Despite what doctors have long recommended, regularly taking calcium and vitamin D does not prevent fractures in older people who have broken a bone or who have osteoporosis, according to two large studies released.

People with osteoporosis are often encouraged to consume as much calcium and vitamin D as possible to strengthen their bones and to lower the likelihood of injuries.

But the new studies, involving thousands of elderly people in Britain who had symptoms of the disease, found that those who took calcium and vitamin D tablets were just as likely to break a bone as those who took neither.

The authors of the reports, published in The Lancet and the British medical journal BMJ, said their findings suggested that for people with brittle bones, relying solely on the supplements might not be enough.

"Our trial indicates that routine supplementation with calcium and vitamin D3, either alone or in combination, is not effective in the prevention of further fractures in people who have had a recent low-trauma fracture," the authors of the Lancet study wrote.

Other experts called the findings important but cautioned that they did not apply to most people.

Just people older than 70, a majority of them women with previous bone injuries, were included in the studies.

Because vitamin D and calcium are widely considered early preventive measures, it is no surprise that the studies failed to find them effective in people who already had osteoporosis or signs of it, said Dr. Steven R. Goldstein, a professor of obstetrics and gynecology at the New York University medical center.

"This is something we've known for a long time, that calcium and vitamin D alone are not enough to treat osteoporosis," Dr. Goldstein said. "Once you've developed frank osteoporosis and a bone fracture, you're going to need medication, not simply vitamin D and calcium to prevent fractures."

Osteoporosis causes bones to become thinner and occurs primarily in the elderly.

About 10 million Americans older than 50 develop the disease each year, and 1.5 million break bones because of it. Osteoporosis can be treated with drugs like biphosphonates that prevent the bones from deteriorating, but doctors often recommend taking supplements or eating foods rich in calcium and vitamin D to build up bone mass.

The largest of the two studies, in The Lancet, followed more than 5,o00 Scots who had broken bones in the last decade. Separated in four groups, the subjects received large doses of calcium, vitamin D, a combination of the two or a placebo. After several years, the researchers found that all four groups had roughly the same number of fractures, mostly to the hip.

Dr. Joan McGowan, an expert on osteoporosis at the National Institutes of Health, said it was surprising that there were not fewer fractures among the people who took the calcium and vitamin D, but that neither was seen a replacement for medication." Nobody thinks calcium and vitamin D are as potent as some of the other bone-active drugs that we have available," Dr. McGowan said.