Vitamin D supplements and cancer incidence and mortality: a meta-analysis

Background: Observational studies suggest that effects of vitamin D may be stronger for cancer mortality than for incidence. Yet, existing randomised controlled trials (RCTs) of vitamin D supplementation have limited power to examine the relationships as their primary end points are not cancer incidence or mortality. Methods: Meta-analyses of RCTs of vitamin D supplementation and total cancer incidence and mortality were conducted. Results: Over 2-7 years of duration, vitamin D supplementations had little effect on total cancer incidence (400-1100 IU per day, summary relative risk (RR) = 1.00, 95% confidence interval (CI) = 0.94-1.06, I-2 = 0%; four RCTs with combined 4333 cases), but significantly reduced total cancer mortality (400-833 IU per day, summary RR = 0.88, 95% CI = 0.78-0.98, I-2 = 0%, three RCTs with combined 1190 deaths). Conclusions: Over 2-7 years of duration, the benefit of vitamin D supplementation may be limited to cancer mortality.