Vitamin D is essential for the regulation of the immune system. In recent years, the role of vitamin D in the control of several autoimmune conditions such as inflammatory bowel disease (IBD), celiac disease, type 1 diabetes mellitus (T1DM), and others has been investigated. The aim of this review was to define the level of knowledge on vitamin D's role in these disorders, as well as the preventive and therapeutic role of vitamin D supplementation. Relevant studies published over the last 20 years were identified via a PubMed/Medline (http://www.ncbi.nlm.nih.gov/pubmed/) search using the keywords: vitamin D, autoimmune disease, and prevention. Vitamin D deficiency or impaired function of the enzymes necessary for its activity has been shown to affect the onset and severity of the autoimmune diseases examined. Vitamin D supplementation appears useful in the support therapy of IBD. Its role in celiac disease, autoimmune hepatitis, T1DM, and autoimmune thyroiditis is unclear. In conclusion, further studies are needed to define whether vitamin D is a cause or a result of the most common autoimmune, extra-skeletal diseases, such as IBD. Vitamin D should be provided to all newborns during their first year of life. Afterwards, the vitamin D supplementation regimen should be tailored to the presence of risk factors for vitamin D deficiency and/or specific disease
