The body responds to a calorie deficit by slowing down the metabolism and burning muscle tissue. That leads to weakness, sluggishness, slow times. In girls, it can also result in cessation of menstrual periods, which in turn leads to loss of bone density and frequent stress fractures.

Cold is not without its risks to runners, of course, especially ones who don't head south when winter visits their neighborhood. Even pooh-pooh-ers of frozen lungs and lovers of dark jogs over permafrost have been known to be careful about certain hazards.

For most teenage runners, the right foods means a varied diet, decreasing the amount of fat found in the typical American diet and replacing those calories with carbohydrates. Avoid saturated fats, such as those found in fried foods, and eat plenty of fresh fruits and vegetables.

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