The combination of calcium and vitamin D supplements does not cut the risk of breast cancer for postmenopausal women, contrary to some previous studies, U.S. research suggests.

The findings are based on data from a randomized, doubled-blind, placebo-controlled trial of 36,282 postmenopausal women.

They were published online Tuesday in the Journal of the National Cancer Institute.

The women were all participants in the long-running Women's Health Initiative study in the United States. They were randomly assigned to take either 1,000 milligrams of calcium plus 400 international units (IU) of vitamin D daily, or a daily placebo, over seven years.

Rowan Chlebowski of the University of California Los Angeles Medical Center and colleagues found that the incidence of invasive breast cancer was similar in the supplement and placebo groups, with 528 and 546 cases, respectively.

The findings are significant because some research had suggested higher calcium intake, and especially vitamin D intake, was associated with lower breast cancer risk. But results from such observational studies were inconsistent.

The study marks the first attempt to evaluate the relationship between the supplements and breast cancer with a randomized trial - a more rigorous approach.

In an accompanying editorial, doctors Corey Speers and Powel Brown of Baylor College of Medicine in Houston applauded the researchers' efforts to test the question in such a rigorous manner.

Although they agreed the data do not support an association between calcium and vitamin D supplementation and breast cancer incidence, they argued for more research.

"Because pre-clinical, epidemiological, and clinical trial results of vitamin D supplementation are conflicting, additional studies will be needed to determine whether vitamin D plus calcium will prevent breast cancer. However, this article by Chlebowski [and colleagues] offers an important first step in addressing this issue," the editorialists wrote.

Breast cancer is the most common form of cancer among Canadian women. The Canadian Breast Cancer Foundation estimates that in 2008, 22,400 women in Canada will be diagnosed with the disease.