The gradual thinning of the bones that commonly occurs after menopause can be slowed significantly if women who ordinarily eat little calcium take a calcium supplement, according to a report published today.

While a barrage of advertising claims have suggested as much, solid evidence had been lacking. The study showed, however, that women who are already eating enough calcium in their diet do not benefit from taking a supplement.

American women in their forties, fifties and sixties consume an average of 475 milligrams of calcium a day, far less than the current recommended daily allowance (RDA) of 800 milligrams. Gradual bone loss, or osteoporosis, which often begins after menopause, can lead to debilitating fractures of the hip and spine.

The study compared two types of calcium supplements in 301 women between ages 40 and 70, whose diets ordinarily contained less than 650 milligrams of calcium a day. One type was calcium carbonate, the most commonly used supplement. The other was calcium citrate malate.

The two-year study, published in today's New England Journal of Medicine, found that daily calcium supplements appeared to help only those women whose diets contained very little calcium -- less than 400 milligrams a day -- and who had been through menopause more than five years earlier.

Women in that category who took a daily supplement of 500 milligrams of calcium citrate malate had significantly less bone loss from the spine, hip and wrist than those who took a placebo. A similar group who took 500 milligrams of calcium carbonate had less bone loss from the wrist, but they lost bone from the spine and hip at the same rate as the placebo group.

Women who had gone through menopause recently -- within five years of entering the study -- suffered significant bone loss from the spine whether or not they took dietary supplements. Women with intermediate calcium intakes, ranging between 400 and 650 milligrams a day, also did not appear to benefit from supplements.

Procter & Gamble, which paid about one-third of the study's $1.5 million cost, hired a New York public relations firm to trumpet the results to news organizations. The form of calcium that performed better in the study -- calcium citrate malate -- is commercially available only in Procter & Gamble's brand of orange juice called Citrus Hill Plus Calcium.

But researchers said that, rather than proving the superiority of that form of calcium, the study's main contribution is to show that calcium supplements can benefit some women, and that the benefit varies depending on the diet and how much time has elapsed since menopause.

"We chose the size of the supplement {500 milligrams} to bring them up to the level of the RDA," said Bess Dawson-Hughes, chief of the calcium and bone metabolism laboratory at the Agriculture Department's Human Nutrition Research Center at Tufts University, and the study's principal author. "When we did that, we saw a very substantial benefit to their bones."

She said her first advice to women who want to apply the findings would be "to change their diets so that they are having three servings of calcium-rich foods a day, every day." She added that a desire to cut down on dietary fat should not prompt women to cut out dairy products. "There are many dairy products that don't have a lot of fat," she said.

The entire recommended amount of calcium, for example, can be consumed in two cups of plain yogurt, or four ounces of cheddar cheese. Green leafy vegetables are also good sources. Three cups of cooked turnip greens or kale also supply the entire recommended amount.