Hot temperatures will help your body burn more calories.
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The relationship between your body's metabolism and outside temperatures is complex. Although it may seem like your metabolism slows in the summer, this actually isn't the case. Warm weather can actually create a slight increase in your metabolism.
In the summertime you're likely to spend more time outdoors enjoying the sunshine. Just 10 minutes in the sun during the middle of the day, if you're fair-skinned, will give you far more vitamin D than your recommended daily allowance. Vitamin D gives a boost to your metabolism by preserving muscle tissue and decreasing fat accumulation. In fact, women with low levels of vitamin D gain more weight than their healthier counterparts, according to a Kaiser Permanente study published in the "Journal of Women's Health" in 2012.
The warmer the temperatures, the more energy your body has to expend in order to cool you down. During the summer, you'll burn more calories just by your body trying to cool off. This is why people who live in tropical climates tend to have as much as 20 percent higher resting metabolic rates than people who live in milder climates.
Your metabolism may appear to increase if you exercise outside in the heat during the summer. During a cardio workout, your heart will have to expend more energy to bring oxygen to your muscles. It also has to work harder to send blood through your body. But the extra calories you're burning won't make a huge difference. The calories will be enough for a bite of ice cream or a bite of a candy bar, but not much more.
If you think warmer weather is causing your metabolism to slow down, it's probably just a result of other side effects you're having from the warm temperatures. For example, people who exercise outside are more likely to fatigue early, slow down and even cut their exercise shorter during times when the temperatures are extra hot. Although your metabolism isn't slowing down, this does mean you will burn fewer calories due to less exertion, which can result in your losing less weight overall.