The Fat-Burning Effects of HIIT Workouts Are Exaggerated
Here’s a closer look at the science on cardio and weight loss and what it all means for you.
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People who lift regularly will often say that if you want to get lean, you shouldn't bother with steady-state cardio—it’s a waste of time. The key to losing fat as fast as humanly possible is to do high-intensity interval training (HIIT) instead, they argue.
That's not necessarily wrong: HIIT can be a very efficient way to spend your time in the gym. In some cases, just 30 minutes of HIIT each week can improve various markers of cardiometabolic health to the same extent as two and a half hours of traditional cardio. The fat-burning benefits of HIIT, however, have been greatly exaggerated, and it doesn’t work nearly as well for fat loss as some like to claim. Here’s a closer look at the science on cardio and weight loss and what it all means for you.
For starters, as some Tonic contributors have argued in the past, cardio by itself is not a very effective way to lose fat. Back in the '90s, researchers at the George Washington University Medical Center set out to determine if adding aerobic exercise to a low-calorie diet accelerates weight loss. After looking at 493 studies, they found that diet and exercise provides only a marginal benefit compared to diet alone.
The average weight loss after a 15-week program of aerobic exercise was seven pounds. Over the same period, dieting cut weight by roughly 17 pounds. When exercise and diet were combined, average weight loss was 20 pounds—just three pounds more than diet alone.
As part of the HERITAGE Family Study, one of the largest studies of its kind, researchers tracked more than 500 men and women as they embarked on a 20-week exercise program. Following a grand total of 60 workouts, the average amount of fat lost was slightly less than two pounds, prompting scientists to admit that aerobic exercise “is not a major factor” in weight loss.
That's not to say steady-state cardio can't deliver results, but you need to do a lot of it. In one trial, a group of obese men who did 60 minutes of brisk walking or light jogging every day lost, on average, 13 pounds of fat. While losing a little over one pound of fat per week is a decent result, the men taking part in this trial exercised for more than one hour, every single day, for three months. They were also obese and carrying around a lot of extra weight, which means they burned a massive amount of energy during their daily workouts. It’s the equivalent of a normal-weight individual walking around with a heavy rucksack strapped to their back.
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But let's get back to HIIT for a moment. HIIT is often praised as being uniquely effective for fat loss—more so than steady-state cardio. That’s mainly on the basis that your body continues to burn calories at an accelerated rate after the workout is over. The size of this “afterburn effect,” as well as the extent to which it contributes to weight loss, however, have both been highly exaggerated.
Researchers from Colorado State University, for example, found that HIIT led to an average of 226 extra calories being burned over the course of the day. And that’s not just the calories burned after the workout. It’s the calories burned both during and after exercise. What’s more, HIIT had no impact on resting metabolism when it was measured 23 hours after exercise. All of the calories were burned during and immediately after the workout itself.
By way of comparison, an Appalachian State University study shows that 45 minutes of steady-state cardio at 85 percent maximum heart rate burned a little over 700 calories—519 during the workout itself and 190 after it had finished.
In 2017, a team of Australian scientists published a meta-analysis on the subject of HIIT, steady-state cardio, and fat loss. (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. 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. It’s also worth pointing out that exercise does have some potential downsides as far as fat loss is concerned. For some, a workout that burns lots of calories will stimulate your appetite, so you end up replacing the calories you’ve worked so hard to burn. There’s also a phenomenon known as moral licensing, where being “good” gives you permission to be “bad.” In other words, you may end up eating more food after a high-calorie workout because you feel like you “earned” it.
What’s more, some studies show that if you burn lots of calories via exercise, you tend to spend less energy elsewhere. Put differently, your body attempts to “cap” the amount of energy you use in day. Beyond a certain point, increasing the number of calories you burn in the gym won’t automatically lead to more fat being lost.
To sum it all up, there are many different ways to do cardio. All of them have their place at different times and for different people. While HIIT is a long way from being a magic bullet as far as fat loss is concerned, no type of exercise will do much to shift the weight in the absence of a good diet.
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