Background: The several meta-analyses of the effect of vitamin D on depression have produced inconsistent results and studies dealing with anxiety were not incorporated. There has been no comprehensive analysis of how results are affected by the nature of the sample or the dosage and duration of supplementation. The study is aimed to investigate whether vitamin D supplementation reduces negative emotions and to analyze the possible influence of sample and regimen. Method: We conducted a systematic review and meta-analysis of randomized controlled trials comparing the effect of vitamin D and placebo on negative emotion. Databases were searched for relevant articles published before February 2019. Results: The analysis covered 25 trials with a total of 7,534 participants and revealed an effect of vitamin D on negative emotion (Hedges' g = −0.4990, 95% CI [−0.8453, −0.1528], p =.0047, I2 = 97.7%). Subgroup analysis showed that vitamin D had an effect on patients with major depressive disorder and on subjects with serum 25(OH)D levels ≤50 nmol/L. The pooled data from trials of vitamin D supplementation lasting ≥8 weeks and dosage ≤4,000 IU/day indicated that vitamin D had an effect. Conclusions: Our results support the hypothesis that vitamin D supplementation can reduce negative emotions. Patients with major depressive disorder and individuals with vitamin D deficiency are most likely to benefit from supplementation. But to interpret the results with high heterogeneity should still be cautious.
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