There’s a good chance you’ve heard of the afterburn effect.
It refers to the fact that your body continues to burn calories at an accelerated rate even after your workout is over.
You might also see it called oxygen debt, recovery energy expenditure, or excess post-exercise oxygen consumption (EPOC).
It’s a controversial subject, mainly because there’s still a lot of uncertainty about how big the afterburn effect is, how long it lasts, and what kind of contribution it makes to weight loss.
In the most recent study on the subject, researchers from Colorado State University presented some interesting new data on the number of calories burned both during and after an interval training workout .
For two days, subjects taking part in the study lived in a metabolic chamber. On one of the days, they performed a sprint interval workout that involved pedaling as fast as possible on a stationary bike for 30 seconds. They did five sprints in total, with each sprint separated by four minutes of recovery where they pedaled slowly against very little resistance.
The figure below shows minute-by-minute energy expenditure during a rest day and a day beginning with a single bout of sprint interval training (SIT). While there’s a big spike in energy expenditure during and immediately after the workout itself, it soon dropped back to normal.
How many extra calories did the subjects burn?
500… 750… 1000?
The average increase in energy expenditure on the interval training day was 225 calories. And that’s not just the calories burned after the workout. It’s the calories burned both during and after exercise.
I couldn’t find a picture of 225 calories, so here’s one that shows you what 200 calories looks like. It’s not even a whole chocolate muffin.
In short, interval training had no impact on resting metabolism when it was measured 23 hours after exercise. All of the calories burned came during and immediately (2-3 hours) after the workout itself.
By way of comparison, an Appalachian State University study, using a very similar study design, found that 45 minutes of steady-state cardio (cycling at 85% maximum heart rate) burned just over 700 calories – 519 during the workout itself and 190 after it had finished .
One of the strengths of both studies is the fact that the researchers used a metabolic chamber, rather than a Douglas bag or metabolic cart system, to estimate the number of calories burned.
With the latter, measurement time is typically limited to 15–30 minutes and then extrapolated to a 24-hour period. Metabolic chambers allow for energy expenditure to be measured continually, which makes them a lot more accurate.
I should point out that there is an anaerobic component to energy expenditure that wasn’t measured in either study.
As exercise intensity increases, the anaerobic component makes an increasingly larger contribution to calorie expenditure. Leaving this out, as many studies do, results in an incomplete estimate of the number of calories burned.
Christopher Scott, a Professor at the University of Southern Maine, has published a number of studies on this very subject.
Back in 2005, he reported on research appearing to show that the anaerobic contribution to energy expenditure during a bout of interval training would have bumped up the number of calories burned by around 65%, compared to a 10% increase for treadmill walking .
However, when he was asked about it in a later interview, Scott does hint at the fact that those numbers were probably wrong.
“I wouldn’t so much agree with the numbers on that,” he says. “But what it did do was paint a picture that we are possibly way off in terms of looking at the total energy expenditure of a weightlifting activity or a sprinting activity.”
So there is some kind of anaerobic contribution to energy expenditure, particularly during intense exercise. But nobody is really sure how big it is.
One of the other issues with most of the research on EPOC is that it’s looked at subjects in a state of energy balance.
In other words, the people taking part in the studies were given more food to compensate for the extra calories they burned during the workout.
Why does this matter?
If you’re in a calorie deficit (which you’ll need to be if you want to lose fat), the afterburn effect is going to be lower than it otherwise would be. In one study, two days of dieting reduced the post-exercise calorie burn by 40-50% .
In fact, some studies show that when an increase in physical activity results in a calorie deficit (which it’ll need to if you want to lose weight), the post-exercise rise in metabolism is wiped out completely .
So keep in mind that most estimates of calorie expenditure during and after exercise are just that, estimates. They don’t take into account the anaerobic contribution to energy expenditure, and will also be heavily influenced by the type of diet you’re eating.
Researchers don’t actually measure calorie expenditure during exercise. It’s more of a prediction, based on the best tools and methods available to them at the time. And just like stock market analysts, football pundits and weather forecasters, chances are that some of their predictions will prove to be wrong.
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ABOUT CHRISTIAN FINNChristian Finn holds a master's degree with distinction in exercise science, is a certified personal trainer and has been featured on BBC TV and radio, as well as in Men's Health, Men's Fitness, Fit Pro, Zest, and Perfect Body magazine.
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