BOSTON — For years, doctors have been telling older women to take calcium and vitamin D tablets to protect their bones as they age.

Now, the biggest study ever to examine the value of that timeworn advice suggests the supplements convey only limited protection. Calcium and vitamin D failed to protect against most fractures in the mostly low-risk women.

At the same time, the supplements did seem to offer some benefit against hip breaks among women over 60 and those who took the pills faithfully.

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.

Even so, he and other experts are urging women to stick with government advice to keep taking the supplements anyway. "We don't want to send the message to people to throw away their calcium pills, which was my wife's first reaction," Lasser said.

"We still do believe ... that maintaining an adequate calcium intake will lay the foundation for bone health," added lead author Dr. Rebecca Jackson at Ohio State University.

The findings, published today in the New England Journal of Medicine, were a long-awaited offshoot of the big national study of diet and hormone therapy known as the Women's Health Initiative.

The outcome affects 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.

For women over age 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, which probably further cut 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."