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.
Baseline 25-hydroxyvitamin D levels was determined in all 1,067 breast cancer cases and 1,067-matched control subjects.
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.