It seems that the prevailing notion in the fitness industry is that slower is better when it comes to weight loss, or “fat burning.” The misconception is that low-intensity exercise is more beneficial for reducing body weight and specifically body fat, since a higher percentage of fat is being burned for fuel. While this may be an attractive hypothesis, there is no documented evidence to support the contention that the type of fuel (carbohydrate or fat) burned during exercise has any preferential impact on the rate or amount of body fat loss.

Let’s look at the facts. Your body utilizes primarily carbohydrates (CHO) and fats for fuel during exercise. Proteins play a negligible role, except during exhaustive exercise bouts. The intensity of exercise and type of fuel being burned is reflected in what is called the respiratory exchange ratio (RER), which varies between .70 and 1.0. The lower the intensity of exercise, the lower the RER value and the greater the percentage of fat being utilized (see Table 1). Conversely, the greater the intensity, the higher the RER and the greater the percentage of CHO being utilized. At low exercise intensities, the body doesn’t need to be very efficient and, therefore, can take the time to mobilize free fatty acids from the adipose tissue to be burned as fuel. At higher intensities, the body must create energy very quickly and efficiently. Stored carbohydrates (sugars) are better adapted to meet this need. While the concept of keeping exercise intensity low in order to mobilize and selectively burn a higher percentage of fat may sound logical, the concept does not hold up mathematically and, more importantly, has never been verified in the laboratory.

Data collected in the laboratory should help to illustrate this point. In a study conducted at the University of Wisconsin-La Crosse, a group of subjects walked on a treadmill for 30 minutes at a self-selected, comfortable walking pace. Subjects walked at an average of 3.8 mph (16 minutes/mile pace), burned approximately 8 kcal/min, and their RER value was .88. From Table 1, we can see that they were expending 59 percent of their calories from carbohydrates and 41 percent of their calories from fat. When asked to run for 30 minutes at a comfortable pace, subjects ran at an average of 6.5 mph (nine minutes/mile pace) and expended approximately 15 kcal/min. Their RER value averaged .93, they were expending 76 percent carbohydrates and 24 percent fat. Figure 1 illustrates the impact on total caloric expenditure and substrate utilization. Not only did running result in a greater total caloric expenditure, but also a slightly higher total number of calories from fat in the same period of time.

The only reported study which has investigated the relationship between exercise intensity and weight loss was conducted by Ballor and colleagues and published in the American Journal of Clinical Nutrition (Volume 51, pp. 142-146, 1990). Twenty-seven obese females were randomly assigned to either a low-intensity or high-intensity exercise group, with both groups exercising three days per week for eight weeks. The low-intensity group cycled at 50 percent V02max for 55 minutes per session, while the high-intensity group cycled at 80 to 90 percent of V02max for 25 minutes per session. It was found that both groups lost approximately 12.5 pounds over the course of the study and had similar reductions in lean body mass and fat weight, regardless of the intensity of exercise. An additional finding was that only the high-intensity group significantly improved their V02max as a result of training.

The question is, then, does low-intensity exercise have a role? Of course it does. But one must look at the goal of the exercise program. If the goal is to improve overall health and reduce cardiovascular mortality, any sort of activity is beneficial. The recent recommendations published jointly by the Centers for Disease Control and Prevention, the American College of Sports Medicine, and the President’s Council on Physical Fitness and Sports recommend that all Americans get at least 30 minutes of moderate-intensity physical activity most days of the week. This equates to walking approximately two miles per day, four to five days per week. If the goal of the exercise program is specifically to lose weight, the aim should be to maximize total energy expenditure. We have seen in our example that this is best accomplished via higher-intensity work.
If this last statement is true, then why do most recommendations, especially for the obese, center around utilizing low-intensity, long-duration exercise to meet that end? The answer is twofold. First, if we prescribed high-intensity exercise for overweight and/or unfit people, a high percentage of them would become injured, forcing them to drop out of the program. Second, we know that adherence to high-intensity exercise is much lower than compliance to low- or moderate-intensity exercise. If people don’t perceive exercise or activity to be fun, or at least tolerable, they’re never going to stay with it. This was the professional intent for advocating low-intensity, long-duration exercise in the first place — to get people more active and to keep them active.

If weight loss is a major goal for an individual, the exercise regimen that burns the greatest number of calories in the shortest period of time should be what is prescribed. Determination of what activity the person enjoys, as well as what they can orthopedically and psychologically tolerate, are the main factors that should guide the exercise prescription. Consideration of the specific fuel burned during exercise should play no role in the exercise prescription process. Until proven otherwise, when it comes to weight loss, a calorie burned is a calorie burned, regardless of its source.

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