Today, calcium has become the "in" nutrient. It is being added to cereals, juice and even milk. Old antacid pills have suddenly become new successful calcium pills. Calcium fortification of food and direct calcium supplementation in tablet form have become a multimillion-dollar business.

Health-conscious American women are consuming calcium to protect their bones. Many men and some women consume calcium to lower their blood pressure or to prevent it from getting higher than normal. Some people are taking calcium supplements to lower their risk for cancer.

Is calcium the new wonder nutrient? Or is all this just a fad created by the advertising industry to sell a new product? The truth, of course, lies in between. ::

Ninety-five percent of the calcium in the body is contained within bone. All of the bones within our bodies serve two major functions. They form the solid structure that allows us to bear weight and to perform the thousands of functions that require the support of our skeleton, and they contain a vast reservoir of calcium available to the cells of every other organ of the body. Every cell in the body requires calcium to function.

Without calcium, nerve cells would not transmit impulses, muscle cells would not contract, the heart would not beat and the kidneys would not exorcise waste products.

Without calcium, life is impossible, and therefore the amount reaching and leaving each cell is carefully regulated. The gastrointestinal tract plays a role by regulating the amount of calcium absorbed by the body. The kidney plays a role by producing a hormone from Vitamin D and by excreting more or less calcium. The parathyroid glands play a role by sensing the amount of calcium in our blood and regulating that amount between narrow limits.

But the most important role in maintaining a constant calcium supply to the tissues is played by our bones. Calcium molecules flow into and out of our bones in response to the needs of our body. All through our growing years, the net flow of calcium is into our bones. They grow larger and stronger and, of course, they contain more calcium.

As a woman reaches her thirties and a man his late forties or fifties, the net flow of calcium is out of bone. Thus, calcium is actually lost from the body.

When a woman reaches menopause, calcium begins to be lost much more rapidly because of a decline in estrogen -- a hormone that, among other things, helps maintain bone integrity by keeping calcium in place. Thus, the rapid bone loss that leads to osteoporosis in many older women is because of lack of estrogen at the beginning of menopause.

Osteoporosis occurs when bone becomes brittle because too much calcium has been lost. Whether or not this will occur depends on a number of factors. First and foremost is the amount of bone you start with. The more money you have in the bank when things go wrong, the more you can withdraw without becoming bankrupt. Therefore, large women get less osteoporosis than small women. Black women get less osteoporosis than white women because they are bigger-boned.

The amount of weight-bearing exercise affects the rate of calcium loss of bone. Heavy women are less prone to osteoporosis than slim women because their bones bear more weight.

Certain life-style practices can also deplete calcium from bone. Both alcohol use and smoking fall into this category.

Finally, the number of times a woman has been pregnant (the more often, the lower her risk) and the time of menopause (the earlier, the higher her risk) are very important factors.

Even the consumption of certain drugs for medical purposes can increase a woman's risk of osteoporosis. Cortisone, a drug that is used in the treatment of severe asthma and other illnesses, causes rapid bone loss.

So far, I have not mentioned calcium intake. Of course, it too plays a role, but perhaps not as great a role as recent publicity would have us think.

During the growing years, calcium is most important. If calcium intake is inadequate, less bone will be formed and the reserves will not have reached their limit before the long period of bone loss sets in. Thus, infants, children, adolescents and pregnant and lactating women should consume a diet rich in calcium. It is during these periods of a woman's life that calcium supplements can do most good.

After menopause, the problem usually is not too little calcium but rather too little estrogen.

To be sure, if your diet is deficient in calcium it will aggravate the situation. Thus, a dietary calcium deficiency in a post-menopausal woman should be corrected to prevent her bone loss from being aggravated. And in the woman who already has osteoporosis, calcium supplements may slow progression somewhat but will not cure the disease. ::

Recently, research has revealed another important role of calcium in maintaining good health. Studies of large populations within the United States as well as abroad have associated low calcium intake with high blood pressure.

Calcium supplementation has been shown to prevent high blood pressure in strains of animals genetically prone to this condition.

Clinical trials using calcium supplements have lowered blood pressure in a significant number of people who were already suffering from high blood pressure.

How might calcium work in lowering high blood pressure? Blood pressure is really the pressure exerted by the blood on the walls of the arteries. The more rigid the artery, the higher the overall pressure (both systolic and diastolic).

Calcium causes relaxation of the smooth muscle cells within the wall of the arteries, and the most prevalent current thinking is that this may be the mechanism by which calcium may lower blood pressure.

The data are too limited at present to be sure just how important calcium supplementation may prove to be in the prevention and treatment of high blood pressure. My own opinion is that the data are very impressive and that ultimately calcium supplementation may play a more important role in the control of hypertension (high blood pressure) than it will in the control of osteoporosis.

Some studies have related cancer of the colon to low calcium intake, and animal studies are currently investigating this relationship. Some studies in humans are investigating the effect of calcium supplementation on the cells of the gastrointestinal tract. The research is still in the early stage, and no conclusions are yet warranted. At present, I see no reason for calcium supplementation in an attempt to prevent colon cancer. ::

Should you take a calcium supplement? To decide, you must first determine how much calcium you are getting from your food. If you eat generous amounts of dairy products, you probably don't need a supplement. If you do not and you fall into a high-risk group for osteoporosis or hypertension, you might wish to take a supplement. If you do, calcium carbonate or citrate (500 to 1,000 milligrams of calcium per day) are the best sources.

Myron Winick, MD, is the Williams professor of nutrition at the College of Physicians and Surgeons, Columbia University, New York.