Our expert says:
Glycogen is always the source of energy first, but so is fat. There is no "switch" that suddenly makes you go from one to the other, at any exercise intensity. You always use both, just at different levels depending on the exercise intensity.
Best thing is to read the following article: http://www.sportsscientists.com/2010/01/exercise-and-weight-loss-part-3-fat.html
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