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 advice suggests the supplements convey only limited protection. Calcium and vitamin D failed to protect against most fractures in the mostly low-risk women.
Even so, Dr. Norman Lasser at New Jersey Medical School 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 Dr. Rebecca Jackson at Ohio State University, lead author of the study.
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 fragile bones due to osteoporosis. One of two women will suffer such a fracture in her lifetime.
For women 50 and older, 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.
But researchers say women of all ages fared well when they followed the plan religiously. For them, there was a 29 percent decrease in hip fractures.
Dr. Dorothy Lane, who led a portion of the trial at Stony Brook University Hospital, where 4,000 Long Island women participated, said she stands by the supplements, despite the pills' failure to thwart spinal and wrist fractures.
"I think it would still be advisable to take the recommended amount of calcium to improve bone density and to maintain bone health," Lane said, noting hip fractures' link to disability and death.
Dr. Elizabeth Nabel, director of the Women's Health Initiative, stressed the importance of obtaining calcium through foods. She said daily exposure to sunlight helps achieve adequate vitamin D levels. The nutrient forms under skin when exposed to sun. Vitamin D increases calcium absorption.
Calcium supplements are "a good start, but women at higher risk need to know it's not enough," wrote Dr. Joel Finkelstein of Massachusetts General Hospital in an editorial published today in the New England Journal of Medicine.
Advertising may have falsely reassured many women that the supplements are going to protect them, Finkelstein said.
"Women come to see me all the time saying, "How can I possibly have osteoporosis? I exercise and I take calcium and vitamin D,' " he said. "The ads for calcium have given many women the impression that they are protected against osteoporosis. The message of the study is that calcium and vitamin D by themselves are not enough."
As a therapy to protect against osteoporosis, he said, supplements are "pretty weak." Women who have the condition should consider taking one of the seven prescription drugs on the market that have been shown in rigorous clinical trials and approved by the Food and Drug Administration to prevent fractures, he advised. Six of the drugs inhibit bone breakdown and one spurs the growth of new bone.
The findings are the latest in a series of recent major studies that have failed to find a clear association between diet, dietary supplements and health. Data from the same study last week showed that low-fat diets failed to protect women against heart disease, colon cancer or breast cancer.
"These negative results might give people the feeling that diet doesn't matter," said Volker Mai of the University of Maryland. "Diet does matter. The problem is we don't know enough about diet to know what specifically matters."
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.
Some researchers said the effect would have been clearer with higher doses of vitamin D, perhaps up to 1,000 units daily. The vitamin helps the body absorb calcium and promotes muscle health, reducing falls.
The study did show a significant side effect with the diet supplements: a 17 percent increase in the risk of kidney stones. But several doctors downplayed that risk, saying hip fractures are typically much worse than kidney stones.
The study also checked whether the supplements might help prevent colon cancer, and the results showed no benefit. That wasn't a big surprise partly because past studies had not signaled much benefit.
Still, the researchers plan to check participants in future years, because colorectal cancer can take 10 to 20 years to develop.
Information from the Associated Press, Newsday, Washington Post and New York Times was used in this report.