Depressive disorder is a mood disorder and is the most common mental disorder worldwide. It causes symptoms that affect how a person feels, thinks, and carries out daily activities. Vitamin D plays an important role in restoring the balance of neurotransmitters involved in depressive disorders. We were interested in the correlation between oral vitamin D supplementation and the reduction of the risk of depressive disorder in adults and the effect of vitamin D supplementation on depressive symptoms in adults already suffering from depressive disorder, and whether this effect varies according to the baseline serum 25-hydroxyvitamin D level. To answer the research questions, we conducted a systematic review following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. We searched the PubMed and Web of Science databases for literature using search strings and selected 20 randomised controlled trials, taking into account inclusion and exclusion criteria. After the selection, we assessed the quality of the studies using the checklist for randomised controlled trials published by the Critical Appraisal Skills Programme. We analysed the studies according to individual parameters, where we compared the country and year of publication of the selected studies, the research plan, dosage and duration of the intervention, as well as data about research participants. The studies were then divided into four groups, which were compared on the basis of the severity of depressive disorder and the initial serum 25-hydroxyvitamin D level. We found that vitamin D supplementation in adults without depressive disorders was unlikely to affect the likelihood of developing depression, independent of baseline serum 25-hydroxyvitamin D levels. Nevertheless, vitamin D supplementation could have a significant impact on the treatment or alleviation of depressive symptoms in patients with moderate to severe depressive disorder and sufficient serum 25-hydroxyvitamin D levels. Some studies suggest a correlation between vitamin D supplementation in adults with low serum 25-hydroxyvitamin D levels and depressive symptoms, but we have not confirmed this. We suggest that further research should be conducted on individuals with depressive disorders, and we advise standardisation of research methods and harmonisation of populations. The heterogeneity of research has contributed to the inconsistent findings in our thesis.
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