And other top myths and truths about running in the cold.

Winter running breeds some interesting misconceptions, so we decided to get the straight story. We enlisted the help of one of the key scientists studying cold-weather workouts: John Castellani, Ph.D., research physiologist at the U.S. Army Research Institute of Environmental Medicine in Natick, Mass. See if you can separate the truths from the lies below—and stay warm and fit all winter.

Winter running burns more calories. True or False. Sorry, put the French fries down. Unless you’re running through snow or mud, you’re not burning any more calories than when you run in any other season. Sure, research shows that shivering and very heavy clothes do cause you to burn more calories. But by “heavy clothes,” researchers are talking about army boots and hiking gear, not your winter shell with titanium thermo-regulating technology.

Cold makes you pee more, so you’re more likely to get  dehydrated.
True or false. Well, the first half is true: Cold can create what researchers call cold-induced diuresis (CID), meaning you pee more when your body meets cold air or water. When your skin gets cold, blood is shunted away from your skin and redirected to your core. “With more blood in the thorax, the heart says, ‘I have too much fluid on board and need to get rid of some of it,’” says Castellani. But exercise, even at a moderate intensity, prevents CID.

Moving the blood to the core also makes your body think you have enough fluids on board. You need to be smart about replacing what you’re losing, but don’t go crazy: Unless you’re overdressed, you won’t need as much water as you would on a 90-degree day with 80 percent humidity.

Covering your head is the key to warmth. True or false. “You hear claims that you lose 50 to 80 percent of heat through your head,” says Castellani. “If that were true, you could put on a hat and run naked and you’d stay perfectly warm.” It’s true that blood vessels in your head don’t constrict when it gets cold, so you are continually losing heat through it. “But your head comprises only about 8 to 9 percent of the surface area of your body,” he says. Bottom line: “Your head is an extra avenue of heat loss that needs to be taken into account,” Castellani says.

You should do your warm-up inside. True or False. “If it’s really cold out, we suggest people try to warm up indoors and then head out,” says Castellani. “I like to define really cold as less than 10 degrees F.” And it’s smart to cool down inside, especially if you’re really sweating, so sweat doesn’t freeze. “Otherwise, warm-up and cool-down can proceed as normal.”

When it’s below 32 degrees F, you need three layers to stay warm. True or False. There’s no “always.” The number of layers varies widely from person to person, says Castellani. No matter how many you wear, they need to transport sweat from the skin to the air so it can evaporate. Water has 70 times the convective heat transfer of air, meaning that the same reason you want to be wet when you’re working out in the summer is the same reason you want to avoid it in the winter.


Did You Know?

Did You Know?