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Just a week after announcing that a low-fat diet doesn't keep older women from developing colon cancer, researchers have disappointing news about another potential prevention measure. Taking calcium and vitamin D doesn't appear to help either, researchers from the Women's Health Initiative reported Thursday.

In their study of more than 36,000 healthy women past menopause, those who took regular supplements of calcium and vitamin D had the same risk of colon cancer as women assigned to take a placebo (dummy pill). The findings were published in the New England Journal of Medicine.

They come as a bit of a disappointment because several other studies have suggested that calcium and vitamin D might, in fact, be protective. Most of those, though, were observational studies that couldn't really determine cause and effect, or studies of people at high risk of colon cancer because they'd already had colon polyps.

The new study is different in some important ways, said lead author Jean Wactawski-Wende, PhD, of the University at Buffalo, part of the State University of New York.

"This is the first long-term clinical trial to specifically look at colorectal cancer in a group of women who are not at especially high risk," she explained. "After 7 years, there was no apparent protection from supplementing with 1,000 mg of calcium and 400 IU of vitamin D3."

But the new result isn't necessarily the final answer on whether calcium plus vitamin D might be useful against colon cancer, other experts said. For one thing, the study may not have been long enough.

"Colon cancer takes about 10 to 15 years to develop, and follow-up of the study was only 7 years," noted Marji McCullough, ScD, RD, a nutritional epidemiologist with the American Cancer Society.

The study participants will be tracked for another 5 years, though they'll no longer be required to take supplements, to see if any differences arise later.

It's also possible the doses of calcium and vitamin D were too low to make a difference. The earlier studies that did see some benefit used more calcium, McCullough said. And there's now some evidence that it might take much higher doses of vitamin D -- around 1000 IU -- to influence cancer.

Another issue: many women in the placebo group also took calcium or vitamin D supplements on their own, and therefore may have had some protection already. In other words, the differences between the groups may not have been big enough to see the impact of the supplements, McCullough said.

Wactawski-Wende agreed that this factor may have hindered the study's ability to find a benefit. But she said analyses of smaller groups of study participants didn't suggest that. For instance, even women who started the study getting less than 800 mg of daily calcium weren't protected from colon cancer, though they should have benefited the most from taking a supplement. And there was no difference between women who were most vigilant about taking all their supplements and the participants on placebo.

"The findings were remarkably consistent among the subgroups," she said. "But no one study can answer all the questions."

Future studies may well look into different doses of both calcium and vitamin D, and may focus on different groups of people -- like those at high risk of colon cancer. For now, the American Cancer Society does not recommend taking calcium or vitamin D in hopes of preventing colon cancer.

All this doesn't mean women should quit taking calcium and vitamin D, though. Both are necessary for good bone health.

"I wouldn't want people to come away from this believing calcium and vitamin D aren't important nutrients," said Wactawski-Wende. "They're still critically important."

In fact, the study was primarily intended to look at the effects of these supplements on bone density and fractures. That analysis did find some benefit. Women who took calcium and vitamin D had higher hip bone density (stronger hip bones) than those on placebo. And women over 60 who took supplements were less likely to break a hip. Those results are published in a separate paper in the same issue of the journal.

Citation: "Calcium Plus Vitamin D Supplementation and the Risk of Colorectal Cancer." Published in the Feb. 16 New England Journal of Medicine (Vol. 354, No. 7:684-696). First author: Jean Wactawski-Wende, University at Buffalo.