Exercise Intensity, Fat Burning, and the "Afterburn" Effect

Traditionally, it was thought that low intensity exercise was more beneficial for fat loss
than high intensity exercise. And for good reason since physiologically the body burns a greater percentage of fat calories is at a lower intensity. It was therefore thought that this equated to more fat loss.

However, this line of thinking failed to consider the fact that even though high intensity exercise might have a relatively lower percentage of fat calories expended, more total calories are expended during high intensity exercise for a given duration.

This, in turn, would increase the absolute number of fat calories expended during exercise of a given duration. In addition, low intensity exercise fails to increase the excess post-exercise oxygen consumption (EPOC), or as I like to call it – the “afterburn”, as greatly as high intensity exercise, especially high intensity exercise of longer durations.

A greater EPOC or “afterburn” leads to an increase in calories expended following the conclusion of the exercise bout. As exercise intensity increases, the magnitude and duration of the “afterburn” increases.

This has been shown in numerous studies. For instance, a study in the journal Metabolism, had subjects complete exercise bouts at intensities of 29%, 50% and 75% of their VO2max for a period of 80 minutes. The greatest EPOC was reported following the highest exercise intensity (75% VO2max) with 30 liters of oxygen consumed or 150 calories burned.

(Note: it is well understood in all exercise physiology and nutrition texts that for every liter of oxygen consumed, approximately 5 calories are burned.)

Additionally, the duration of EPOC following the highest intensity exercise was significantly longer when compared to the lower-intensity bouts (10.5 hours versus 0.3 and 3.3 hours). That’s an astounding difference!

Now some people might say that 150 calories over the course of 10 hours isn’t very significant. This may be true but it becomes very significant when this type of training becomes part of your fitness routine. Assuming everything else to be the same, an extra 150 calories burned at rest on a daily basis would equate to a 1 lb fat loss every 23 days!

Exercise Intensity and Your Resting Metabolic Rate

The most important aspect of how high intensity exercise causes weight and fat loss does
not necessarily occur during the exercise itself. A higher exercise intensity can also cause an acute increase in resting metabolic rate (RMR) for up to 24 hours post-exercise (as shown by Bielinski and colleagues in a study in the American Journal of Clinical Nutrition). An increase in RMR will automatically increase caloric expenditure throughout the day since your RMR accounts for up to 75% of your total daily energy expenditure.

Additionally, even though high intensity exercise does not use as much relative fat as a fuel source during exercise, fat is burned to a greater extent following exercise in order to replenish the glycogen stores depleted during the high intensity exercise.

What Role Does Caloric Restritcion Play in Fat Loss?

Some researchers have concluded that exercise without caloric restriction is not an effective means of absolute weight loss because exercise preserves or, in the case of resistance or high intensity cardiovascular exercise, increases fat-free mass. Reduce the number of calories coming and increase the amount of calories you expend and your weight loss becomes a whole lot easier.

Researchers, such as those from a 1995 study in the Journal of the American Dietetic Association, have also found that high intensity exercise facilitates fat loss as well as preserving fat-free mass at a greater rate than low intensity exercise. One of the reasons for this is that high intensity exercise promotes the release of growth hormone - a hormone necessary for muscle development. Furthermore, high intensity exercise, unlike extreme duration low intensity cardio, does not allow you to exercise for excessive durations that could potentially cause the body to tap into muscle protein reserves for fuel.

References:

Bielinski, R., Schutz, Y., & Jéquier, E. (1985). Energy metabolism during the postexercise recovery in man. American Journal of Clinical Nutrition, 42, 69-82.

Grediagin, A., Cody, M., Rupp, J., Benardot, D., & Shern, R. (1995). Exercise intensity does not effect body composition change in untrained, moderately overfat women. Journal of the American Dietetic Association, 95, 661-665.

McArdle, W. D., Katch, F. I., & Katch, V. L. (1996). Exercise physiology: Energy, nutrition, and human performance (4th ed.; pp. 151-152, 408-411, 541-542). Baltimore: Williams & Wilkins.