The Roles of Vitamin D in Increasing the Body's Immunity and Reducing Injuries due to Viral Infections: With an Emphasis on its Possible Role in SARS-CoV-2 (COVID-19)

Background: It is known that vitamin D can increase the body’s immunity against some viral infections. Many people worldwide have vitamin D deficiency; therefore, this has become a public concern whether vitamin D is an important factor protecting against COVID-19 infection. In this paper, the data about the roles of vitamin D in immunity and recovery from viral infections, especially novel Coronavirus disease (COVID- 19), are reviewed. Methods: The electronic databases of Pubmed, Google Scholar, Research Gate, Excerpta Media Database (EMBASE), and Medical and Health Education (Medrix) were searched. Results: Vitamin D is considered an important factor in immune homeostasis. Various effects have been considered for this nutrient on the immune system, particularly because of vitamin D receptor (VDR) and Cytochrome P450 Family 27 Subfamily B Member 1 (CYP27B1) expression in most of the immune cells. Vitamin D can increase cellular immunity, reduce cytokine storm, and enhance antioxidants production. It also has modulatory effects on Angiotensin-converting enzyme 2 (ACE2) receptors and might have protective functions against acute lung injuries, including COVID-19 infection. However, there are some articles against this positive effect. Conclusion: Vitamin D supplementation is reported to be effective in the enhancement of the immune system and might be effective in the treatment and prevention of COVID-19 infection, especially in those with its deficiency. However, it should be considered that vitamin D deficiency shows the overall health status of the patients and cannot be considered specific for COVID-19 infection.