A large multicenter randomized clinical trial reported that supplementation with calcium and vitamin D did not reduce breast cancer risk in the overall population.
However, exploratory analyses suggest the effect of vitamin D and calcium supplements on breast cancer risk may vary according to a woman's initial supplement use.
"Our findings suggest that calcium and vitamin D supplementation may reduce breast cancer risk in some women, but more research is needed to clarify these results," said Rowan T. Chlebowski, MD, PhD, Medical Oncologist at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center (LA BioMed) and the study's lead author. "We can't yet make a general recommendation about how much calcium and vitamin D individuals should take each day as supplements."
This study, part of the Women's Health Initiative, included 36,282 postmenopausal women without prior breast cancer at 40 centers in the United States. Breast cancer incidence was compared between 18,176 women randomly assigned to receive 1000 mg of elemental calcium carbonate and 400 IU of vitamin D3 daily, and 18,106 who received a placebo.
After a median follow-up time of seven years, breast cancer incidence did not differ significantly between the two groups overall (2.9% with breast cancer in the calcium/vitamin D group versus 3.0% in the placebo group). Among the women who did develop breast cancer, mean breast tumor size was smaller in the calcium/vitamin D group (1.54 cm versus 1.71 cm, p=0.05).
Women were allowed to continue personal use of calcium and vitamin D supplements as well as those provided in the randomized trial. Among the 19,115 women not reporting any supplement use at entry, the risk of developing breast cancer was reduced by 18% in the calcium / vitamin D group compared with women who received a placebo (Hazard Ratio (HR) 0.82, 95% Confidence Interval (CI) 0.70-0.97). In contrast, no reduction was seen in women already using any calcium and vitamin D supplements at entry (HR 1.14, 95% CI 0.96-1.36). The interaction p value was 0.008, suggesting a different effect in these two subgroups.
Dr. Chlebowski explained that "planned analyses of vitamin D metabolites in the blood may help interrupt the findings."
Previous observational studies have suggested that vitamin D and calcium may protect against breast cancer. Because natural sources of vitamin D are limited (mostly sun exposure and oily fish), vitamin D is added to some foods, such as milk. The Institute of Medicine has concluded that data are insufficient to set an official recommended daily allowance (RDA) for vitamin D, but advises most adults to get at least 400 IU per day (600 IU for those 70 year years or older). Currently, about a third of U.S. women regularly use supplements containing calcium and vitamin D.