At the most basic level, workouts need energy and as you are working out, the body is looking for that energy in the form of ATP. Carbs, Fats & Proteins are the body’s fuel and are converted to ATP when the body needs this energy.
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Now, ATP is created either aerobically or anaerobically, depending on the workout. In aerobic workouts (think endurance running, long distance cycling etc), the body is able to create large amounts of ATP via oxygen reacting with fats and glucose and keep producing it for extended periods of time. As long as there is a continual supply of fuel (eg. fats and carbohydrates stored in the body) and oxygen, aerobic activities can continue for long periods. Eventually, however, your body can’t absorb oxygen at the same rate and it flips into the anaerobic energy system. The lack of oxygen activates specific enzymes that drive ATP breakdown products into IMP, and further into hypoxanthine, xanthine and uric acid. Hypoxanthine is the last step before ATP degradation reaches the "point-of-no return."
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In anaerobic workouts (sprinting, baseball, soccer, weightlifting etc), the body requires a large amount of energy quickly and will not have sufficient oxygen to create a large amount of ATP. It will require glucose to access ATP. The anaerobic processes cannot continue indefinitely as the stores of ATP become depleted Again, The lack of oxygen activates specific enzymes that drive ATP breakdown products into IMP, and further into hypoxanthine, xanthine and uric acid. Hypoxanthine is the last step before ATP degradation reaches the "point-of-no return."
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