Calcium and magnesium are the "earth alkali" minerals found in the earth's crust, usually as salts that are fairly insoluble. (The word calcium comes from the Latin calc, meaning "lime," as in limestone, a calcium carbonate substance.) Dolomite, a calcium-magnesium earth mineral combination that is a little more soluble and usable by the body than some other forms, is a commonly used calcium supplement.

Many other nutrients, vitamin D, and certain hormones are important to calcium absorption, function, and metabolism. Phosphorus as well as calcium is needed for normal bones, as are magnesium, silicon, strontium, boron possibly, and the protein matrix-all part of our bone structure. The ratio of calcium to phosphorus in our bones is about 2.5:1; the best proportions of these minerals in our diet for proper metabolism are currently under question.

Calcium works with magnesium in its functions in the blood, nerves, muscles, and tissues, particularly in regulating heart and muscle contraction and nerve conduction. Vitamin D is needed for much calcium (and phosphorus) to be absorbed from the digestive tract. Along with parathyroid hormone and calcitonin secreted by the thyroid, vitamin D helps maintain normal blood calcium levels.

Maintaining a balanced blood calcium level is essential to life, especially for cardiac function. A normal blood calcium level is about 10 mg. percent-that is, about 10 mg. per 100 milliliters (ml.) of blood. Of that, approximately 5.5 mg. are in ionic form as Ca++, about 4 mg. are bound to carrier proteins, and about 0.5 mg. is combined with phosphate or citrate. If there is not enough calcium in the diet to maintain sufficient amounts of calcium in the blood, the parathyroid glands will be activated to release more parathyroid hormone (PTH), which will then draw calcium out of the bones as well as increase intestinal absorption of available calcium. So even though most of our body's calcium is in the bones, the blood and cellular concentrations of this mineral are maintained first. This is why, with nearly 30 percent of people in this country eating calcium-deficient diets, osteoporosis (a loss of bone substance) is so prevalent.

Elderly people usually have less calcium in their diets than others do, and calcium deficiency particularly affects postmenopausal women. But low dietary calcium is only one factor in the huge and complex topic of calcium bioavailability from foods, calcium absorption, and osteoporosis. Many factors are involved in making calcium available for its many essential functions. Vitamin D is, of course, most essential to calcium absorption, though this may be less necessary when the calcium chelates, such as calcium aspartate or calcium citrate, are used. Some clinical studies need to be done to see which calcium supplements are readily transported into the body and how vitamin D may affect them. Many doctors do not consider this important absorption issue and prescribe an oyster shell or a dolomite or bonemeal source as a calcium supplement. Frequently, calcium lactate or calcium carbonate (such as Tums) pills-which are more alkaline and slowly absorbed-are prescribed without suggesting additional vitamin D and magnesium, so important for calcium balance and metabolism. A woman who needs more calcium could be taking an extra gram a day without really getting much of it into her blood or bones.