Vitamin D and Calcium Supplement Use and High-Risk Breast Cancer: A Case–Control Study among BRCA1 and BRCA2 Mutation Carriers

SIMPLE SUMMARY: The relationship between vitamin D and calcium supplement use and breast cancer among women in the general population is not clear. Furthermore, whether such an association exists among women at high risk due to a BRCA1 or BRCA2 mutation has not been investigated. Thus, we evaluated the association between vitamin D and/or calcium supplement use and breast cancer in this high-risk population. In this case–control study, we identified 134 women diagnosed with breast cancer and 276 women without breast cancer. Women with a BRCA mutation who consumed vitamin D-containing supplements had 46% lower odds of having breast cancer compared to those who did not take any supplements. Increasing vitamin D and calcium supplement intake was inversely associated with the odds of having breast cancer. Higher vitamin D and/or calcium supplement intake may be associated with lower breast cancer risk in this high-risk population. ABSTRACT: The role of vitamin D and calcium use in the development of breast cancer among women in the general population is not clear. Furthermore, whether vitamin D and calcium supplement use are associated with breast cancer in high-risk populations has not been evaluated. Thus, we evaluated the association between vitamin D and/or calcium supplement use and breast cancer among women with a pathogenic variant (mutation) in BRCA1 or BRCA2. BRCA mutation carriers enrolled in a longitudinal study were invited to complete a supplemental questionnaire on lifetime supplement use. Cases included women with a prevalent diagnosis of invasive breast cancer, and controls had no history of breast cancer. Vitamin D and calcium use were categorized as never/ever use, and as tertiles of supplement intake (total average daily supplement use). Unconditional logistic regression was used to estimate the odds ratio (OR) and 95% confidence intervals (CIs) of breast cancer. This study included 134 breast cancer cases and 276 controls. Women who used vitamin D-containing supplements had 46% lower odds of having breast cancer compared to those who never used supplements (OR 0.54; 95% CI 0.31, 0.91; p = 0.02). Increasing vitamin D and calcium supplement intake was inversely associated with the odds of having breast cancer (p-trend = 0.04). Findings were suggestively stronger among BRCA1 mutation carriers; however, analyses were limited by small strata. These findings suggest a potential inverse association between vitamin D and calcium supplementation and BRCA breast cancer. Additional studies are warranted to confirm these findings and accurately inform clinical care guidelines.