The 'good nutrition' part of this equation involves eating a wide variety of foods that are high in complex carbohydrates, moderate in protein, and relatively low in fat. The purpose of eating a wide variety of foods is to assure that the athlete is exposed to all the nutrients critical to health and performance. One of the nutrients we're concerned about is calcium, because it is needed for strong bones. Inadequate calcium intake is associated with weak bones that are more prone to development of stress fractures, an injury all too common in gymnastics. But having enough calcium by itself is not enough.
Vitamin D is needed to assure that the calcium being consumed is properly absorbed into the body.
Calcium, besides its well-known importance in bone health, is also an important component of the blood. Vitamin D controls the way the body uses calcium by assuring that the blood level of calcium stays constant. If dietary calcium is inadequate, then calcium is taken from the bones to keep the blood level constant, and if dietary calcium is adequate, then vitamin D increases the absorption of calcium and puts more in the blood. The extra calcium in the blood is used to build bones. This is especially important when bones are developing and growing during childhood and young adulthood.