The notion behind fat-burning zones is that the harder you work, the more energy you burn, but a greater proportion of that energy is carbohydrate (glycogen). When sitting quietly, you burn only fat but not much. When you walk, jog, run, and then sprint, you burn more energy, but with greater intensity a higher percentage is glycogen, so at maximum capacity you burn only glycogen. The fat-burning zone is thus a low-to-moderate level of activity that burns about as much fat as glycogen, but it’s sort of a fiction because there is a wide range of variation between individuals of moderate
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