When you head outdoors for a cold-weather workout, you may be wondering if your body is working harder as it tries to keep you warm. You're sweating less than you would during warmer months, but depending on the temperature, you may be shivering, and that has to burn loads of calories, right?
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To a degree, says Zac Schlader, PhD, associate professor in the department of kinesiology at Indiana University and a fellow for the American College of Sports Medicine. According to Schlader, the big question is: Just how cold is it?
"When we work out in cold weather, we wear a lot of clothes, and our skin gets cold, but as we start to exercise, we ultimately get warm," he says.
In this situation, what happens to the body isn't much different from what happens when you exercise in cooler temps, wearing shorts and a T-shirt. Sure, your skin will be a little colder at first, but you'll be able to produce enough heat with exercise so you don't become hypothermic, which means a drop in core body temperature.
The exception, Schalder says, is when you start exercising in very cold weather — like 20 degrees Fahrenheit — in shorts and a T-shirt. At that point, you might start to burn calories trying to stay warm, in addition to calories burned from exercise.
"The amount of heat you generate with exercise may not be enough to maintain your body temperature," he says. "In that case, there would be different physiological responses."
Exposing yourself to the extreme cold when not properly dressed could lead to hypothermia and frostbite. So if you're working out in the cold, make sure to wear appropriately warm layers, including a beanie and gloves.
Shivering Burns Calories
One way your body tries to maintain its core temperature is to warm itself up is through shivering — whether you're sitting at a bus stop or logging miles in subzero temperatures.
"Shivering is analogous to sweating in the heat," Schlader says. "You sweat to get rid of heat and you shiver to generate heat." But while sweating doesn't burn calories, shivering does.
It's hard to say exactly how many calories shivering burns, but a February 2014 study published in Cell Metabolism suggests that shivering for 10 to 15 minutes may be comparable to an hour of moderate exercise. But, Schlader points out, just being cold isn't a reliable method of calorie burn for weight loss. Plus, he says, "shivering is really uncomfortable."
Brown Fat Also Affects Calorie Burn
Until recently, it was thought that only small rodents and human babies had brown adipose tissue — brown fat — which helps the body stay warm without shivering. But now, Schlader says, it's clear that adults have brown fat, too.
Research has shown that cold temperatures activate the body's brown fat stores, which generate heat without having to do any additional work, Schlader says. "The beauty of it is you can increase heat production without feeling miserable [from shivering]," he says.
So how many calories does brown fat burn when it's trying to warm you up? "That's the million-dollar question," Schlader says.
The amount of extra calories burned through brown fat activation or shivering, Schlader says, is not astronomically high. "If you go out for a 30- to 45-minute run, I don't think we're talking about more than 100-calorie difference," he says. "I wouldn't say, oh good, you can eat that extra cheeseburger."
And let's say it's so cold that you cut your four-mile run down to three miles. You'd burn more calories from running a fourth mile than from your body trying to warm itself up. Plus, to date, studies looking at whether brown fat activation alone is an effective tool for weight loss seem to have come up short, Schlader says.
"If you go outside without any clothes on and just stand there in 40 degrees for four hours [definitely not advisable!], will you burn more calorie than sitting inside? The answer is yes. The body has to produce heat. But if I were to exercise inside versus going outside, dressed appropriately, the evidence suggests I wouldn't be burning more calories in the cold," Schalder says.
In other words, yes, working out in the cold burns calories, but not necessarily more than in other conditions. Again, the exception is if you're not properly dressed to work out in the cold.
"If you're not wearing enough clothes, your body increases heat production, and that extra heat has to be coming from somewhere," Schlader says, and that's from either brown fat activation or shivering.
Acclimating to Cold Weather
Just like you can train your muscles to get stronger, you may be able to "train" your body's brown fat stores to get better at creating heat, thus burning calories, by exposing yourself to cold weather.
A very small — only six subjects — March 2014 study in the Journal of Clinical Endocrinology and Metabolism found that four weeks of cold exposure resulted in a 45-percent increase of brown fat activity. And an August 2013 study in the Journal of Clinical Investigation also found that frequent cold exposure can increase brown fat activity and subsequent calorie burn.
But more research is needed when it comes to brown fat and calorie burn, Schlader says. "The investigation into brown fat is really interesting, but it's pretty early as far as understanding the importance of these changes being observed."
For example, much of what's been observed when it comes to cold temperatures activating brown fat is at a very mechanistic molecular level and hasn't been translated to a whole-body level, yet.
"That doesn't mean it won't, but we're just at the tip of the iceberg — no pun intended," Schlader says. "It's too early to say, go sit in a cold room and you'll lose weight."
- Journal of Cell Metabolism: "Irisin and FGF21 are Cold-induced Endocrine Activators of Brown Fat Function in Humans"
- Journal of Clinical Endocrinology and Metabolism: "Increased Brown Adipose Tissue Oxidative Capacity in Cold-acclimated Humans"
- Journal of Clinical Investigation: "Cold Acclimation Recruits Human Brown Fat and Increases Nonshivering Thermogenesis"
- New England Journal of Medicine: "Functional Brown Adipose Tissue in Healthy Adults"