Low-intensity exercise is often promoted in “fat-burner” aerobics classes as a way to maximize fat metabolism. Such claims are misleading. It’s true that fat burning provides a higher proportion of the calories used in low-intensity workouts (about 50 percent) than in intense workouts (40 percent). But since you burn far more calories during intense exercise, you end up burning more fat.
So if you exercise for 30 minutes at low intensity and burn 200 calories, about 100 of those come from fat. If you exercise for 30 minutes at twice the intensity, you may burn 400 calories, with 160 of them coming from fat. Thus, you still burn more fat when working out intensely. In addition, in the hours after a high-intensity workout, your metabolic rate stays slightly elevated, so you continue to burn a few extra calories.
If you work out twice as long at low intensity as at high intensity, you’ll burn more fat but about the same number of calories. To lose a pound, you have to burn, on average, 3,500 more calories than is needed to maintain your current weight.
Lower-intensity exercise does have some advantages. People are often more willing and able to do it and stick to it. There’s also less chance of injury. And lower-intensity exercise can promote weight loss—you just have to do it longer. But it’s not a better way to lose weight than more intense workouts.