Fartlek, or speed play, is variable-pace running that emphasizes creativity. During a 30-minute run, choose objects to run to - telephone poles, trees, buildings, other runners, whatever. Make choices that mark off different distances, so your pickups vary in length from 15 to 90 seconds, and modify your pace to match the distance.

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.

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.

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