Unlike proteins, which are used as building materials, carbohydrates are used for energy, particularly for high-intensity exercise. Sports nutritionist Dr. Michael Colgan, author of "Optimum Sports Nutrition," calls carbohydrates "premium fuel." I've never heard a better definition. Fats are also used for fuel, but the difference is that fats don't burn as efficiently as carbohydrates. It's a common misconception that fat is a more efficient fuel source, but it's not - it's simply a more concentrated fuel source (nine calories per gram for fat versus four calories per gram for carbohydrate). Carbohydrates are the body's preferred and most efficient energy source. Whenever carbohydrates are restricted, energy levels and performance usually decline.
Fat is stored in the body as a backup energy source (like a "reserve fuel tank"). A 185-pound man with 18% body fat has 116,500 calories stored in his "reserve tank." Your body can also store carbohydrates, but in much more limited quantities. Carbohydrates are stored in your muscles and liver in the form of glycogen - About 400 grams of glycogen can be stored in the muscles (1600 calories) and approximately 100 grams (400 calories) in the liver.
Your body is always burning a mixture of carbohydrate and fat for fuel. During low intensity, long duration exercise, most of your energy comes from body fat. Most of your energy also comes from fat while you are at rest (Although you don't burn many calories worth of fat when you're laying on the couch). During short bouts of high intensity exercise such as sprinting or weight lifting, glycogen (carbohydrate) is the main fuel source. Your primary fuel source also tends to change depending on which fuel is more readily available.
Your body can easily use fat for fuel and even in lean people, there's enough fat stored to last a long time. However, carbohydrates are always the limiting factor in exercise and athletic performance because carbohydrates are the more efficient fuel source. Exercise burns up muscle glycogen very quickly and if you fail to replace it every day by eating high carbohydrate foods, your glycogen stores quickly diminish. Within about three days of a severe carbohydrate cutback, your muscle glycogen will be almost totally depleted.
Glycogen is your primary energy source for weight training and high-intensity exercise, so this explains why your energy will be low and your workouts will suffer when you don't eat many carbohydrates (And if you don't have the energy to work out hard, you won't be burning much fat, will you?) Low carbohydrate diets are seldom appropriate for athletes or anyone else involved in serious training.
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