Fat Oxidation During Exercise

During exercise, your body breaks down fat, carbohydrates and protein to fuel your body. Of the three, fat delivers the most energy and is your body's preferred source of fuel during prolonged exercise. Fat is stored in fat cells between and within muscle fibers. Oxygen is required to break down this fat so that your body can utilize it for fuel during exercise. The amount of fat oxidation that occurs during your exercise session is primarily dependent upon the intensity and duration of your workout.

  1. Low to Moderate-Intensity Exercise

    • During low- to moderate-intensity exercise, fat oxidation is the primary source of energy for your body. The low work demand on your muscles gives your body ample time to bring oxygen in, break down fat and produce energy. An article published in "The Physician and Sportsmedicine" journal notes that fat oxidation is highest when exercising at 65 percent of your VO2 max, which is the maximal amount of oxygen your body can take in during physical exertion. So exercising at just more than half of your maximum capacity will burn the most fat for energy.

    High-Intensity Exercise

    • As the intensity of exercise increases, energy sources gradually shift from fat oxidation to carbohydrate oxidation primarily due to the fact that the breakdown of carbs is easy, quick and doesn't require oxygen. During high-intensity exercise, nearly 100 percent of the energy is from carbohydrates. If the exercise session is prolonged, even at high intensities, your body gradually shifts back to fat oxidation for energy.

    Duration of Exercise

    • The duration of your exercise session is the second factor in how much fat oxidation fuels your workout. The longer you exercise, the more your body must transition into utilizing fat oxidation for energy. As your exercise session progresses, carbohydrate stores become depleted and your body must rely more and more upon your abundant supplies of fat. Generally, once you pass the 20-minute mark, the transition into higher levels of fat oxidation begin.

    Encouraging Fat Oxidation

    • As your fitness improves, your body becomes more efficient with its energy stores and its use of fat oxidation. If you don't feel like your body is progressing as much as you'd like, you can nudge it along with interval training. Interval training incorporates periods of high-intensity exercise with rest periods. This allows you to train at high-intensities for much longer stretches and ultimately to train your body to run off of fat oxidation at higher work capacity than it otherwise would. This can be very beneficial when training for long distance events such as marathons and triathlons.