Depending on the duration of exercise, amino acids may be counted on to generate as much as 6 to 10 percent of the fuel with the remainder split between fat and carbohydrate. The use of amino acids for energy is mostly a consideration for higher-level endurance athletes. This would include people who train seriously several times a week for extended periods such as a couple of hours. This is one reason why marathoners often look very lean but not as muscular as sprinters or milers, for example. One of the most significant reasons that more and more amino acids are used for energy is because cortisol levels in the blood are increased as the higher intensity activity is endured. Cortisol can cause the breakdown of muscle protein and the freed amino acids can be used for energy. Some amino acids will be used directly by muscle to make ATP, while others will circulate to the liver and be converted to glucose.
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