Abstract
Vitamin D is synthesized by human skin cells exposed to sunlight and also obtained from nutritional sources. It has dual nature and serves as a vitamin and an immunomodulatory hormone. Metabolically, vitamin D is responsible for calcium and phosphate homeostasis, bone resorption, and maintenance of a healthy and mineralized skeleton. As a hormone, its activated form (1,25-dihydroxyvitamin D), binds with the vitamin D receptor (VDR), triggers the regulation of more than 100 genes, many of those associated with the immune system, Hence, it plays a critical role in the regulation of the key components of both, the innate and adaptive immune systems. Deficiency of 25-hydroxyvitamin D has been linked with an increased risk of autoimmune and respiratory diseases such as rheumatoid arthritis, type 1 diabetes, multiple sclerosis, tuberculosis, and influenza. Recent pharmacogenomic studies have shown that variation in vitamin D receptor gene expression alters the response of different individuals to treatment with vitamin D. Introduction of vitamin D promotes the synthesis of antimicrobial and antiviral proteins in the cell and improves the cellular levels of calcium and phosphorus, eventually promotes autophagy to remove viruses and bacteria from the cells. This review specifically aims at establishing a concrete relationship between vitamin D deficiency and increased susceptibility to various respiratory diseases. We also aimed to explore the possibility of using vitamin D supplementation programs to improve immunoprotection in individuals prone to respiratory illnesses.