| Summary: | Vitamin D administered pre-diagnostically has been shown to reduce mortality. Emerging evidence suggests a role of post-diagnosis vitamin D supplement intake for survival among cancer patients. Thus, we conducted a meta-analysis to evaluate the relationship. PubMed and Embase were searched for relevant observational cohort studies and randomized trials published through April 2022. Summary relative risk (SRR) and 95% confidence interval (CI) were estimated using the DerSimonian–Laird random-effects model. The SRR for post-diagnosis vitamin D supplement use vs. non-use, pooling cohort studies and randomized trials, was 0.87 (95% CI, 0.78–0.98; <i>p</i> = 0.02; <i>I</i><sup>2</sup> = 0%) for overall survival, 0.81 (95% CI, 0.62–1.06; <i>p</i> = 0.12; <i>I</i><sup>2</sup> = 51%) for progression-free survival, 0.86 (95% CI, 0.72–1.03; <i>p</i> = 0.10; <i>I</i><sup>2</sup> = 0%) for cancer-specific survival, and 0.86 (95% CI, 0.64–1.14; <i>p</i> = 0.29; <i>I</i><sup>2</sup> = 0%) for relapse. Albeit not significantly heterogeneous by variables tested, a significant inverse association was limited to cohort studies and supplement use during cancer treatment for overall survival, and to studies with ≤3 years of follow-up for progression-free survival. Post-diagnosis vitamin D supplement use was associated with improved overall survival, but not progression-free or cancer-specific survival or relapse. Our findings require confirmation, as randomized trial evidence was insufficient to establish cause-and-effect relationships.
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