Fat Oxidation During Exercise

During exercise your body uses oxygen to break down carbohydrate, fat and protein to produce energy. This process is called oxidative metabolism. Fat requires more oxygen than carbohydrate when used for energy during exercise. Your body's ability to use fat as an energy source during exercise is dependent upon the intensity and duration of exercise and your cardiovascular fitness. Low-to-moderate-intensity exercise and high-intensity interval training can improve your body's ability to use fat.

Moderate-Intensity

In an article published in 2002 in "Medicine and Science in Sports and Exercise," researchers determined that maximal fat utilization occurs at approximately 65% of an individual's VO2 max. VO2 max is the maximum volume of oxygen your body is able to consume and use during exercise. Moderate-intensity whole-body exercise -- including cycling, walking or running -- triggers a high fat oxidation rate, as reported in the June 2007 issue of "The Journal of Physiology."

High-Intensity

High-intensity interval training involves high-intensity intervals interspersed with recovery intervals of low-intensity exercise. In the April 2007 issue of the "Journal of Applied Physiology," researchers found that women participating in high-intensity interval training increased their fat oxidation by 36%. In an article published in the March/April 2009 issue of "NSCA's Performance Training Journal," Gaff summarized the positive effect high intensity interval training has on increasing skeletal muscle oxidative capacity and fat oxidation after six weeks of training.

Duration

During low-intensity exercise, fat is the primary source for energy production. After about 20 minutes, your body relies more on fat than carbohydrate when exercise intensity stays in the low to moderate intensity range. If you increase your exercise intensity, your body may recruit more carbohydrate than fat to continue exercise. As your cardiovascular fitness improves, your body will have a greater ability to use fat at higher intensities.

Considerations

The use of fat as an energy source depends on fat mobilization from your body's fat stores through specific enzymes and hormones. High lactic acid and insulin levels inhibit fat use during exercise. If you are new to exercise, begin slowly with low-intensity exercise for at least 20 minutes. Gradually increase your duration and intensity to minimize lactic acid levels. If you already exercise, try adding high-intensity intervals to increase fat use during workouts.