Is Working Out in the Cold Really the Key to Burning More Fat?

The hidden secret to exercising for weight loss, according to Johnny Adamic, is doing it in the cold. He tells me of Ray Cronise, a former NASA scientist turned health guru who lost 30 pounds in six weeks by going on “shiver walks,” sleeping in the cold, and taking cold showers. There are many potential takeaways one could glean from this story.

As half the founding team of Brrrn, a boutique exercise studio in New York City that markets itself as “the world’s first cool temperature fitness concept,” Adamic, unsurprisingly, views Cronise’s tale as a testament to the power of cold. Brrrn’s website claims that “the body burns more calories in cooler temperatures than in ambient or hot temperatures,” and that “when exposed to cooler temperatures”—64°F or lower—“the body may turn to burning fat as fuel to stay warm.”

“We have two theses when you walk into Brrrn,” Adamic says. First, you’ll begin the process of shivering, which stimulates heat production through the involuntary shaking of skeletal muscles. You might also undergo “nonshivering thermogenesis,” which occurs when the body’s stores of brown fat—more on this later—produce heat in response to cold temperatures. Both of these processes burn calories, which is what weight-loss enthusiasts want.

I had to run to the Brrrn studio from a stalled subway train, so I'm not shivering as I wait for the class to start, even though I selected the coldest of their three options. (Rooms are chilled to 65°F, 55°F, or 45°F.) During the session itself, though, in which we alternate between rope and weight exercises over blaring pop music, the frigid temperature is barely noticeable; I immediately start sweating. Later, Adamic will tell me that thanks to an earlier yoga class that ran late, the class was warmer than it should have been.

It’s a good workout—challenging but easily tailored to one’s individual fitness level. If Brrrn’s website is to be believed, it was also more effective than if I had done the same moves in warmer conditions.

Science, however, says the picture is more complicated.

Researchers have long been interested in the connection between cold and metabolism. This kicked into high gear back in 2009, when brown adipose tissue, or heat-generating brown fat, was discovered in adults; previously, it was thought to exist only in infants. (Because they can’t shiver, babies depend on brown fat to keep themselves warm.)

It was an important revelation, because in mice brown fat generated a lot of heat and was associated with an increased metabolic rate. There was also evidence that exposure to cool temperatures could produce more brown fat, says Aaron Cypess, a clinical investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, who was one of the researchers to first identify brown fat in adults. This effect has since been demonstrated in humans, too, leading to the intriguing idea that brown fat could help combat obesity and obesity-related conditions.

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That’s just a theory, however. At this point, there are far more questions about brown fat than answers. Researchers can’t say for sure that activating brown fat—or developing more of it—could help with weight loss. In part, that’s because while we know brown fat uses ordinary white fat as fuel, estimates of how many calories it burns vary widely. “We don’t know yet exactly how much heat it generates,” says Cypess. “It’s a sufficiently wide range that we can’t make definitive statements. We can't say it's irrelevant, and we can’t say it’s really important.”

Besides, working up a sweat while out in the cold negates the whole burns-more-calories theory. Through shivering and brown-fat activation—again, impact on your figure TBD—cold temperatures can cause the body to burn more calories, but it “only really happens when you’re at rest,” says Cara Ocobock, a biological anthropologist at the University of Albany. When you exercise, your body generates heat, which means that when you're out in the cold, your body doesn't have to burn extra calories in order to keep you warm. "That's what the physical activity is doing," she says.

And yet, the perception persists that working out in winter burns more calories than a comparable workout in the spring or summer. That myth partly stems from Ocobock’s own research, in which she studied 53 participants who lived in the mountains for months at a time. In the winter, they burned far more calories than in any other season. The results have been featured in a number of articles and blog posts touting the calorie-torching effects of winter workouts.

This is the wrong takeaway, she says: Participants in the study burned more calories in the winter because they engaged in more vigorous physical activities, including cross-country skiing, sledding, and shoveling snow. “It didn’t have to do with it being cold. It had to do with the actual difficulties of navigating a winter environment in the mountains,” Ocobock says, laughing.

All these caveats and unanswered questions don’t make for a very convincing case, which is where these cold-as-the-magic-ingredient pitches come in. Cypess finds anecdotes like Ray Cronise’s weight-loss journey to be frustrating, since they aren’t well-designed studies that do the work of removing confounding variables. “That’s what distinguishes what we do—it’s not easy to do these experiments because so many things have to be subtracted,” he says. Ray, of course, wasn't held to this same standard.

This is not to say that cold workouts are a bad idea! While there’s no good evidence they burn more calories, all three experts I spoke with said there are some indications that working out in the cold can help boost endurance, likely because you sweat less and therefore get less dehydrated and fatigued. (For hot-yoga fans, apologies, but there's “no basis” for its benefits, Cypess says. He finds the idea of choosing to work out in a heated room baffling.) In other words, what Brrrn has done here is the same thing that many fitness studios (and startups of all varieties) do: Translate complicated, ambiguous data into shiny marketing copy and pitch the entire package to busy professionals.

Ocobock has two pieces of fitness advice, which are boring but sound: Always be wary of gurus, and focus on choosing a physical activity that you like enough to do regularly. If that’s working out in a chilled room or trekking through the mountains, great. If it’s hot yoga, Cypess’s skepticism notwithstanding, great. It can even be—wait for it—indoors. “You need to find something you enjoy doing enough that you can continue doing over the long term,” she says. When it comes to temperature, though, don’t sweat it so much. (Sorry.)