The role of vitamin D in depression and anxiety disorders: a review of the literature.

BACKGROUND: Prevalence of mental health disorders continue to increase worldwide. Over the past decades, suboptimal vitamin D (VD) levels and gut dysbiosis have been associated with neurological dysfunction and psychiatric disorders. METHODS: In this review, we examined the available literature on VD and mental health disorders, particularly depression and anxiety, in both clinical and pre-clinical studies. RESULTS: Our extensive review failed to find a link between VD deficiency, depression, and anxiety-related behavior in preclinical animal models. However, strong evidence suggests that VD supplementation may alleviate symptoms in chronically stressed rodents, with some promising evidence from clinical studies. Further, fecal microbiota transplantations suggest a potential role of gut microbiota in neuropsychiatric disorders, although the underlying mechanisms remain to be fully elucidated. It has been postulated that serotonin, primarily produced by gut bacteria, may be a crucial factor. Hence, whether VD has the ability to impact gut microbiota and modulate serotonin synthesis warrants further investigation. CONCLUSIONS: Taken together, literature has suggested that VD may serve as a key regulator in the gut-brain axis to modulate gut microbiota and alleviate symptoms of depression and anxiety. The inconsistent results of VD supplementation in clinical studies, particularly among VD deficient participants, suggests that current intake recommendations may need to be re-evaluated for individuals at-risk (i.e. prior to diagnosis) of developing depression and/or anxiety.