Exercise Science

A few months ago I reported on a study that determined the level of intensity at which bodyfat was maximally oxidized, or burned, during exercise’which turned out to be 64 percent of maximum oxygen intake (VO2max). That’s a moderate level of exercise. A new study concurs with those findings; it found maximum fat burning during exercise at 63 percent of VO2max.1

The subjects of the new study were cyclists, all of whom had at least three years of training experience, which is significant because the body adapts to regular exercise through physiological changes. Examples of such changes include increased activity of oxidative enzymes that burn fat; more mitochondria in cells, where fat is actually burned; and more muscle capillaries, which deliver the oxygen needed to spark fat burning.

Trained athletes can tap into fat stores more rapidly than inexperienced athletes or recreational exercisers. The reasons have to do with the exercise-induced physiological changes, plus the improved cardiovascular efficiency that comes with regular training. In addition, new research shows what many people have long suspected’world-class athletes are genetically different. They still need to train, but their bodies are highly efficient compared to those of most people.

Physiologists have long known that once exercise reaches a certain level of intensity, the body can’t process fat fast enough and must rely on glucose circulating in the blood and stored glycogen. The new study also suggests that the limitation on fat burning under high-intensity-exercise conditions is caused by a reduction in free fatty acid availability to muscles. Researchers have proposed various explanations for that. One has to do with increased muscle acidity: As exercise intensity rises, the exercise switches from aerobic to anaerobic, with the latter relying on carbs rather than fat. A by-product of anaerobic exercise metabolism is increased acid production in muscle due to lack of sufficient oxygen. The acidity, in turn, blunts the action of oxidative enzymes that burn fat in cells.

Another possibility involves lack of carnitine activity. As most bodybuilders know, L-carnitine is an amino acid by-product that ferries fatty acids into the mitochondria, where fat is burned in a process called beta-oxidation. As exercise intensity increases, however, the available carnitine acquires an acetyl group, becoming acetyl-L-carnitine, which isn’t as effective for fat burning.

If you eat carbs during or just before a workout, a substance called malonyl coenzyme-A increases, and that inhibits the muscle-enzyme complex that works with carnitine to promote fat burning during exercise.

Besides, not everyone burns fat at the same level. While the average maximum fat-burning intensity level is at 63 percent of VO2max, some exercisers reach it at 85 percent, a relatively high level of intensity. Some of the subjects in the study burned four times more fat than others even though all were working at the same level of intensity.

What’s eaten before exercise may account for the variability in fat burning. The fewer carbs you take in before exercise, the greater the level of fat burning. (That relates to aerobic exercise, by the way, since weight training is anaerobic, relying on muscle glycogen, or carb stores.) Another factor is the ability to process oxygen during exercise. The more efficient the process is, the greater the ability to tap into fat stores. That’s the primary reason you can burn more fat with continued exercise’you develop a more efficient system of oxygen delivery to muscles.

Developing cardiovascular efficiency isn’t the same as burning maximum levels of fat during exercise. To increase cardiovascular fitness, it’s best to gradually increase exercise intensity, as in training at near-maximum pulse rate during endurance work. That trains the heart most efficiently and prods it for more oxygen.

Burning fat during training requires you to avoid slipping into anaerobic metabolism, which occurs when exercise intensity reaches a higher level. So for most people fat burning during aerobics is most efficient at a moderate exercise intensity, or about 63 percent of maximum heart rate. In cardiovascular training the range is up to 85 percent of maximum heart rate.

1 Achten, J., et al. (2003). Maximal fat oxidation in trained men. Int J Sports Med. 24:603-608.