When people start popping Tums as a prevention against brittle bones and "The Calcium Bible" becomes a Book-of-the-Month Club selection, it's clear that calcium has become a topic of great interest. With good reason, since calcium deficiency is widespread in the United States today.

According to the National Institutes of Health, the average daily intake of dietary calcium is about 450 to 550 milligrams -- well below the National Research Council's recommended dietary allowance of 800 milligrams. Other government studies have shown that up to 50 percent of men age 18 to 34 and more than 70 percent of all adult women have inadequate calcium intake.

Osteoporosis is one of the primary results of this deficiency of dietary calcium.

A disorder in which the bones lose calcium and become brittle, osteoporosis affects as many as 20 million people in this country, causing more than 1 million fractures a year in people over 45 and costing an estimated $3.8 billion annually. Eight times more common in women than men, osteoporosis causes pain, disability and from 15,000 to 30,000 deaths a year.

Blood pressure also seems to be affected by calcium. Recent studies have shown that people with high blood pressure (hypertension) tend to have lower calcium intake than people with normal blood pressure. Some people with hypertension have an abnormally high level of calcium excretion.

Women with osteoporosis also have high blood pressure two to three times as often as other women. And some researchers believe that increasing the amount of calcium in the diet may actually protect people from hypertension.

Low calcium intake has also been linked to loss of the jaw bone, which holds the teeth, resulting in gum disease, improperly fitting dentures and poor dental health.

Widespread calcium deficiency has been recognized for some time, says Patricia Housman, author of "The Calcium Bible" (Rawson Associates, $13.95), but it took an April 1984 NIH Consensus Conference on Osteoporosis to really focus national attention on the problem. That conference, says Housman, was the "watershed event" that stimulated much of the heightened concern about calcium in the diet.

Ninety-nine percent of the body's calcium is found in bones and teeth, while the remaining 1 percent regulates such functions as normal heartbeat, blood clotting, nerve conduction, muscle contraction, hormone secretion and activation of certain enzymes.

Eating the proper amount of calcium is important because less than 40 percent of calcium intake is absorbed from the intestinal tract. And as the body works to keep a constant level of calcium in the blood, calcium can be taken or "leeched" from the bones if there is not enough in the diet.

Contrary to popular belief, bone growth continues into adulthood. Adolescence brings a big growth spurt, but peak bone mass is not achieved until about age 30 to 35.

Many adults don't realize that their bones are living tissues that are constantly being broken down and reformed. This process, called remodeling, is a cycle that lasts three to four months and continues throughout life.

After 35 however, more bone is broken down than is formed, causing loss of bone mass. Women start out with less bone mass than men, and then lose bone earlier and more rapidly. After menopause, bone loss accelerates so quickly that an average woman in her seventies has lost half her bone mass while a man the same age suffers only about 14 percent loss.

Adult women have a higher requirement for calcium than the recommended dietary allowance (RDA) for the other groups. According to the NIH conference, premenopausal and estrogen-treated women require about 1,000 milligrams of calcium, while postmenopausal women who are not treated with estrogen should have 1,500 milligrams of calcium in their daily diet.

Men over 18 need 800 milligrams a day, according to the National Research Council. As they age, they do not lose as much bone mass as women.

The NIH panel concluded that "it seems likely that an increase in calcium intake to 1,000-1,500 mg a day beginning well before menopause will reduce the incidence of osteoporosis in postmenopausal women. Increased calcium intake may prevent age-related bone loss in men as well."

While inadequate calcium in the diet is the primary cause of osteoporosis, some less well known "calcium culprits" can adversely affect calcium balance.

*Vitamin D deficiency decreases the absorption of calcium in the intestine. Supplements of vitamin D are usually not advised since most people can obtain the 400 international units needed each day from sunshine or from fortified milk.

*Excess protein in the diet can be a silent calcium "robber." Studies have shown that a protein intake over 56 grams for men and 44 grams for women increases the excretion of calcium in the urine. Too much meat can mean extra bone loss.

*Caffeine is another calcium culprit. While "safe" limits of coffee are constantly debated, it is known that people who drink large amounts of coffee lose more calcium than people who do not drink coffee.

*High alcohol intake can also affect calcium balance by decreasing the absorption of calcium in the intestine. Again, "safe" limits are controversial, but it's best to avoid taking calcium supplements with alcoholic beverages and to limit your alcohol intake to moderate amounts.

*Smoking has been associated with increased loss of bone, although there is no direct evidence that smoking causes bone loss.

While all of these things can rob calcium from the body and begin the process that will cause osteoporosis in later life, adjustments in the diet or a calcium supplement seem to be able to prevent, or at least slow down, osteoporosis. But preventive actions must be taken decades before the disease appears.