In recent years, there has been a focus on the calorie burn after training. Colloquially referred to as the “afterburn effect” and more technically as EPOC (Excess Post-Exercise Oxygen Consumption), a number of popular training approaches have been advocated to try to leverage it for fat loss. But there is a long held issue regarding the absolute magnitude of EPOC and how much of a contribution it actually makes. I want to address this issue by examining the following paper.
Background on EPOC
As stated above, EPOC stands for the Excess Post-Exercise Oxygen Consumption and this represents the calories burned after a workout has been completed. For years EPOC was thought to be related to the “oxygen debt” from exercise, essentially the difference in how much oxygen was needed during exercise and how much was available. We now know that it’s related to a host of other processes in the body but those aren’t practically that relevant.
What is relevant from a weight or fat loss point of view is how large the EPOC is or can be and whether or not it does or even can contribute meaningfully to the overall calorie expenditure of an individual. In looking at a massive review on EPOC several things became apparent. The first is that intensity was far more important than duration in terms of the EPOC created, at least in term of the percentage increase.
However, even there the absolute magnitude of EPOC after even high intensity was relatively limited amounting to perhaps 30-50 calories total. This is the equivalent of one-half an apple or so and clearly insignificant in the big picture. This is especially true when you compare it to the number of calories burned during the exercise itself.
Based on that data, the researchers concluded:
The manipulation of energy balance for these individuals should not be concerned with generating large EPOCs but focused on both the energy expended during the actual exercise and the design of programmes that enhance compliance.
The compliance issue is actually a big one. The studies showing a large relative EPOC typically use High-Intensity Interval Training (HIIT), a type of activity that alternates period of near maximal intensity work with lower intensity work. And while not all research agrees, it’s unlikely that the typical beginner will tolerate or sustain those types of intensity on a consistent basis.
There is the additional practical fact that, even if HIIT can be done acutely, it can’t be done every day. Whether for fat loss or otherwise, anyone who wants to exercise frequently can’t perform HIIT at every workout.
That said, the study I want to examine today seems to run contrary to the above conclusion as it found that the EPOC following “vigorous activity” was significant. So let’s look at it.
The Impact of Vigorous Activity on EPOC
The study recruited 10 male subjects, aged 23-33 years who were capable of bicycling for 45 minutes continuously. Body composition was measured via DEXA and the subjects all underwent VO2 max testing using a fairly standard protocol. Energy expenditure was measured in a metabolic chamber, a very specialized piece of equipment that, well, measures metabolic expenditure.
Following testing, the subjects performed two “exercise” sessions. In the first they sat quietly in a metabolic chamber to get a baseline level of energy expenditure. In the second they performed the actual exercise. This allowed the researchers to measure the additional calorie burn over baseline following the exercise.
The exercise bout consisted of 45 minutes of cycling at 57% of their maximum wattage and this put them at roughly 70% of their VO2 max. The 45 minute duration was chosen to represent a sort of “midpoint” for average recommendations. There was a 4 minuet warm-up and cool-down before the actual 45 minute exercise. The subjects stayed in the metabolic chamber after the ride which is how EPOC was measured.
Food intake was controlled which is a strength of the study. Many early studies did not control for food intake and when they measured energy expenditure, they were invariably measuring the Thermic Effect of Food (TEF) rather than any impact of exercise itself. It is critical to note that the subjects were kept in energy balance, that is they were fed more to compensate for the energy expenditure of the exercise.
The exercise bout itself burned a total of 519±60 calories which is roughly 11 calories/minute over 45 minutes. For all but the most elite trained athletes, this is definitely a vigorous intensity. Over the next 14 hours, including 3 hours of sleep, the subjects burned an additional 144±50 calories with the majority of that occurring in the first 9 hours after the exercise bout.
This EPOC represented a whopping 27% of the actual exercise bout energy expenditure, far higher than what was reported (usually 15% or less) in the LaForgia review I examined previously. And the total impact of the exercise bout was an energy expenditure of ~750 calories above and beyond the resting condition.
