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The Final Nail in the HIIT EPOC Coffin

After an exercise workout is completed, oxygen uptake (VO2) remains elevated for some time over resting levels. This phenomenon is called, straightforwardly enough, "Elevated Post-exercise Oxygen Consumption" or EPOC. The duration and total amount of EPOC varies according to the duration and intensity of the workout. The increased VO2 means a increase in calories burned after the workout as well.

A frequent rationale for promoting High Intensity Interval Training (HIIT) programs for weight loss is the assertion that HIIT workouts burn more calories for an extended time AFTER the workout. A perfunctory use of the Google will quickly uncover dozens of examples of these claims:

"HIIT increases EPOC (excess post-exercise oxygen consumption) resulting in an elevated fat loss state for up to 24 hours after you finish your workout – something you won’t get from lower intensity exercise."

And another:

"Long, slow, low-intensity cardio will produce a small EPOC effect that will at best, last only a couple of hours after the workout. Basically the 400 calories you burn during the workout will be all that you burn, because the EPOC effect afterwards is negligible. However, studies have shown that after HIIT workouts, the EPOC effect can still be found up to 38 hours after the workout - basically your body is burning fat to fuel the restorative processes for 38 hours AFTER your workout! So, even though you may have only burned 250 calories during your HIIT workout, the total calories burned after will be far greater.  Those calories, plus the 250 burned during the HIIT workout surpasses the 400 burned during the slow, steady-state workout by a LONG, LONG way."

Actually, the "studies" point out that these claims are exaggerated, to say the least, and that the effect of EPOC for all workouts is modest at best. There are plenty of good reasons to include HIIT workouts in your routine, but a supposedly elevated EPOC is not one of them. 

A 2006 study reviewed all the research that had been done up to that time on EPOC. The authors concluded that the additional calories burned from EPOC for endurance exercise was about 7% of total calories expended during the workout. Calories from EPOC for HIIT workouts were almost double, approximately 13% of total calories during the workout.

LaForgia J et. al. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. J Sports Sci. 2006 Dec;24(12):1247-64.

Double the calories sounds pretty good, however, when comparing the effects of something like EPOC, one must look at the TOTAL calories burned, not the percentage increase and not the "duration" of the "afterburn". Looking at the actual numbers tells a different story.

80 kg individual does a 60 min endurance workout and burns 640 calories. Add 45 calories for the 7% EPOC for a total of 685.

Same person does a 20 min HIIT workout and burns about 300 calories. Even with the extra EPOC of 13%, that's only an extra 40 calories, for a total of 340.

There is no "afterburn" that is going to make up those extra 300 calories.

(For a more in-depth discussion of the LaForgia study, see Lyle McDonald's excellent review: 

http://www.bodyrecomposition.com/research-review/research-review-effects-of-exercise-intensity-and-duration-on-the-excess-post-exercise-oxygen-consumption.html)

More evidence was presented in a recent article on the website of the National Strength and Conditioning Association. Several studies were reviewed (including the LaForgia study already discussed) and showed that, even when EPOC remains elevated for 9 hours following a HIIT workout, total calories burned from EPOC only ranged from 18 to 80 calories TOTAL. The only exception was a recent study from Appalachian State University that reported a total EPOC of 190 calories over 14 hours).

As if that weren't enough, a review article published by Melanson et al in 2009 showed that exercise training does not affect 24 hour total body fat oxidation, even though it increases fat metabolism in muscle.

Exerc Sport Sci Rev. 2009 April ; 37(2): 93–101. doi:10.1097/JES.0b013e31819c2f0b.

Other studies have shown either no difference in the 24-hr rate of fat use following exercise, regardless of intensity, or, even when there was a small increase in fat oxidation following high-intensity exercise, the total amount of fat burned was trivial.

The conclusion of the NSCA article was that research:

"...suggests that EPOC is not a primary contributor to the energy cost of exercise and that it does not alter substrate use enough to be of consequence in a weight loss program."

Again, the purpose of this is not to denigrate HIIT workouts. They can play an important role in many workout programs, and, when the time comes, they are essential in order to accomplish significant body recomposition. However, like so much else in the fitness world, the benefits of HIIT are sometimes overhyped and their application frequently overgeneralized. People need different types of workouts depending on their age, body fat levels, fitness level, fitness experience and overall health. When starting a weight loss program, total energy expenditure is still a primary goal of the exercise component. Endurance cardio workouts, submaximal interval workouts, and even endurance strength workouts are usually the better choices for beginners.

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