Calcium has long been known to help prevent osteoporosis, the brittle-bone disease that afflicts many older Americans, especially women. Calcium is particularly helpful in girls and women under 30, whose bones are still forming, and in post-menopausal women, who tend to lose calcium from their bones. In one study, children who took a 700-mg. calcium supplement in addition to a calcium-rich diet had denser bones than those who ate the diet alone. Calcium supplements have also been found to reduce bone loss in post-menopausal women, especially those who get less than 400 mg. of calcium a day from food, reports Beth Dawson-Hughes, M.D., a calcium/osteoporosis researcher at Tufts University.