# Which Uses More Energy, Exercising Hot Or Cold?

**URL:** <https://boards.straightdope.com/t/which-uses-more-energy-exercising-hot-or-cold/347670>\
**Category:** Factual Questions\
**Created:** [March 9, 2006, 8:42am UTC](https://boards.straightdope.com/t/which-uses-more-energy-exercising-hot-or-cold/347670 "2006-03-09T08:42:41Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Scissorjack](https://avatars.discourse-cdn.com/v4/letter/s/c67d28/32.png) [@Scissorjack](https://boards.straightdope.com/u/Scissorjack)\
**Post date:** [March 9, 2006, 8:42am UTC](https://boards.straightdope.com/t/which-uses-more-energy-exercising-hot-or-cold/347670/1 "2006-03-09T08:42:41Z")

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This question occurred to me as I was dutifully on my nightly run, now that summer’s sliding into autumn and the evenings are getting nippy. Assuming that all other factors such as speed and distance are constant, which burns up more energy, exercising in hot or in cold weather? My run is about 10 km, which conveniently takes about an hour: so which uses more joules, running 10 km/h for one hour in 20 degree heat or running the same speed and time in 10 degree heat?

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**Author:** ![Sublight](https://avatars.discourse-cdn.com/v4/letter/s/439d5e/32.png) [@Sublight](https://boards.straightdope.com/u/Sublight)\
**Post date:** [March 9, 2006, 9:30am UTC](https://boards.straightdope.com/t/which-uses-more-energy-exercising-hot-or-cold/347670/2 "2006-03-09T09:30:17Z")

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I dunno, but Cecil did have [this to say](http://www.straightdope.com/classics/a2_313.html) (emphasis added):

> [@](#):
>
> Now for the specifics. You do burn up calories faster when it’s cold, although after a while your body adapts and the effect is reduced. Not that it matters much. Unless you exercise at the same time, you’re not going to be burning the right calories. When your metabolism is in “rest” mode, you burn mostly glucose, the simple sugar that’s found in your blood. Your fat cells remain untouched, and you don’t lose any weight. But when you engage in sustained, strenuous (i.e., aerobic) exercise, your system shifts gears and you start metabolizing body fat. This typically occurs about 15 or 20 minutes into your workout, when you experience the familiar phenomenon known as “hitting your stride.” Suddenly, things seem easier. That’s because fat is a more efficient fuel–it provides 18 times more energy than glucose. **Working out in the cold (within safe limits, of course) will burn even more fat, although curiously enough so will working out in the heat–your body uses up a lot of calories trying to keep you cool, too.** Once fat metabolism gets started, it takes at least an hour, and maybe as long as four or five hours, before your body switches back to “rest” mode. So you can safely eat a big meal after working out and be confident you’ll burn it off pretty fast.

If I had to guess, I’d say working out in 10 degrees would burn more calories than 20 degrees, but 30 degrees might burn more, too. I don’t know the actual numbers.