Calcium is the major component of bone, providing structural skeletal support to the human body (see 00033). The approximately 2-3 kg of bone calcium in each person also provides a storage reservoir for the small percentage of ionized calcium that allows muscle to contract, nerves to communicate, enzymes to function, and cells to react. The body has developed several hormonal mechanisms, including vitamin D, parathyroid hormone, and calcitonin, to protect the small amount of ionized calcium in the blood from changing drastically. Tight control of blood calcium levels is needed because unduly low blood calcium might result in uncontrolled tetanic muscle contractions and seizures, while high blood calcium levels may cause kidney stones and muscle calcifications. To increase blood calcium levels, vitamin D and its metabolites increase calcium absorption from the intestinal tract, parathyroid hormone increases calcium reabsorption from the kidney, and both increase resorption of calcium from the bone.
During the early years of life, calcium is deposited in the bone as it grows, but after about the 3rd decade, there is a steady decline in bone calcium. This is especially marked after menopause in women, when estrogen declines, and often leads to bone loss (osteopenia) to below a threshold that predisposes women in particular to fractures (osteoporosis). Osteoporosis is not just a disease of the elderly, and may occur in much younger patients, especially athletic young women, those with anorexia nervosa, those on steroids and other medications, and in anyone on prolonged bed rest, including astronauts experiencing long periods of weightlessness.
Dietary calcium is often seen as the most limiting factor in the development of peak bone mass, and strategies to increase dietary calcium have been promoted. Other factors in the development of bone mineral include height, weight, racial background and inheritance, gender, activity, vitamin D deficiency, parathyroid hormone deficiency, vitamin A, vitamin K, growth hormone, calcium, phosphorus, and magnesium. Phosphorus, the other major component of bone mineral, is relatively common in the diet.
In the 1997 DRIs, Als of calcium were raised from 800 to 1300 mg in 9-18 year olds. Only a small percentage of the population takes in the RDA for calcium. The estimated average calcium intake in American women is only about 500-600 mg a day, and is much lower in the developing world (as low as 200 mg a day). From calcium tracer studies performed since the 1950s, intestinal calcium absorption ranges from 10% to 40% of ingested calcium, with a higher percentage absorption with lower calcium intakes. A large percentage (usually 7080%) of dietary calcium is from milk and dairy products, which provides about 250 mg calcium per 8 oz (240 ml) glass of milk, and most studies show better absorption from dairy products than from vegetable sources. However, many people, especially non-Caucasians, develop relative lactose intolerance after childhood, and are reluctant to increase their dairy food intake.
Thus, attention has focused on whether supplementation or fortification with calcium, especially during adolescence, will ensure achievement of peak bone mass. Calcium supplementation in adolescent females has shown short-term increases in bone mineral density, but this may be because it increases mineralization in a limited amount of tra-becular bone, and it remains to be seen whether this leads to long-term improvement or protection against future fractures. Also, most studies still assume that increased bone mineral density is synonymous with reduced fracture risk, although fractures may depend on many other factors such as optimal bone architecture and lack of falls. Although the majority of scientific opinion probably favors increased dietary calcium intake in adolescence, the factors that control bone mineralization are not yet completely understood, and long-term protection against eventual bone loss and fractures remains to be demonstrated by randomized clinical trials.
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