So, the percentage of fats you burn will be higher at lower levels of exertion (60% MHR) but the total number of fat calories burned will always be less working at lower heart rates than if you work at heart rates exceeding 75% MHR. For example, an average person might perform an exercise at 60% MHR and they may burn 1000 total calories, 70% of the total calories are derived fat - 700 fat calories burnt alone (the other 300 calories primarily come from carbohydrate). If this person performs the same exercise, but at a higher percentage (e.g. above 75% MHR), they will burn around 2500 calories. So at this higher heart rate the percentage of fat burned might be 40% e.g. 1000 calories of fat which is 40% of 2500 – so the total number of fat calories burned is higher even though the percentage is lower. So, the percentage of fat being used to fuel the body is irrelevant. You’re only interested in the total number of fat calories that you burn off and that amount will always be more when you train at above 75%MHR as opposed to at 60%MHR.
COACH G