CHICAGO—Taking calcium and vitamin D supplements does not reduce breast cancer incidence in postmenopausal women, according to data from a randomized, doubled-blind, placebo-controlled trial published online Nov. 11 in the Journal of the National Cancer Institute.
Several observational and preclinical studies have suggested that vitamin D supplements may reduce breast cancer risk, but results have been inconsistent. Researchers evaluated breast cancer incidence as a secondary endpoint in the Women's Health Initiative study in which 36,282 postmenopausal women were randomly assigned to take 1,000 mg of calcium plus 400 IU of vitamin D daily or a daily placebo. (The primary endpoint of the study was effect of the supplements on hip fracture.) Baseline 25-hydroxyvitamin D levels were determined in all 1,067 breast cancer cases and 1,067 matched control subjects.
Researchers found that the incidence of invasive breast cancer was similar in the supplement and placebo groups, with 528 and 546 cases, respectively. Additionally, 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, said the authors. Furthermore, 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.
