De novo vitamin D supplement use post-diagnosis is associated with breast cancer survival

PurposeExperimental laboratory data have indicated a protective effect of vitamin D on breast cancer progression, while epidemiological evidence is growing. Using pharmacy claims data, this study investigates the association between vitamin D supplement use initiated after a breast cancer diagnosis and associated mortality.MethodsWomen aged 50-80years with a record of invasive breast cancer were identified on the National Cancer Registry Ireland database (n=5417). Initiation of de novo vitamin D post-diagnosis was identified from linked national prescription data (n=2581, 49%). Multivariate Cox proportional hazards models were used to estimate adjusted HRs (95% CIs) for breast cancer-specific mortality.ResultsThere was a 20% reduction in breast cancer-specific mortality in de novo vitamin D users (modelled as a time-varying variable) compared to non-users (HR 0.80; 95% CI 0.64-0.99, p=0.048) and the reduction was greater at 49% (HR 0.51; 95% CI 0.34-0.74, p<0.001), if vitamin D was initiated soon after the breast cancer diagnosis (within 6months).ConclusionsIn this large national breast cancer cohort, de novo vitamin D use post-diagnosis was found to be associated with a reduction in breast cancer-specific mortality. Vitamin D, therefore, has the potential as a non-toxic and inexpensive agent to improve survival in breast cancer patients. Findings support the need for RCTs exploring the effect of vitamin D supplementation on breast cancer survival.