The skin is a central organ for vitamin D metabolism, representing both the site of its synthesis and a target organ. Vitamin D regulates both proliferation and differentiation of keratinocytes. Vitamin D is also involved in regulating the synthesis of ceramides, a key component of the corneocyte lipid envelope, which acts as an epidermal barrier, protecting the skin from chemical, physical and microbiological agents. Vitamin D also carries out several actions on the skin’s immune system. Among these there is the induction of the synthesis of antimicrobial peptides such as hCAP18/LL-37 and β-defensin and it inhibits antigen presentation by Langerhans cells, whilst inducing the formation of regulatory T lymphocytes. Atopic dermatitis (AD) is the most common inflammatory skin disease, affecting up to 20% of the paediatric population and 5% of the adult population. Several epidemiological studies have shown an inverse correlation between AD prevalence and latitude, reduced exposure to sunlight and hypovitaminosis D. Most observational studies and meta-analyses have shown that vitamin D levels are lower in adults and children with AD than in controls. Vitamin D supplementation, either oral or secondary to exposure to UV radiation, is generally associated with an improvement in AD. Serum vitamin D dosing is recommended for patients affected by AD.
