The biggest study ever to examine the value of the supplements suggests they convey only limited protection against broken bones. They failed to protect against most fractures in the mostly low-risk women, but seemed to offer some benefit against hip breaks among women over 60 and those who took the pills most faithfully.
The outcome could affect an enormous number of people, since an estimated 10 million Americans have break-prone bones thanks to osteoporosis. One of two women will suffer such a fracture in her lifetime.
Doctors, who have long taken the value of these supplements almost as an article of faith, tried to put the findings as positively as possible.
But some disappointment seeped out at the margins. The study is "not as ringing an endorsement of calcium as one might like," said one of the researchers, Dr. Norman Lasser at New Jersey Medical School.
The study's findings were published Thursday in The New England Journal of Medicine. They were a long-awaited offshoot of the big national study of diet and hormone therapy known as the Women's Health Initiative.
For women older than 50, federal guidelines recommend 1,200 milligrams of bone-building calcium and 400-600 international units of vitamin D daily from diet and, if needed, supplements.
The seven-year study of 36,282 women ages 50 to 79 gave half the participants 1,000 milligrams of calcium and 400 units of vitamin D, while the other half took dummy pills.
However, many were also taking their own supplements before the research began, and they were allowed to keep doing so, whether they were assigned to the test group or the comparison group. These extra supplements may have helped the women stay healthy but ironically diluted the findings, since any benefit is harder to show against a backdrop of fewer fractures. Also, women in the study were taking hormone pills, likely further cutting the number of fractures.
The study showed better hip bone density in the group given supplements, but they ranked no better statistically in avoiding fractures of all kinds.
However, some benefit seemed apparent. Women over age 60 reduced their chances of hip fracture by 21 percent with the supplements. And those who took their supplements most regularly lowered their risk by 29 percent.
"There's probably a small benefit," said Dr. Joel Finkelstein, of Massachusetts General Hospital, who wrote an editorial to accompany the study. "It's a good start, but women at higher risk need to know it's not enough."
Many experts downplayed the meaning of the negative finding. Dr. Bess Dawson-Hughes, a Tufts University vitamin expert who helped shape the dietary guidelines, said they should remain unchanged for now.
"You put people who don't need it together with people who aren't taking it, and you find nothing — and that really isn't all that surprising," she said.