The Utilization of Fat & Carbs During Exercise

Fats and carbohydrates, or carbs, are the primary fuels for your muscles when you're working out. Fats are metabolized more slowly than carbs and are the preferred energy source during long-duration, low-intensity exercise, such as walking. As exercise intensity increases, muscles need energy more quickly and the fuel mix shifts toward faster-burning carbohydrates.

During the first 10 seconds of exercise, your muscles draw on immediately accessible stored energy in the form of two compounds: ATP, called the energy currency of the cells, and creatine phosphate. Muscle cells store very little of these compounds. For you to continue exercising, they must quickly start metabolizing carbs and fats to replenish ATP stores. Because carbs are metabolized more quickly than fats, they provide most of the energy at the beginning of any exercise session.

Within a few minutes of starting to exercise, your muscles return to burning fats as well as carbs. The intensity of the exercise determines which fuel predominates. Fat metabolism is slow, but because fat contains much more energy per gram than carbohydrate, it's also very efficient. During low-intensity exercise, such as walking, your muscles burn mostly fat. Since your body stores much more fat than carbohydrate, fat also provides most of the energy for long-duration events, such as distance running.

When you exercise at a higher intensity, such as during interval training, your muscles need energy more quickly. Because carbs are metabolized faster than fats, they form a larger portion of the fuel mix. Carbohydrate metabolism can take place anaerobically -- without oxygen. Fats, however, can only be metabolized using oxygen. During very high-intensity exercise, when your circulatory system cannot provide oxygen to your muscles fast enough for aerobic metabolism to fully meet their energy needs, carbs become even more important as a fuel source.

If your goal is to burn fat while exercising, you might think you should stay with low-intensity exercise to maximize fat metabolism. However, you will expend more energy in total if you exercise at a higher intensity. Although the percentage of the calories that come from fat will be lower, you will burn more total fat than you would if you stayed with lower-intensity exercise because the total calorie expenditure will be higher.