Deficiency or insufficiency of vitamin D is a problem all over the world because it is not known exactly what is the therapeutic dose for treatment in some chronic diseases. A recently published meta-analysis discovered that vitamin D significantly reduced total mortality. In most diseases, vitamin D deficiency is implicated. The final metabolic product of vitamin D is a pleiotropic, potent, maintenance and repair substance, that targets more than 200 human genes in a wide variety of tissues. Vitamin D has as many mechanisms of action as well as target genes.

Several serious illnesses are associated with vitamin D deficiency, such as cancer, heart disease, multiple sclerosis, diabetes, autism, and a host of other illnesses. It is very important to elucidate the role of vitamin D in chronic diseases because the relationship is not yet fully understood. Little is known about the mechanisms responsible for the onset and progression of these diseases. The possible role of vitamin D in immunotherapy has also been studied. In the context of the mechanism and clinical effect of subcutaneous immunotherapy, the natural activity of vitamin D seems to be highly attractive. The literature has highlighted that there is an important role for vitamin D (more specifically, calcitriol) in the body’s immune system. Vitamin D should always be an adjuvant treatment in patients with chronic diseases and never replace standard treatment. Vitamin D blocks the features of the adaptive immune system that would lead to autoimmunity when it is present.

The main aim of this Special Issue is to define an integrative mechanistic model for a clearer understanding of the relationship between vitamin D and chronic disorders. We look to answer multiple questions, such as when and how vitamin D is implicated in the evolution and prognosis of chronic disorders, and if diet is the best therapy for controlling vitamin D deficiency or insufficiency. Papers addressing the reasons why some patients develop mild forms of diseases and others severe, and whether these responses depend on individual susceptibility or external influences are also welcome. We welcome both original research and review articles.

Potential topics include but are not limited to the following:

• The role of vitamin D on dendritic cells
• Monocytes and vitamin D
• Vitamin D and oxidative stress
• Vitamin D and autoimmunity
• Micronutrient deficiency and vitamin D
• Vitamin D and allergies
• Molecular actions of vitamin D metabolites
• Immunomodulatory effect of vitamin D