May 30 (HealthDay News) -- Premenopausal women who get a lot of vitamin D and calcium may cut their risk of breast cancer by almost a third, Harvard Medical School researchers report.

"Adequate intakes of calcium and vitamin D are necessary for women in keeping up their health, and additionally, these two nutrients may help prevent breast cancer development, especially among premenopausal women," said lead author Jennifer Lin, an assistant professor of medicine.

Her team's report appears in the May 28 issue of the Archives of Internal Medicine.

In the study, Lin's team collected data on more than 10,500 premenopausal and almost 21,000 postmenopausal women age 45 and older who were part of the Women's Health Study. The data included information on what they ate and the dietary supplements they took.

Over an average of 10 years, 276 premenopausal women and 743 postmenopausal women went on to develop breast cancer.

The researchers found that premenopausal women whose intake of vitamin D and calcium was high had about a 30 percent lower risk of developing breast cancer. However, they didn't find this association for postmenopausal women.

Animal studies have also found an association between calcium and vitamin D intake and breast cancer prevention, Lin's group noted.

"Calcium and vitamin D may confer protection against breast tumorigenesis," Lin said. "However, more studies are necessary to investigate the potential utility of these two nutrients in breast cancer development," she added.

One expert stressed that the evidence for a protective effect of vitamin D and calcium is still not clear.

"I really don't think that one can say from this study that the effect is only in premenopausal women, because there are a number of factors in the study that may have limited the ability to see the effect," said Victoria Stevens, an epidemiologist at the American Cancer Society. "This is particularly true for vitamin D, because most vitamin D comes from exposure to sunlight, which they did not take into account."

At this point in time, a recommendation that women take vitamin D or calcium to decrease their breast cancer risk is not warranted, she said.

"I don't think we should go that far," Stevens said. "The society doesn't recommend the use of any vitamin supplements" to prevent breast cancer.

"I don't think this study says that we should change that," Stevens said. "At this point, the evidence doesn't support anything more than getting a good diet and maintaining good levels of physical activity," she said.

SOURCES: Jennifer Lin, Ph.D., associate professor of medicine, Brigham and Women's Hospital and Harvard Medical School, Boston; Victoria Stevens, Ph.D., epidemiologist, American Cancer Society, Atlanta; May 28, 2007, Archives of Internal Medicine