Fit or Fiction: Does Exercising in Hot Weather Burn More Calories?

Do you burn more calories when the weather's hot and you sweat a lot? -- Jane, Fair Lawn, N.J.

When you go out for a walk in 100-degree weather, don't confuse the sweat that pours off your body with fat melting. It's just water. You burn about the same amount of calories on a steamy, muggy day as you do in a comfortably air conditioned gym. If you step on the scale immediately after a workout in the hot sun, you will initially weigh less, but as soon as you rehydrate -- a fancy way of saying drink up -- you replace the water weight. You should be drinking to quench your thirst when you workout in warmer weather anyway; and you should be doing everything you can to keep your body as cool as possible to avoid heat-related illness.

Ditto for hot yoga classes. A super-heated room may offer some advantages in terms of muscle flexibility and suppleness, but you'll burn about the same number of calories as an equivalent class in a cooler environment. Don't be fooled by marketing that says otherwise.

As for exercising in a rubber or plastic suit for the purposes of losing weight (people are still doing this!) it's silly, ineffective and dangerous practice. I'll go so far as to say it can even be deadly. Besides increasing the chances of severe dehydration and causing rapid-onset heat stroke, the toxins from the suit can enter your blood stream and poison you. Just ask comedian Martin Lawrence. In 1999, he fell into a three-day coma after collapsing from heat exhaustion while jogging in 100-degree heat wearing a sauna-suit. He did recover, although there were reports that at one point his body temperature rose to a sizzling 107 degrees and he was on a respirator.

For the record, the only time extreme temperatures significantly rev up calorie burn is when it is so cold you start to shiver. Shivering is an involuntary clenching of muscles; its purpose is to generate heat and warm you up. When your teeth are chattering and every muscle in your body is tight and tense, you burn nearly four times more calories than usual. And when it's so cold that, in addition to shivering, you have to hop from foot to foot and rub your hands together to keep warm, you burn up to 400 additional calories per hour. That said, I certainly don't encourage you to wear a bikini in Iceland.

And since we're on the topic of working out in hot weather, this is a good time to share tips. I'll go first: Check out how to run in hot weather and exercising in the heat. Before I go out for the run in the summer, I wet down a bandanna and put it in the freezer for a few hours; I place it around my neck and as I run it melts and drips cool water all over me. Feels good! Got any other keep-cool workout tips you want to share? Post here or tweet me.