The Role of Vitamin D in Rare Diseases-A Clinical Review.

Background/Objectives: Patients suffering from rare diseases are particularly vulnerable to vitamin D deficiency. The role of vitamin D status in rare disease management remains insufficiently investigated and employed in routine clinical practice. Methods: This review analyses current data on vitamin D status in selected rare diseases of organs involved in vitamin D metabolism: skin (epidermolysis bullosa, morphea), liver (autoimmune hepatitis, primary biliary cholangitis, primary sclerosing cholangitis), kidney (Alport syndrome, Fabry disease), and cystic fibrosis as a model of a systemic rare disease. Additionally, this review critically examines potential drug-vitamin D interactions in the context of rare disease patient polypharmacy. Results: Evidence suggests that vitamin D deficiency is prevalent in rare disease patient populations, often at once exacerbating and being simultaneously exacerbated by the underlying condition. Vitamin D deficiency correlates with worse clinical outcomes and lower quality of life across the examined diseases. Immunoregulatory properties of vitamin D appear relevant for rare diseases with autoimmune components. Conclusions: An urgent need for developing disease-specific clinical practice guidelines, implementing routine vitamin D monitoring in rare disease patient care, and introducing tailored supplementation under the principles of precision medicine is emphasized.