Fat Burning: Low-Intensity Or High-Intensity Exercise?

An oft-quoted 2010 study suggested that low-intensity exercise is more effective for weight loss than high-intensity exercise. In this study, conducted by researchers at the University of Udine, Italy, 20 obese adolescents were placed on a diet and exercise program for three weeks. Half of the subjects engaged in a low-intensity exercise program (at 40 percent VO2 max) while the other half engaged in a high-intensity exercise program (at 70 percent VO2 max). After three weeks the changes in weight and fat mass were averaged within the two groups and compared between them.

And guess what? It was discovered that members of the low-intensity exercise group lost 37 percent more weight and almost twice as much fat. The authors of the study believe this happened because the subjects burned a lot more body fat during low-intensity exercise than during high-intensity exercise, in which they burned mostly carbohydrates.

So there you have it: low-intensity exercise is more effective for weight loss than high-intensity exercise.

Not so fast. Past research has shown that the body’s fat-burning mechanisms do not function as well in obese individuals as in normal-weight individuals, while the fat-burning mechanisms in aerobically fit, normal-weight individuals function exceptionally well. Consequently, runners are able to burn a lot more fat at somewhat higher exercise intensities. The 40 percent VO2 max intensity at which members of the low-intensity group exercised in this study is nothing more than a brisk walk for the trained runner. We need to go harder than that — somewhere around 55 to 60 percent VO2 max, or a steady jog—to maximize fat burning.

What’s more, even the 70 percent VO2 max intensity at which members of the high-intensity group exercised in this study is not particularly high. There are many runners who can run entire marathons at this intensity and above. Research has shown that normal-weight and especially fit individuals burn a lot more fat after truly high-intensity exercise, such as interval sessions at 100 percent VO2 max and above, than after low- or moderate-intensity workouts.

In fact, one recent study reported significantly greater total fat loss in a group of individuals placed on a very high-intensity interval training program than in a group of individuals who performed much longer workouts at moderate intensity, even though both groups burned equal calories during the workouts themselves. The 70 percent VO2 max “high” intensity used in the present study simply wasn’t high enough, as the researchers who conducted it found no significant difference in the rates of post-exercise fat burning between the two groups.

What they did find, however, was that VO2 max improved in the high-intensity exercise group, whereas it did not change in the low-intensity group. In other words, members of the high-intensity group got fitter and members of the low-intensity group did not, despite losing more weight. This is significant because fitness has an important bearing on weight loss. The fitter you become, the faster you go at any given intensity, and the faster you go, the more calories you burn per minute at any given intensity.

For example, suppose you start a running program in which you always run at 70 percent VO2 max, and initially this corresponds to a pace of 10:00 per mile for you. After several weeks, your fitness will have improved so much that you are able to run, say, 9:30 miles at the same 70 percent VO2 max intensity. And after several more weeks, you may find yourself running 9-minute miles at the same intensity. Assuming your total time commitment to running doesn’t change, you’ll be burning far more calories in your training at this point than you were when you started.

Thus, I believe that if the study under discussion had been, say, 12 weeks long instead of three weeks, and if pace had been adjusted continuously throughout the program to account for fitness improvements in the high-intensity group, the final results would have been reversed: that is, the high-intensity group would have lost more weight than the low-intensity group.

So there you have it: high-intensity exercise is more effective than low-intensity exercise for fat loss.

Not so fast. The most effective way to exercise for fat loss is to create a program that combines high-intensity and low-intensity exercise. That’s because each type of exercise promotes fat loss in different and complementary ways. Also, while high-intensity exercise is more effective for fat loss than low-intensity exercise on a minute-per-minute basis, low-intensity exercise has the greater overall capacity to produce weight loss because it’s possible to do so much more of it.

For example, you might only be able to handle five hours of exercise per week on an all-high-intensity exercise program, whereas you could perhaps handle three times that amount on an all-low-intensity program, and you’d surely lose more weight on the latter plan.

High-intensity and low-intensity exercise are like bagels and cream cheese: so much better together than either is alone.