Defying years of conventional medical wisdom, researchers report that calcium and vitamin D pills regularly consumed by millions of woman provide limited protection from broken bones.
The supplements seemed to reduce the risk of hip fractures in women over 60 who faithfully took them. But the pills did not reduce spine or wrist fractures, and increased the risk of kidney stones, researchers said.
The study, published today in the New England Journal of Medicine, raises questions about the benefits of taking supplements to prevent osteoporosis, a condition marked by weakened bones and a heightened risk of fractures that commonly afflicts women after menopause.
Concerns about the disease have fueled sales of calcium supplements to the tune of nearly $1 billion year.
Despite the study's weak support for calcium supplements, most researchers continued to endorse taking them if needed to meet federal guidelines for calcium and vitamin D.
"It's like the old expression: It couldn't hurt," said Dr. Norman Lasser of the University of Medicine and Dentistry in New Jersey and one of the study's authors.
He acknowledged that the report "couldn't be called a ringing endorsement" of calcium.
Federal guidelines recommend that women over 50 consume 400 units of vitamin D and 1,200 milligrams of calcium daily, equivalent to four 8-ounce glasses of milk. Some studies have found that as few as 5 percent of older women now meet those requirements.
The $18 million study was part of the Women's Health Initiative, a federal study that a few years ago showed hormone treatment after menopause conferred more risks than benefits. Last week, the study challenged the benefits of low-fat diets.
The calcium study tracked 36,000 women for seven years. Half of the women received 1,000 milligrams of calcium and 400 units of vitamin D daily. The rest received a placebo.
Researchers expected a positive result because calcium and vitamin D, which aids the absorption of calcium, have long been assumed to increase bone strength.
They were surprised when the results showed no significant difference. Those taking supplements experienced hip fractures at a rate of 14 of every 10,000 women compared to a rate of 16 of 10,000 for those taking placebos.
Only when researchers parsed the data to examine a subgroup of women - those over 60 who were the most diligent taking their pills - did they find some evidence that the supplements could prevent hip fractures.
Those taking supplements had hip fractures at a rate of 10 of 10,000 women compared to a rate of 14 of 10,000 for women in the placebo group.
Gauging the importance of the results was difficult. Researchers said one problem was that all the women were taking relatively high amounts of calcium in their diets to begin with.
What's more, women in the placebo group who had been taking calcium pills before entering the study were allowed to continue, potentially improving their bone health.
Another problem, researchers said, was that a high percentage of women had trouble sticking to the supplement regimen, which required them to take pills twice each day. By the end of the study, just 59 percent of the participants were faithfully taking their pills.
Dr. Joel S. Finkelstein, an endocrinologist at Massachusetts General Hospital in Boston, said in an editorial accompanying the study that the supplements also might have shown greater benefit if the women received higher dosages of vitamin D.
Given the difficulties, "it was a miracle we got the results we did," said Lasser.
Dr. Elizabeth Nabel, director of the National Heart, Lung and Blood Institute, said the results strongly suggested that women over 60 follow government recommendations for calcium and vitamin D to maintain healthy bones. Women should take supplements only if they cannot get enough calcium through their diets and vitamin D through exposure to sunlight, she said.
The decision for women in their 50s was less clear.
Marcia Stefanick, a researcher at the Stanford University School of Medicine and one of the study's authors, said healthy woman in their 50s with a good diet don't need to take the pills. She said 800 milligrams of calcium daily - the amount most women on a placebo consumed - appeared adequate for middle-aged women.
In a related study, researchers said calcium also offered no protective effects from colon cancer.
It was possible that the seven-year study did not last long enough to see an effect because cancer can take many years to develop, researchers said.
Denise Gellene writes for the Los Angeles Times.