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; p = 0.02; I2 = 0%) for overall survival, 0.81 (95% CI, 0.62-1.06; p = 0.12; I2 = 51%) for progression-free survival, 0.86 (95% CI, 0.72-1.03; p = 0.10; I2 = 0%) for cancer-specific survival, and 0.86 (95% CI, 0.64-1.14; p = 0.29; I2 = 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.
Keywords: cancer-specific survival; cohort study; meta-analysis; overall survival; post-diagnosis; progression-free survival; randomized controlled trial; relapse; vitamin D supplement use.