You burn most available fat when you engage in aerobic activity because you're doing it with air, where you find the oxygen. Remember what I said at the start. If you do aerobic activity, reduce cardio exercises, you will burn most of your fuel from stored body fat. During low to moderate intensity cardio, you can use 85% fat and 15% glucose. However, conditions may change during your workout as you increase or decrease intensity or speed. Aerobic metabolism and oxidative phosphorylation is usually used when heart rate is below 120 bpm. If you slowly increase your heart rate to 140, you may still have oxidative phosphorylation, depending on the condition, but use a little less oxygen and a little more glucose. It depends on whether the intensity you are doing now still consumes oxygen and to what extent. Can you breathe or are you behind? These are the signs! From one point (more than 120-130 heartbeats) it gradually switches to anaerobic metabolism. You start relying on glycolysis by breaking down carbohydrates in addition to oxidative phosphorylation. And you can downshift again or use a blend depending on the type of exercise, intensity, speed, fitness level, and whether you're recovering air.
