Working out when you're cold isn't recommended, though it has nothing to do with fat burning. It's really because you can sustain an injury to your muscles or joints if they're not properly warmed up before you start pushing them through a challenging workout. Working out when your body is cold can still burn fat, and the amount that's burned can even increase, depending on how cold you are.
How Cold Are You?
When your body gets cold, it's first reaction is to send warm blood inward, to your organs. This level of cold doesn't in itself require much energy or effort. However, when you're cold enough to shiver, your body starts burning calories through the muscle contractions that make you shake and tremble. Your muscles try to generate heat to keep your body from becoming colder, and that heat generation requires burning fuel, or calories. Julia Valentour, M.S., says in her article for The American Council on Exercise that shivering can burn approximately 400 calories per hour -- but you need to continue shivering in order to burn those calories.
Exercise Heats You Up
If you're cold when you start to work out, whether it's enough to make you shiver or just enough to make you feel a little uncomfortable, you won't stay chilled for long. Exercising raises your body temperature through the energy you use to lift weights, pedal the stationary bike or run a few miles. When your heart starts pumping, the blood courses through your body, carrying heat to all parts and chasing away the cold.
More Effective Than Exercising Cold
It probably seems counterintuitive, but working out when you're warm or even hot is more effective for burning fat than exercising when your body is cold. Your cardiovascular system has to make more of an effort to keep you cooled down than it does to heat you up. So when your body is already warm and you heat it up with exercise, your heart will have to pump more blood to your skin to encourage you to sweat, using up more fuel than if you were working out cold.
Warm Up First
When the whole goal is to burn fat, knowing that you'll increase what you burn if you don't work out cold is good reason to warm up before you start exercising. That is, of course, in addition to the safety aspects of preparing your body for a workout to avoid injuring yourself. Many people equate stretching before a workout with warming up, but that's not accurate and isn't even safe. You should never stretch cold muscles. Instead, jog in place or jump rope for five to 10 minutes before you do anything else to get the blood flowing to your muscles and joints. Then you can perform your stretches and flex and rotate all your joints before diving into your workout.
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