A new study has revealed that taking calcium and vitamin D supplements does not reduce breast cancer incidence in postmenopausal women.
Some studies have suggested that vitamin D supplements may reduce breast cancer risk, but results have been inconsistent.
In the new study, Rowan Chlebowski, M.D., Ph.D., of the Los Angeles Biomedical Research Institute at Harbor-University of California Los Angeles Medical Center and colleagues evaluated breast cancer incidence as a secondary endpoint in the Women's Health Initiative study in 36,282 postmenopausal women who were randomly assigned to take 1,000 mg of calcium plus 400 IU of vitamin D daily or to daily placebo.
The researchers found that the incidence of invasive breast cancer was similar in the supplement and placebo groups, with 528 and 546 cases, respectively.
Also, 25-hydroxyvitamin D levels were only modestly associated with dietary and supplement vitamin D intake and were not associated with breast cancer risk in a nested case-control analysis.
These findings call into question recommendations for evaluation of higher vitamin D dosage in future studies, according to the authors.
In addition, as 25-hydroxyvitamin D levels were strongly associated with leanness and high physical activity, which both influence breast cancer risk, prior observational studies relating vitamin D levels to breast cancer risk may have been influenced by these factors.
"The main findings do not support a causal relationship between calcium and vitamin D supplement use and reduced breast cancer incidence," the authors said.
The study is published online November 11 in the Journal of the National Cancer Institute.