The major macronutrients in the food you eat are carbohydrates, fats and proteins. Each of these provides energy for skeletal muscle function, but carbohydrates and fats provide most of your energy; protein is just a minor contributor. According to exercise physiologists and authors Jack Wilmore, David Costill and Larry Kenney, the macronutrients consist of carbon, hydrogen and oxygen atoms, and proteins also have nitrogen atoms. Inside your body, these atoms break apart and release energy which is converted to adenosine triphosphate (ATP). Your muscles then use ATP as fuel to energize their movements. Carbohydrates, fats and proteins are each processed differently to form ATP.
Carbohydrates
Although carbohydrates constantly fuel your muscles, they are especially important when you perform high-intensity physical activity or exercise, according to exercise nutritionists William McArdle, Frank Katch and Victor Katch. While you rest or perform light physical activity, most of the carbohydrates in your body are stored in your muscles and liver as glycogen, a sugar molecule. Then, when you become more active and your muscles need more energy, they quickly can convert glycogen into glucose, a simple sugar, through a process called glycogenolysis. Glucose, then, must go through glycolysis, a separate process, to eventually form ATP.
Fats
The fat stores in your body are much larger than the carbohydrate stores. In fact, according to Wilmore, Costill and Kenney, fats in your body can provide about 27 times the energy of carbohydrates. However, your body cannot convert fats into energy as quickly as carbohydrates. Therefore, fats can only provide significant energy while you rest or perform light- to moderate-intensity physical activity or exercise; when your muscles do not need energy at a fast rate. The reason fats cannot be converted to ATP quickly is because they must go through a process called lipolysis, according to Wilmore, Costill and Kenney. During this process fats are broken down from their original form, triglycerides, into glycerol and free fatty acids (FFAs). The FFAs then enter your blood stream where they are carried to the muscles that need them. Once they enter your muscle fibers, they are broken down further, eventually producing ATP.
Proteins
Proteins are rarely used for energy. They are only used significantly if you exercise for a very long time or are truly starving, according to Wilmore, Costill and Kenney. If needed, proteins can be converted into FFAs through a process called lipogenesis so the FFAs can be converted into ATP.
References
- "Physiology of Sport and Exercise (Fourth Edition)"; Jack H. Wilmore, David L. Costill and W. Larry Kenney; 2008
- "Sports and Exercise Nutrition (Third Edition)"; William D. McArdle, Frank I. Katch and Victor L. Katch; 2009
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