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How Does the Body Burn Fat?

By Carolyn Robbins ; Updated August 14, 2017

Peruse a shelf at the grocery store and you'll see half a dozen items with prominent "fat-free" or "reduced fat" labels. Of all nutrients, fat is subject to the most scrutiny -- and with good reason. A high-fat diet is associated with an increased risk of heart disease and other complications. Nevertheless, fat plays an essential role in the human body. It gives you the feeling of satiety, preventing overindulgence, and is integral in the absorption of certain nutrients, including vitamins A, D, E and K.

Walking 500 Miles

Gram-for-gram, fat is an energy powerhouse. One gram of fat has 9 calories, compared with the 4 calories found in 1 gram of protein or carbohydrate. On average, the human body reserves 50,000 to 60,000 calories in fat stored as triglycerides. Walking one mile burns only 100 calories, so the energy potential available in stored fat is enormous. If you stored the same amount of energy in carbohydrates instead of fat, you would weigh 100 more pounds, according to the Gatorade Sports Science Institute

The Role of Exercise

Your body stores fat in two places -- in adipose tissue, a conglomeration of fat cells, and in muscle fibers as intramuscular triglycerides. During exercise, your body metabolizes both triglycerides and carbohydrates stored in the form of glycogen and glucose. Carbohydrate stores are more readily metabolized because they are easily mobilized and yield quick energy. As you progress from low to moderate intensity exercise, however, the rate at which your body burns or oxidizes total fat increases.

Feel the Burn

The process by which your body burns fat for energy can be summarized in a basic equation -- oxygen plus fat yields carbon dioxide, water and energy. However, the biochemistry of fat metabolism is involved and complex. For instance, fats have to be hydrolyzed or broken apart into compounds called glycerol and fatty acids. These components enter an elaborate cyclical series of reactions, including the citric acid cycle, the Krebs cycle and the tricarboxylic acid cycle, for your body to gain energy.

Fighting Evolution

While nomads benefited from the body's super efficient fat storage system, it can be problematic in the modern world. With food readily available, there is no need to store thousands of kilocalories of fat, but it's hard to fight the body's evolutionary wiring. To become a more efficient fat burner, Len Kravitz, exercise science professor at the University of New Mexico, recommends a combination of aerobic and resistance activities. Focus on aerobic workouts, which burn the most calories. Depending on your physical fitness level, that may mean short, high-intensity workouts or long, slower workouts.

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