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Question: “At what point do you start to burn muscle rather than fat when working out? Is there a specific time frame that should be followed?”
Answer: This question applies particularly to cardiovascular exercise. As a rule, the body typically uses glycogen as the main source of energy in the first twenty minutes of cardiovascular exercise. So to get in the fat-burning zone, one would want to exercise for a period that extends well beyond twenty minutes. There are, however, additional guidelines to follow. In general, the body most effectively used fat as an energy source when the heart rate is between 50-65% of maximum - it will also use glycogen as an energy source in this heart rate range. When exercise is performed above this target heart rate, muscle (or more appropriately, protein) and fat are used in conjunction with glycogen as the primary energy sources. Of course, there are exceptions to these guidelines. For example, if a person already has an extremely low level of body-fat, then this person's body would likely use protein as an energy source, even at lower heart rates. Ensuring that body protein stores are filled (through a high-protein diet) can aid in ensuring the muscle tissue of the body does not atrophy during exercise at high heart rates.
If we are talking about resistance training, one would want to make sure that body protein stores are high by way of a high-protein diet. One would also want to make sure that glycogen stores are up before a workout and are replenished immediately after a workout, so that the body does not use protein to fuel energy demands.