1. Heredity—each person has a genetic potential for height, that is, a maximum height, with genes inherited from both parents. Many genes are involved, and their interactions are not well understood. Some of these genes are probably those for the enzymes involved in cartilage and bone production, for this is how bones grow.
2. Nutrition—nutrients are the raw materials of which bones are made. Calcium, phosphorus, and protein become part of the bone matrix itself. Vitamin D is needed for the efficient absorption of calcium and phosphorus by the small intestine. Vitamins A and C do not become part of bone but are necessary for the process of bone matrix formation (ossification). Without these and other nutrients, bones cannot grow properly. Children who are malnourished grow very slowly and may not reach their genetic potential for height.
3. Hormones—endocrine glands produce hormones that stimulate specific effects in certain cells. Several hormones make important contributions to bone growth and maintenance. These include growth hormone, thyroxine, parathyroid hormone, and insulin, which help regulate cell division, protein synthesis, calcium metabolism, and energy production. The sex hormones estrogen or testosterone help bring about the cessation of bone growth. The hormones and their specific functions are listed in Table.4. Exercise or “stress”—for bones, exercise means bearing weight, which is just what bones are specialized to do. Without this stress (which is normal), bones will lose calcium faster than it is replaced. Exercise need not be strenuous; it can be as simple as the walking involved in everyday activities. Bones that do not get this exercise, such as those of patients confined to bed, will become thinner and more fragile.