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.