Vitamin D and Clinical Cancer Outcomes: A Review of Meta-Analyses.

The relationship between vitamin D status or supplementation and cancer outcomes has been examined in several meta-analyses. To address remaining knowledge gaps, we conducted a systematic overview and critical appraisal of pertinent meta-analyses. For meta-analyses of trials, we assessed their quality using AMSTAR-2 (A Measurement Tool to Assess Systematic Reviews), strength of associations using umbrella review methodology and credibility of evidence using GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) criteria. Meta-analyses of observational studies reported inverse associations of 25OHD with risk of cancer incidence and cancer mortality and, particularly for colorectal cancer, fulfilled some of Bradford-Hill's causation criteria. In meta-analyses of trials, vitamin D supplementation did not affect cancer incidence. However, we found credible evidence that vitamin D supplementation reduced total cancer mortality risk, with five out of six meta-analyses reporting a relative risk (RR) reduction of up to 16%: RR, 0.84 (95% CI, 0.74-0.95). The strength of the association, however, was classified as weak. This was true among meta-analyses of high, moderate, and lower quality (AMSTAR-2-rated). Trials did not include large numbers of vitamin D-deficient participants; many tested relatively low doses and lacked sufficiently powered data on site-specific cancers. In conclusion, meta-analyses show that, although observational evidence indicates that low vitamin D status is associated with a higher risk of cancer outcomes, randomized trials show that vitamin D supplementation reduces total cancer mortality, but not cancer incidence. However, trials with larger proportions of vitamin D-insufficient participants and longer durations of follow-up, plus adequately powered data on site-specific common cancers, would provide further insight into the evidence base.