You could retreat to the climate control of your gym this winter, but if you brave the same workouts outdoors, science says that you'll end up with a better body. For starters, you'll boost your metabolism the minute you step outside: When you exercise in temperatures below 64 degrees, you increase the amount of calories you burn, according to a study at Maastricht University in the Netherlands. That's because chilly temps trigger something called nonshivering thermogenesis. This means that your body isn't so cold that it's shivering, but behind the scenes, it's stoking the metabolism fire to keep you warm. (But How Cold Is Too Cold to Run Outdoors?)
You'll also torch more fat. Several studies point to the effect of cold on brown adipose tissue (brown fat), whose main job is to keep you warm. Expose your body to cold, and you'll activate brown fat, which changes unhealthy white fat that collects around your belly, butt, and hips into beige fat, allowing it to burn calories for heat.
Researchers at Northern Arizona University also discovered in an animal study that training in cold weather for a few months increases your VO2 max (a measure of how fit you are) and running speed. "For most exercise situations, cold is safer and more accommodating than the summer's heat," explains Bill Brewer, the director of exercise science at the Rochester Institute of Technology in New York. "And it lets people work harder and longer."
Best of all, you can get these benefits without feeling as if you're a snowman throughout your session. Here's how to keep comfortable so you can focus on getting a killer workout. (Are you a runner? Try these Cold Weather Running Tips from Elite Marathoners.)