Vitamin D and chronic respiratory diseases: a complex relationship

Chronic pulmonary diseases, including infectious and inflammatory diseases, have been associated with significant morbidity and mortality. The role of vitamin D is not only limited to bone and mineral regulation, but the vitamin D has a role in many aspects in regulating various other functions like immunity, inflammation, host defense, and repair. Vitamin D enhances innate immunity by stimulating antimicrobial peptides cathelicidin and thus promoting the elimination and killing of various organisms involved in the chronic infectious disease. Several studies have shown the association of vitamin D deficiency with the increased incidence and severity of various chronic pulmonary infectious, obstructive, and restrictive lung diseases. Some other studies also showed a controversial role of vitamin D in various chronic pulmonary diseases. This chapter aims to explore the role of vitamin D, its association with chronic respiratory diseases, and highlight the current evidence in this area.