficiently produce energy by burning fat during exercise. These results, however, do not suggest that you lose weight more quickly, and several experts advise that those interested in weight loss are better served by running with some gas in the tank, so to speak.
Fueling Around
When you exert your muscles, they burn a blend of fuels. Initially, for short bursts of activity, your muscle cells burn a molecule called ATP. Your muscles constantly synthesize new supplies of ATP, using carbohydrates, fatty acids and protein. When you briefly exert muscles, they primarily use two types of carbohydrates to produce ATP: glucose in your blood and glycogen stores in your muscles and liver. As you continue to exercise for longer periods, your muscles tap, to a greater extent, fat stores. If protein is available in your bloodstream, muscles will use protein, and during endurance exercise, you tap fat stores more. However, as long as carbs are available, your muscles prefer to burn them because carbs use oxygen more efficiently. When other fuels are less available, your muscles also can tap protein from lean muscle for fuel.
A Calorie is a Calorie is a Calorie
Although the fuel blends differ, depending on the intensity of the exercise and the availability of oxygen and carbohydrates, the amount of calories expended is determined chiefly by the duration, type and intensity of the exercise, not by the type of fuel burned. You must burn 3,500 calories to lose 1 lb. of weight. How much weight you lose is determined by how much you burn, not what you burn. If you burn 900 calories running for an hour, you've burned the equivalent of almost 1/4 pound. It's the same weight, no matter if it's protein, carbs or fat. By the age of 6, you knew that a pound of feathers weighs the same as a pound of lead. The same principle tells you a pound of carbs weighs the same as a pound of fat, which weighs the same as a pound of protein.
The Confusion
The confusion probably began when a few excitable bloggers read studies that described how exercising when glycogen stores are depleted forces the muscles to burn fat for fuel during endurance exercise. And further, doing this on a routine basis enhances muscles' capacity to burn fat and store glycogen. Burning more fat sounds as if it would help with weight loss. However, the actual research results don't focus on weight loss, but rather focus on enhancement of performance. Muscles adapt to repeated carbohydrate-deprivation during exercise so they store carbs better and burn fats more efficiently during exertion. These results bear on energy use during athletic performance, not on weight loss. Research has not generally examined weight loss.
Food for Thought
Some health and exercise experts, such as Len Kravitz from the University of New Mexico, suggest that you may deprive your brain of energy by running on an empty stomach. You're more vulnerable to injury. And lacking energy, you may not perform at the same intensity, therefore burning fewer calories. The bottom line is, you might improve your fat metabolizing capacity during endurance exercise, but the effect on your weight-loss is likely a wash.
References
- "Journal of Applied Physiology"; Beneficial Metabolic Adaptations Due to Enduracne Exercise Training in the Fasted State; Karen Van Proeyen, et al.; January 2011
- MSNBC.com; To Get Rid of Fat, Don't Eat Before Workout; Maria Cheng; June 2010
- TVNZ.co: Scientist Says to Miss Breakfast Before Gym; May 25, 2010
- University of New Mexico; Calorie Burning: It's Time to Think "Outside the Box"; Len Kravitz,
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