The researchers conclude
“The 24-h net energy expenditure difference between exercise and rest days was 750 kcal, a meaningful quantity over time if two or three such exercise bouts are inserted into the weekly schedule and energy intake is controlled.”
Fair enough. Mind you, the grand majority of the energy expenditure, 73% came from the exercise itself. But the EPOC wasn’t as insignificant as it has typically been found to be in either a relative or absolute sense.
Explaining the Difference
The first issue to address is why this study seems to have found a difference from the previous studies on the topic. That is why, was the EPOC so much higher in both relative and absolute terms. Certainly some of it could be methodological, the use of a metabolic chamber versus other methods of measuring O2 consumption. Looking into that would mean pulling all of the previous studies which I’m not doing.
One big issue to at least consider practically is that the subjects were deliberately kept in energy balance. That is, they were fed an additional 660 calories to offset the energy expenditure of the exercise plus EPOC. The researcher state:
The increased energy intake balanced against energy expenditure (energy flux) has been shown in several studies to contribute to the elevated 24-h energy expenditure on exercise days or in trained individuals.
Basically, maintaining what is called a high energy flux (high activity AND high food intake) seems to benefit daily energy expenditure. But this is only relevant within a maintenance or high calorie situation. The afterburn effect/EPOC claims are made for a dieting situation and there is no reason to expect a similar response when someone is not eating to replace the calories burned. I’m not saying that it couldn’t but the study would have to be repeated without increasing food intake to see if the effect is still present.
I think the main reason for the results are that the workout was HARD. As I stated above, the subjects rode at 57% of their max power output which was 70% of VO2 max which is about 85% of maximum heart rate. During the exercise, heart rate was measured at 163±16 beats per minute. Most people have a maximum sustainable heart rate in the 170-180 range so this is a near maximum effort equivalent to the sweet spot and threshold training methods of endurance training.
Without getting into the weeds of what determines EPOC, the researchers state that:
The magnitude of post-exercise energy expenditure is greatest when the body experiences significant physiologic stress during prolonged and high intensity exercise.
Basically, through various mechanisms, the “afterburn effect” is maximized when you massively disrupt the body’s homeostasis physiologically. And this tends to occur to the greatest degree for workouts that are a combination of high-intensity AND high-volume which this was.
Earlier studies tended to examine EPOC following longer duration low-intensity work or short duration very high-intensity work. Neither of which generate nearly the degree of physiological disruption that the workout examined in this study did. Longer duration exercise is too easy and high-intensity exercise is too short.
In contrast, this workout had the subjects working pretty much near their maximal steady state level (as noted, for most this is 170-180 heart rate) for a near maximal duration. Most can’t go much past an hour at true threshold so 45 minutes at 90% of threshold is a hard workout.
Any endurance athletes that have done it know that it’s hard, uncomfortable work. Quite in fact, compared to lower intensity but longer duration work or higher intensity work, this type of training is probably the hardest. Low intensity work is just dull. It’s hard, it’s just boring and you butt gets sore on the bike. HIIT hurts but the work bouts are too short to make it true suffering. The workout is over before it gets awful.
In contrast, 45-60 minutes at a near maximum intensity is grueling both physically and psychologically. The minute tick by as you’re working near your limits without a break. This is especially true on the bike where the discomfort is localized to the legs. In running, the stress seems a bit more “spread out” and you mainly feel breathing hard than your legs burning.
So the results do make sense. Clearly working at such an intensity that generates maximum physiological disruption can generate a fairly large EPOC. That said, the majority of the calorie burn still came from the exercise itself. The EPOC was just a nice bonus. Of course even that only occurred under conditions of calorie balance. Would it have happened in a deficit? I don’t know.
More importantly, in a practical sense, I doubt many would be willing to do that type of training consistently. If they are weight training, recovery rapidly becomes a problem. For most, cutting the intensity to a more tolerable level but doing an additional 15 minutes would still burn similar numbers of calories. The EPOC would be lost but the net effect would be the same. And the effort would be far more repeatable on a consistent basis. Long term, consistency wins.