During prolonged aerobic (cardiovascular) exercise, muscle protein is broken down and amino acids, mostly alanine and glutamine are released into the blood. Alanine is one of the principal amino acids used to make glucose in the liver and the new glucose can help maintain blood glucose levels and fuel muscle during long aerobic exercise bouts. This process is driven by primarily by cortisol as well as epinephrine, both of which are elevated in circulation during exercise. Cortisol promotes muscle protein breakdown during the exercise while epinephrine promotes the conversion of amino acids to glucose in the liver. Since cortisol is a stress-related hormone, the degree to which this happen depends on how hard you exercising and for how long. Thus for shorter, less intense exercise sessions (for example, walking and casual bicycling) this isn't a consideration; however for recreational and competitive endurance athletes and heavyweight trainers it is. We will explore this further in Chapter 11.
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