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Vitamin D is a fat-soluble secosteroid that has long been thought to be an important regulator of bone metabolism, calcium and phosphorus homeostasis. Its action is enabled by binding to the vitamin D receptor (VDR), after which it modulates the expression of thousands of genes both directly and indirectly. Vitamin D is essential for brain development and mature brain activity, and it has been linked to a variety of neurological diseases, including Parkinson's disease (PD). Nearly two decades ago, it was discovered that patients with Parkinson's disease had a higher rate of vitamin D deficiency than the general population. This finding is intriguing given vitamin D's neuroprotective effect, which can be mediated by neurotrophic factors, nerve growth regulation, or protection against cytotoxicity. Vitamin D deficiency appears to be related to disease severity and progression, as measured by the Unified Parkinson's Disease Rating Scale (UPDRS) and the Hoehn and Yahr (H&Y) scales, but not to age of PD onset or disease duration. Furthermore, lower vitamin D levels in Parkinson's disease have been linked to a higher risk of falling. While the link between vitamin D and motor symptoms appears to be plausible, the results of studies looking into the link between vitamin D and non-motor symptoms are conflicting. Furthermore, there is very little evidence that vitamin D supplementation can reduce clinical manifestations and disability in Parkinson's disease patients. However, given the positive balance of potential benefits versus risks, vitamin D supplementation for Parkinson's disease patients is likely to be considered in the near future, if further confirmed in clinical studies.
