The role of vitamin D deficiency in psychiatric disorders: A systematic review on association with depression and schizophrenia.

BackgroundVitamin D is a neuroactive hormone involved in brain development and immune regulation. It may influence psychiatric symptoms, particularly depression and schizophrenia.ObjectiveTo examine the association between vitamin D levels and the symptoms of depression and schizophrenia.MethodsThe PRISMA guidelines were followed in this systematic review. A literature search was conducted in PubMed, Web of Science, Google Scholar, and Cochrane Library to identify studies published between 2014 and 2025. The studies included case-control, cohort, cross-sectional, and interventional designs that evaluated the levels of vitamin D and psychiatric symptoms in individuals with depression or schizophrenia. The risk of bias was evaluated.ResultsEleven studies met our inclusion criteria. From these results, it can be observed that there is a link between vitamin D deficiency and increased symptom severity in depression and schizophrenia. Lower vitamin D levels are related to higher severity scores of symptoms and inflammatory and neurotrophic biomarkers. However, three studies have reported no significant relationship.ConclusionThis systematic review highlights a potential association wherein vitamin D deficiency may contribute to the pathophysiology or severity of both depression and schizophrenia.