An expert weighs in on the claims behind the fad.
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Selena Gomez was recently asked by Elle about her workout schedule while on tour. At first, her response was routine: She travels with a personal trainer. She goes for hikes and does Pilates. Then, she lost us. “I have a sweat bed, which looks like a burrito that I wrap myself up in, and I sweat for about 45 minutes,” she told the magazine. Wait—does sweating in a bed really offer health perks? To find out we talked with Pamela Peeke, MD, assistant professor of medicine at the University of Maryland and author of Body For Life For Women. (Spoiler alert: She’s not buying it.)

The backstory

Gomez said she started using a sweat bed at a place called Shape House. According to its website, Shape House is an “urban sweat lodge” with three locations in California, and its treatments use infrared heat technology to “burn calories, deepen sleep, improve skin, lift moods, and change lives.”

Gomez agrees with at least some of these claims. She told Elle that using the sweat bed “gets your heart rate up, and it gets everything flowing in your body. It's changed my skin, it's kind of changed my body as well, so it feels really good.” And she’s not the only celebrity fan. According to The Hollywood Reporter, Rooney Mara, Demi Moore, and the Kardashians are all regulars at Shape House as well.

We certainly won’t deny the power of a good sweat session—one that you really work for, at least. But can you possibly get the same benefits from just lying there in a heated bed?

The claims

Shape House’s website states that a session in a sweat bed can burn between 800 and 1,600 calories. This includes the 55 minutes spent in the bed along with the extra calories burned over the next 36 hours as the result of a “metabolic boost.”

It also states that as toxins are released through sweat, the body is better able to eliminate fat that’s been storing these toxins. Sweating out heavy metals may even help correct hormonal imbalances, improve digestion, and help you sleep, the company claims.

And perhaps most surprisingly, Shape House states that during your time in its beds, “your heart receives a workout similar to that of a 10-mile run.” Using a sweat bed is perfect for people who don’t have time to exercise regularly, the company suggests, since it's basically the same thing.

The expert opinion

“If something sounds too good to be true, it probably is—and this is a perfect example of that,” says Dr. Peeke. “All of the assumptions that have been made about this service have absolutely no foundation whatsoever in science. None, zero, zip.”

People who say they feel healthier after spending time in a sweat bed are likely just getting “a boatload of placebo effect,” Dr. Peeke adds. “If I tell you that wrapping you up like a burrito and warming you up is going to elevate your mood and make you sleep better and your whole day is going to be better, then sure, you may feel that way. But there’s no proven evidence that it actually does anything, and I sincerely doubt there ever will be.”

As for weight loss, says Dr. Peeke, the results are purely temporary. You may lose a few pounds in a sweat bed, just as wrestlers lose weight by spending time in saunas. But as soon as you drink water, those pounds come right back.

It’s true that sweating does have some cosmetic benefits. “In order to sweat, your circulation has to be going and the vessels right below your skin have to dilate to get rid of that fluid,” says Dr. Peeke. “So you do get a natural glow, and that’s a nice thing—but that’s all you get, end of story.”

And those claims about toxins? “Let’s not insult your body,” she says. “It knows how to detoxify itself quite nicely, without your help.” The vast majority of so-called toxins (also known as cellular waste) is eliminated through urine and feces, Dr. Peeke says; sweat plays only a very small role.

Finally, there’s the matter of the supposed 1,000-plus calorie burn. “Show me the science,” says Dr. Peeke, who’s also a fellow of the American College of Sports Medicine. “A really hard-core Spin class burns about 500 to 600 calories for most people. And to suggest that just lying there is going to burn more than that is just a farce.”

Yes, the body does burn calories at rest, and raising its temperature may increase that burn slightly. But, Dr. Peeke says, the only way to really boost that burn is to use your muscles.

In other words, save your money and go for a run.