A 2016 randomized controlled trial of Vitamin D supplementation in people with major depressive disorder purportedly saw an antidepressant effect,[1] but this study was retracted in 2021 due to concerns over the validity of the participant data.[2]
A number of observational studies have found that people with depression are more likely to have low vitamin D levels.[3] However, randomized controlled trials don’t consistently find beneficial effects of vitamin D supplementation on depression.[4][5] One reason for this could be that benefits only occur if a person has sufficiently severe depression symptoms. In support of this, two meta-analyses of randomized controlled trials found that vitamin D supplementation reduced the severity of depression symptoms in people with depression, but had no effect on depression symptoms in people without depression.[6][7]