When I go on an exercise machine at the gym, one of the first things I’ll notice is the chart which shows age against your heart rate. A band called the fat burning zone runs along the chart at around 65-70%. Frankly, I have always had an issue with the fat burning zone. I mean, how can you possibly burn body fat by working less intensely? It just doesn’t make sense.
The reasoning behind the fat burning zone is the source of energy your body turns to when exercising. When you are working at a high intensity, your body requires more oxygen than you can breathe in.
And due to the relative lack of oxygen, your body is unable to use fat as a source of energy so it must use carbohydrate stores to provide your body with the energy it requires.
At lower intensity, the fat burning zone, your body relies more on fat stores for energy. You breathe more easily and there is enough oxygen consumed to use your fat stores for energy. But at higher intensity, your body is fed by your carbohydrate stores.
Essentially, what this is saying is that energy (calories) supplied from fat are better than energy supplied from carbohydrates. This is simply not the case. A calorie is a unit of energy, a measured quantity. All calories are equal, and therefore a calorie from fat is the same as a calorie from carbohydrates.
Weight loss is achieved by consuming fewer calories than your body needs. When your body has this calorie deficit, it will burn body fat to make-up the difference. Burning body fat to maintain the energy balance is what causes you to lose weight.
In terms of an Energy (or calorie) balance, the source of the energy is not important, but the relative amounts of calories expanded are. If you have a finite amount of time, you would achieve greater success working at a higher intensity. You could still achieve weight loss with relatively lower intensity exercise, however it would take much longer to burn the same amount of calories compared to high intensity exercise. If you have limited time to exercise, burn body fat by ditching the fat burning zone in favor of high intensity workouts.