Some evidence suggests that vitamin D may reduce breast cancer risk. Despite the biological interaction between vitamin D and calcium, few studies have evaluated their joint effects on breast cancer risk.
The objective was to evaluate the associations and potential interaction between vitamin D and calcium (from food and supplements) and breast cancer risk in a population-based case-control study.
Breast cancer cases aged 25-74 y (diagnosed 2002-2003) were identified through the Ontario Cancer Registry. Controls were identified by using random digit dialing; 3101 cases and 3471 controls completed epidemiologic and food-frequency questionnaires. Adjusted odds ratios (ORs) and 95% CIs were estimated by using multivariate logistic regression.
Vitamin D and calcium intakes from food only and total combined intakes (food and supplements) were not associated with breast cancer risk, although the mean intake of vitamin D was low. Vitamin D supplement intake >10 microg/d (400 IU/d) compared with no intake was associated with a reduced risk of breast cancer (adjusted OR: 0.76; 95% CI: 0.59, 0.98). No categories of calcium supplement intake were significantly associated with reduced breast cancer risk, but a significant inverse trend was observed (P = 0.04). There were no significant interactions involving vitamin D, calcium, or menopausal status.
No associations were found between overall vitamin D or calcium intake and breast cancer risk. Vitamin D from supplements was independently associated with reduced breast cancer risk. Further research is needed to investigate the effects of higher doses of vitamin D and calcium supplements.