November 11 (HealthDay News) -- Although calcium and vitamin D may keep your bones strong, these vital nutrients don't appear to help postmenopausal women lower their risk of breast cancer.

"The message is that there's benefit from calcium and vitamin D for fracture risk, but taking those supplements won't be doing much for breast cancer risk. You wouldn't expect that you're doing it to improve breast cancer outcome," said study author Dr. Rowan Chlebowski, a medical oncologist at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center in Los Angeles.

Results of the study were released online Tuesday and were expected to be published in the Nov. 19 issue of the Journal of the National Cancer Institute.

Previous observational studies had suggested that there might be a link between calcium, vitamin D, and the risk of breast cancer, but results were mixed, and none of the trials were randomized studies, which is considered the gold standard for research.

The current study was a double-blind, placebo-controlled trial that included 36,282 postmenopausal women. All were between 50 and 79 years old, with no history of breast cancer.

The women were randomly assigned to receive either 1,000 milligrams of calcium plus 400 international units daily of vitamin D, or a placebo for an average of seven years.

During the study, the women -- regardless of which group they were assigned to -- were allowed to take additional calcium and vitamin D supplements.

The researchers found no association between vitamin D and calcium intake and the risk of breast cancer.

To further assess any potential role of vitamin D, the researchers compared baseline measures of 25-hydroxyvitamin D levels in the 1,067 women who developed breast cancer during the study to a randomly selected control group of 1,067 women without cancer. Levels of 25-hydroxyvitamin D indicate how much vitamin D is circulating in the body. After adjusting the data to control for physical activity and body-mass index, the researchers again found that vitamin D levels weren't associated with a decreased breast cancer risk.

"This trial is disappointing in that it does not demonstrate a significant cancer preventive effect from vitamin D and calcium in postmenopausal women," said Dr. Powel Brown, director of the cancer prevention program at the Lester and Sue Smith Breast Center at Baylor College of Medicine in Houston.

However, Brown was quick to point out that the study didn't look at premenopausal women. "It may be that the benefit of vitamin D in cancer prevention may be restricted to premenopausal women," he said, adding that the study "doesn't diminish the positive effects on bone health from vitamin D and calcium."

Brown, who co-authored an editorial in the same issue of the journal, said that although the current study was very well-done, he doesn't think that researchers "can definitively say that vitamin D isn't helpful for the prevention of breast cancer," and he hopes that more research will be done to see if higher doses might be more helpful, or if treatment was started earlier, if there might be an effect on breast cancer risk.

SOURCES: Rowan Chlebowski, M.D., Ph.D., medical oncologist, Los Angeles Biomedical Research Institute, Harbor-UCLA Medical Center, Los Angeles; Powel Brown, M.D., Ph.D., director, cancer prevention program, and professor, medicine and molecular and cellular biology, the Lester and Sue Smith Breast Center, Baylor College of Medicine, Houston; Nov. 19, 2008, Journal of the National Cancer Institute