HIIT and the Afterburn Effect: Myth or Fact?

The afterburn effect is certainly real in the sense that exercise (especially intense exercise, such as HIIT) can affect your metabolism for a number of hours after the workout is over. However, the size of afterburn effect, as well as the extent to which it contributes to fat loss, have been exaggerated. 

Here’s a closer look at the science and what it all means for you.

Steady-State Cardio or HIIT: Which Is Better?

In 2017, a team of Australian scientists published a meta-analysis on the subject of HIIT, steady-state cardio, and fat loss [10].

A meta-analysis involves pooling the results from multiple trials on the same subject. Instead of lots of small experiments, you end up with one big experiment, conducted on lots of people. As a result, you’re left with a conclusion that’s more reliable than anything that could have been drawn from each of the smaller studies.

The researchers pooled the results of 28 trials, covering almost 1000 people. After crunching the numbers, they found “no evidence to support the superiority of either high-intensity interval training or steady-state cardio for body fat reduction.”

A separate research team came to much the same conclusion after doing their own analysis of the research [11].

High-intensity interval training and steady-state cardio are similarly effective, they say, and both elicit “modest improvements, and of similar magnitude, in body fat levels and waist circumference in overweight and obese adults.”

They do note, however, that HIIT is a more efficient alternative to steady-state cardio, delivering similar fat-loss benefits with less time spent in the gym.

The most recent review, published in the British Journal of Sports Medicine, reports that HIIT provides a “greater reduction in total absolute fat mass” than steady-state cardio [12].

However, while the difference between HIIT and steady-state cardio was statistically different, the “real world” difference was small.

After 30-35 workouts (10-12 weeks if you’re training three times a week), the average amount of fat loss with HIIT was 3.5 pounds (1.58kg), compared to 2.5 pounds (1.13kg) with steady-state cardio.

Granted, the HIIT workouts were shorter. But still, the overall difference in fat loss between the two types of exercise – one pound of additional fat loss over a 10-12 week period – was relatively small.

How Many Calories Do You Burn After a HIIT Workout?

The ability of HIIT to “put you in fat-burning mode, rev up your metabolism and torch calories for 24 hours after exercise” is not as great as was once believed.

In one study on the subject, researchers from Colorado State University looked at the number of calories burned both during and after a bout of HIIT [1].

For two days, subjects taking part in the study lived in a metabolic chamber. This is a small room that measures how much oxygen you’re breathing in and the amount of carbon dioxide you’re breathing out.

This information is used to estimate how much energy you’re using, as well as where that energy is coming from (fat, carbohydrate or protein).

On one of the days, they performed a HIIT 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 HIIT. 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 HIIT 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, HIIT 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 [2].

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.

How Long Does the Afterburn Effect After HIIT Last?

Some research shows that the afterburn effect after a HIIT workout lasts for up to 24 hours [3]. However, studies done using a metabolic chamber (a more accurate way of estimating post-exercise calorie expenditure) show that the afterburn effect lasts for just 2-3 hours [4].

In fact, most of the calories burned after exercise seem to come in the first hour.

For instance, researchers from Arizona State University compared three different workouts performed on an exercise bike [5]. Here’s what each workout looked like:

• HIIT (four 4-minute intervals at 95% peak heart rate, separated by 3 minutes of active recovery).
• Sprint interval training (six 30-second sprints separated by 4 minutes of active recovery)
• Steady-state cardio (30 minutes at 80% peak heart rate)

They found that energy expenditure in the three hours after exercise was greater with sprint interval training (110 calories) compared to steady-state cardio (64 calories) and HIIT (83 calories).

However, most (around 70%) of those calories were burned in the first hour after exercise. By the third hour, the difference in post-exercise energy expenditure between the different workouts was less than five calories.

So, interval training did have a bigger afterburn effect than steady-state cardio. The workouts were also 5-7 minutes shorter. However, when you add up the total number of calories burned both during and after exercise, it was the steady-state workouts that delivered the best results.

The total number of calories burned both during and after exercise was 348 calories with steady-state cardio, 329 calories with HIIT, and 271 calories with sprint interval training.

Here’s how the researchers sum up their findings:

“Although sprint interval training elicits greater EPOC compared to traditional steady-state cardio, total net energy expenditure (exercise + postexercise) is less than HIIT and steady-state cardio. EPOC is unlikely to be the major contributor to fat loss and body composition changes previously observed following high-intensity interval exercise training.”

It’s also worth pointing out that sprint interval training was poorly tolerated, with three out of 13 subjects unable to complete the study due to nausea, lightheadedness and vomiting.

A team of Canadian researchers report similar results [6]. Post-exercise calorie expenditure was higher with HIIT. But over a 24-hour period, the total number of calories burned with HIIT and steady-state cardio was almost identical.

Granted, the HIIT workout was shorter (18 minutes versus 30 minutes). But the fact there was no difference between the two groups in terms of total calories burned does pour cold water on claims that HIIT will “burn fat like crazy.”

Even with a much longer bout of HIIT lasting 60 minutes, the increase in post-exercise energy expenditure relative to steady-state cardio – 39 calories in total – isn’t particularly dramatic [7]. That’s not going to have a big impact on your rate of fat loss one way or the other.

The Afterburn Effect and Diet

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 that 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% [8].

In fact, some studies show that when an increase in physical activity results in a calorie deficit, the post-exercise rise in metabolism is wiped out completely [9].

So keep in mind that most estimates of post-exercise calorie expenditure will be heavily influenced by the type of diet you’re eating. Numbers for post-exercise calorie expenditure pulled from non-dieting subjects won’t necessarily apply to someone in a calorie deficit.

How HIIT Earned Its Metabolism-Boosting Reputation

So why did HIIT develop such a stellar reputation for raising post-exercise calorie expenditure? Marketing BS? Or flawed research?

Some of it was down to marketing. If one study shows an afterburn effect of 20 calories with HIIT and 10 calories with steady-state cardio, you can make the legitimate claim that HIIT leads to a 100% bigger afterburn effect than steady-state cardio.

Which is technically true, but the extra 10 calories aren’t going to make much difference to your rate of fat loss over time (a fact that isn’t mentioned in the marketing).

But some of it was down to weaknesses in the methods used to estimate post-exercise calorie burn.

Many of the early studies used Douglas bags or metabolic cart systems, where measurement time is typically limited to 15–30 minutes and then extrapolated to a 24-hour period. They’re relatively cheap, but not so accurate.

As metabolic chambers (a small room that allows for energy expenditure to be measured continually) became more widespread, the accuracy of the measurements went up, and it became clear that HIIT wasn’t raising post-exercise energy expenditure to the extent that was once believed.

Final Thoughts

None of this is meant to put you off HIIT, which is a time-efficient way to improve both your cardiovascular fitness and metabolic health.

However, the size of the afterburn effect after HIIT, as well as the extent to which it contributes to fat loss, is not as great as many people have been led to believe.

By itself, HIIT (and aerobic exercise in general) isn’t going to produce radical changes in body composition. For that, you’re going to need to lift some weights and get your diet in order.