The 'Fat-Burning Zone': A Fitness Myth Debunked

There's a lot of misinformation out there about exercise and nutrition. As an ongoing feature, I'll ask experts in those fields about their pet fitness peeves—commonly believed myths that are just plain wrong. This week, I asked Cedric Bryant, chief science officer of the American Council on Exercise, for the facts about the so-called fat-burning zone.

Myth: I will lose body fat more efficiently by working out in the fat-burning zone—doing my aerobic workouts at a low, rather than high, intensity.

Explanation: Many aerobic exercise programs and videos feature low-intensity workouts purporting to maximize fat burning. The argument is that low-intensity aerobic training will allow your body to use more fat as an energy source, thereby accelerating the loss of body fat. While it is true that a higher proportion of calories burned during low-intensity exercise come from fat (about 60 percent as opposed to approximately 35 percent from high-intensity programs), high-intensity exercise still burns more calories from fat in the final analysis.

For example, if you perform 30 minutes of low-intensity aerobic exercise (i.e., at a level of 50 percent of maximal exercise capacity), you'll burn approximately 200 calories. About 120 of those, or 60 percent, come from fat. However, exercising for the same amount of time at a high intensity (i.e., 75 percent of your maximal exercise capacity) will burn approximately 400 calories, and 35 percent of them, or 140 calories, will come from stored fat. So by sticking to the fat-burning zone for their workouts, many individuals are wasting valuable time. Keep in mind that you lose weight and body fat when you expend more calories than you consume, not because you burn fat (or anything else) when you exercise.

Of course, the less intense form of exercise has its benefits as well. For example, because many overweight people tend to find that lower-intensity exercise is more comfortable, they may, therefore, be willing to engage in such workouts. The point to remember is that low-intensity workouts do, in fact, promote weight and fat loss. You just have to do them for a longer period of time.