When you step outside for that cold-weather run, be aware that some major changes are going on inside your body—some good, and others hardly so.

All that shivering from freezing temps actually increases the amount of brown fat in your body—which is great news for your metabolism, researcher Barbara Cannon, PhD, told Time. While brown fat burns energy to keep you warm, white fat stores energy from food. Since shivering activates the brown stuff, you end up burning extra calories.

While brown fat burns energy to keep you warm, white fat stores energy from food. Since shivering activates the brown stuff, you end up burning extra calories.

Not-so-minor detail: The cold is not as beneficial for your heart. In fact, it could even lead to heart attacks and strokes.

“When exposed to cold, the body tries to prevent heat loss by shrinking blood vessels, so you get increased blood pressure and heart rate,” said Shingo Kajimura, PhD. “This is why there are so many 911 calls at 3 a.m. in the middle of winter. Older people get up to go to the bathroom, and when they step out on the cold floor, that stimulates blood vessel constriction and stroke.”

So sure, you might see some metabolic benefits to hanging out in the cold—but when it comes to your pretty-vital-to-your-life heart, you’re probably better off staying nice and cozy in your overly fuzzy, hygge-approved socks.

Use this winter-weather map to plan your healthy travel. Or, stock up on these natural beauty products that are perfect for cold weather.