Dietary Supplement Use after Cancer Diagnosis in Relation to Total Mortality, Cancer Mortality and Recurrence: A Systematic Review and Meta-Analysis.

To study post-diagnosis dietary supplement use in relation to total mortality, cancer mortality and recurrence among cancer survivors. PubMed and Cochrane Library were searched until April 2019 for observational studies (OS) and randomized clinical trials (RCT). Pooled risk ratios (RR) were calculated using random-effects models. Compared to no supplementation, calcium supplementation was associated with lower total (RR = 0.88, 95% confidence interval (CI): 0.77-1.00, I(2)=0%, four OS) and cancer mortality (RR = 0.71, 95% CI: 0.53-0.95, I(2)=0%, three OS) among all cancer survivors, and cancer mortality among colorectal cancer survivors (RR = 0.66, 95% CI: 0.47-0.94, I(2)=0%, two OS). Vitamin D supplementation was associated with lower total mortality (RR = 0.86, 95% CI: 0.76-0.99, I(2)=0%, three OS and two RCT). Among breast cancer survivors, supplementation with vitamin C (RR = 0.79, 95% CI: 0.68-0.92, I(2)=0%, four OS), D (RR = 0.85, 95% CI: 0.72-0.99, I(2)=0%, two OS), and E (RR = 0.76, 95% CI: 0.64-0.90, I(2)=0%, three OS) was associated with lower total mortality, while multivitamins (RR = 0.79, 95% CI: 0.64-0.97, I(2)=0%, two OS), vitamin C (RR = 0.76, 95% CI: 0.64-0.91, I(2)=0%, two OS), and E (RR = 0.69, 95% CI: 0.55-0.85, I(2)=0%, two OS) with lower cancer recurrence. Conclusions: Findings are mostly based on OS. More RCTs are needed to justify any recommendation for use.