Calcium (Ca) is essential to almost every function of the body. Most of these, such as blood clotting and muscle contraction, require only trace amounts. Large amounts of calcium are needed to make bones (which are 18% calcium), so substantial amounts of calcium are needed in the diet during infancy and childhood.
The level of calcium (as its ion Ca2+) circulating in the blood is tightly controlled by:Aging humans lose calcium from their bones so that in time they become fragile, a condition known as osteoporosis. Osteoporosis is estimated to cause some 1.5 million fractures each year in the U. S. One in every three women who survive to age 81 will have suffered a hip fracture because of osteoporosis. (The rate for men of the same age is half that, presumably because men have sturdier bones to begin with.)
Largely because of this growing public health problem, the sale of over-the-counter calcium supplements has become a major growth industry. Most of this may represent money down (quite literally) the drain. Much of the calcium in supplements fails to be absorbed and passes out in the feces. The calcium in the most popular supplements is in the form of calcium carbonate. Limestone and marble are calcium carbonate and some calcium supplements are, in fact, made by powdering limestone (and dolomite, the mineral that gives its name to the mountains of northeastern Italy). The calcium in some other brands comes from powdered oyster shells.
Calcium carbonate is insoluble in water. It is, however, attacked by acid and the hope is that while passing through the stomach, the gastric juice will convert it into soluble forms that can be absorbed.
Much of the calcium that does get absorbed into the blood then travels to the kidneys where it is excreted in the urine (more money down the drain). There really is little to indicate that these supplements halt, let alone reverse, osteoporosis. Perhaps ingesting extra calcium - especially if in a soluble form (milk, milk products, or supplements containing calcium gluconate or calcium lactate) - can slow the inevitable loss of calcium from aging bones.
For women after menopause, treatment with estrogens seems to be the most effective way to avoid osteoporosis. Many physicians feel that any risks of estrogen therapy (e.g., a possible increase in the risk of uterine cancer) are less than the clear hazards of osteoporosis. Time should tell.
In the meantime, those of you who are still young should be sure to get your recommended daily allowance (RDA) of 1000 mg of calcium, preferably in your food. Exercise as well. It might help you develop bone mass and even if it doesn't, it will be rewarding in other ways.
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