Association of calcium and vitamin D supplementation with cancer incidence and cause-specific mortality in Black women: Extended follow-up of the Women's Health Initiative calcium-vitamin D trial

Low circulating vitamin D levels are more prevalent in Black than White individuals. We analyzed the Women's Health Initiative (WHI) calcium plus vitamin D (CaD) randomized clinical trial extended follow-up data to evaluate associations between calcium plus vitamin D supplementation and incident cancer, cardiovascular disease (CVD), and cause-specific mortality endpoints among Black women. Intent-to-treat analysis was performed. Among 3325 Black women in the CaD trial who were randomized into either daily calcium (1000 mg of calcium carbonate) plus vitamin D (400 IU D3) or placebos for an average of 7 years, there were 813 deaths, 588 incident cancers, and 837 CVD events during an average of 15.7 years of follow up (52 230 total person-years). Using Cox's proportional hazards models, we calculated hazard ratios and their confidence intervals for outcomes ascertained during the trial period, posttrial follow-up period and overall periods combined. We found that total mortality, cause-specific mortality, and total cancer incidence were almost identical between CaD and placebo groups. These results suggest that calcium plus vitamin D supplementation does not reduce risks of cancer, CVD, or other major causes of death in Black women overall and, thus, other medical, behavioral or social interventions should be considered to narrow health disparities related to these outcomes. However, other finer endpoints, such as colorectal cancer, warrants further investigation.