This is a great question that is often misunderstood. The body actually has the ability to burn protein, carbohydrates as well as fat for energy. However, we don’t exclusively burn one source of energy over the other (Mahan, 2008). Rather we tap into a combination of sources at varying percentages depending on the activity and the length of our training session.
Generally speaking, the lower the intensity of exercise, the greater likelihood that your body is using more fat as a primary energy source. In fact, as you read this post, you are likely burning greater amounts of fat over other energy sources. Conversely, higher intensity exercises use more glucose, or its stored form (glycogen), from the metabolism of carbohydrates, protein, and fat as the main fuel source. The utilization of one fuel over another has to do with the speed at which the body can break down and use each substance for energy.
Glucose is readily available for rapid mobilization making it a preferred fuel for higher intensity and short burst activities. Fat on the other hand, takes additional time to break down, so the body utilizes a greater proportion of it for low-moderate intensity activities. Fat is also utilized at a higher rate during longer training sessions as the body attempts to conserve glucose fuel.
But it’s important to keep in mind that the rate at which we burn fat for energy doesn’t translate to decreases in actual body fat percentage or weight for that matter; as metabolic processes simply replace energy storage in preparation for your next workout. Rather changes in body weight are directly related to the total calories burned regardless if they came from fat or glucose. Weight loss occurs when we expend more calories than we consume and when we combine that with an optimal exercise program that incorporates elements of cardiovascular, resistance, and flexibility training, it’s a winning combination for an overall leaner body.
Mahan, L. K. (2008) Krauses’s Food and Nutrition Therapy 12th ed., St. Louis, Missouri: Elsevier.