Exercise and adequate calcium both influence bone mass. Smoking, unhealthy eating patterns, and alcohol use detract from bone mass. Excessive salt intake may increase the amount of calcium lost in the urine, and therefore, increase the body's need for calcium. Scientists agree that diets deficient in calcium during childhood and adolescence contribute to the development of osteoporosis, which is not visible until late in life. But it is during the teen years that optimal calcium intake is most important.
Bones grow and incorporate calcium most rapidly then.
By the age of 21 or soon after, calcium is no longer added to bones and a few years later, a steady process of loss of calcium from bones begins. Genetically, people differ in how much calcium is in their bones when they reach maturity, but how much calcium they eat while they are growing has an important influence. The more calcium that is in the bones when loss begins, the longer it will take before the bones become fragile and fracture easily. It is becoming increasingly evident that adequate calcium intake is critical during adolescent years. Adolescents and young adults, ages 9-18, whose bones are growing very fast, need more calcium.