So what you need to remember is that, for anaerobic pathway, pre-stored ATP and glycogen are the only sources of fuel. For the aerobic pathway, all three — carbs, fats and proteins — can be used as fuel. Although aerobically fat appears to be the most important source of fuel, the fact that carbs are more efficient (need fewer oxygen molecules for oxidation) makes them more important as the starting fuel, with fat taking over later.
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