Pediatrician Amanda Leonard, a pediatric sports nutritionist at The Johns Hopkins Children's Center in Baltimore, says doctors should always ask young athletes whether they take dietary supplements. Creatine, for example, can aggravate pre-existing kidney problems.

"I always remind parents -- children and teens the focus should be optimal health, not optimal performance," Leonard says in a statement. "With optimal health, comes optimal performance. It really is that simple."

A healthy diet rich in fruits and vegetables plus a daily multivitamin should provide all the nutrition an active growing body needs. However, endurance training -- such as long-distance running -- requires more calories from both carbohydrates and protein and strength training increases the body's need for protein but beware of overdoing. For instance, too much protein can cause dehydration as well overload kidneys.

Dehydration is a concern for children playing sports -- especially in hot weather -- and can by guarded against by drinking 4 to 8 ounces of water before exercise, 4 ounces every 15 minutes during exercise and 15 to 24 ounces for every pound lost after exercise.