It has become common to hear the phrase “fat burning zone” tossed around in fitness centers. This phrase refers to a specific zone in reference to heart rate. The idea is that if a person exercises at an intensity that causes their heart rate to fall into this specific zone, they will burn fat faster. This zone is usually achieved with low intensity exercise. That’s right; people believe that if they exercise at a lower intensity they will lose fat faster than they would if they exercised at a higher intensity. It is true that at lower intensities of exercise, the human body catabolizes or burns fats as the primary source of fuel. At higher intensities, the body uses carbohydrates as the main energy source. Thus, it is true that lower intensity exercise immediately burns more fat than higher intensity exercise. (Karp)
Here is the catch. When the body burns fat during exercise, it is not burning the fat around your waistline. It is burning small amounts of stored fats in skeletal muscle. So when people say that low intensity exercise burns fat faster, they are technically right, but its not the fat you are trying to burn. Here is another catch; calories. Weight loss always boils down to expending more calories than you consume. When you exercise at a lower intensity for a given amount of time you will always burn fewer calories than you would if you were exercising at a higher intensity. On top of that, high intensity exercise increases resting metabolic rate more so than low intensity exercise. This means that in the hours after a workout, your metabolism is elevated, and burning more calories than it normally would at rest. It is clear that exercising at a higher intensity is more beneficial than exercising at a lower intensity. (Karp)
Karp, J. The fat burning zone: Myths, facts, and suggestions for burning fat faster and more efficiently. IDEA Fitness Journal, 23-25.