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For a quality workout, at what temperature should you run on a treadmill or brave the cold?
—John Kortman, Indianapolis, IN
Cold weather brings a number of risks, from exposure to respiratory ailments and reduced performance. According to the National Weather Service, uncovered skin can get frostbite in 30 minutes or less when the wind chill is -15 degrees or below. However, there are no hard-and-fast rules for frostbite risks at different temperatures—risk depends on each person’s physiology and starting core temperature, among other things. Fingers and feet are especially vulnerable.
Fortunately, your lungs aren’t in danger of freezing. By the time air reaches your lungs, it’ll be warm enough to breathe safely. However, due to its dryness, cold air can irritate the windpipe, especially for people who aren’t acclimated to frigid air. Even for perfectly healthy people, air at temperatures below 0 can contribute to bronchoconstriction.
In addition to bronchoconstriction, cold can reduce performance due to reduced body temperature. Studies show that the legs fatigue more rapidly if body temperature is low, possibly due to less powerful muscle contractions. If your body is not generating enough heat to stay warm, you shouldn’t be running at all, let alone running hard.
Based on limited existing research and anecdotal evidence, I recommend indoor running when temperatures dip below -10 degrees wind chill. Of course, this is an individual decision, based on physiology and the strength of your distaste for treadmills.
Between 10 and -10 degrees wind chill, running outside is usually OK, unless you are particularly vulnerable to cold (like people with asthma). Just be sure to dress warmly, including gloves, a hat and multiple layers.
However, below 10 degrees wind chill, workouts are risky and less productive, due to bronchoconstriction and the restriction of wearing so many layers. To be safe, I recommend my athletes adhere to a threshold of 15 degrees wind chill for doing workouts outside. Above that threshold, train like normal (accepting that you’ll be a bit slower due to cold); below, run outdoors, but don’t do workouts, to avoid airway irritation; below -10 degrees with wind chill, hit the treadmill. However, if you live in Minnesota, you may need to change the thresholds a bit so you don’t miss running outside a third of the year.
Your body generates lots of heat while running, so if you’re smart and safe, you may be able to run in sub-zero conditions. But for quality workouts, be flexible and wait for warmer weather.
Do we burn more or less energy running in the cold versus running in the heat, and should there be any relative change to our calorie intake?
—Pamela Ross, Berkeley, CA
If your body temperature drops during a run, you burn more calories as your body works to stay warm. However, after you warm up, you probably aren’t burning any more calories than usual. Your energy needs don’t change substantially unless you’re extra-cold, which likely means you have underdressed.
A 1991 article from the journal Sports Medicine titled “The Physiology of Exercising in the Cold” indicates that if you don’t dress warmly enough you’ll burn more carbohydrates relative to fat to stay warm.
All things being equal, you will likely burn a few more calories in cold weather. However, mid-run energy needs don’t change appreciably. Think about a hypothetical runner doing a long run at aerobic threshold (where fat and carbohydrate use is around even); the runner starts with 1,000 calories of stored carbohydrates and burns 400 calories of carbohydrates per hour (along with a similar amount of fat). Even if they now burn 500 calories of carbohydrates in winter, they still have a couple hours of stored energy. Plus, most people can only absorb 200 to 400 calories per hour during activity, so taking in more won’t make a huge difference.
As always, the golden rules of nutrition apply: find what works for your body, let hunger be your guide and maintain a general understanding of what is healthy for you based on your goals. Perhaps most importantly, remember that you still sweat in cold weather, especially if you dress well. Just as in hot temperatures, hydration is usually more important than fuel intake. Add some sports drink to the mix, and you’ll probably be made in the shade (or in the cold).
This article originally appeared in the March, 2018 issue of Trail Runner magazine. To get Trail Runner delivered right to your door, subscribe here.
David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. With Megan Roche, M.D., he hosts the Some Work, All Play podcast on running (and other things), and they wrote a book called The Happy Runner.
Have questions for Coach Roche? Email them to zrom@outsideinc.com