Vitamin D a Key Modulator of Innate Immunity and Natural Defense From Acute Viral Respiratory Infections

Calcitriol has been found to induce the transcription of antimicrobial peptides – such as cathelicidin and defensin.

Functional VDR has been identified in almost all immune cells, including neutrophils and antigen-presenting cells.

In vitro studies have shown that vitamin D decreases the expression of pro-inflammatory cytokines and increases the production of antiviral proteins, suggesting an important role in antiviral innate immunity

several in vitro studies suggest that vitamin D plays an important role in local “respiratory homeostasis” either by promoting the expression of antimicrobial peptides or by directly affecting the replication of respiratory viruses

A British cohort study revealed that the prevalence of respiratory infections displayed a strong seasonal pattern in the opposite direction to the pattern for serum 25-hydroxyvitamin D concentrations34. Notably, each 10 nmol/L (4 ng/mL) increase in serum 25-hydroxyvitamin D levels was associated with a 7% lower risk of infection.

Interestingly, in a recent Editorial published in The British Medical Journal raising debate on COVID-19 pandemic, different researchers proposed vitamin D deficiency as a putative risk factor, among others, for novel coronavirus infection. The authors found that the protective effects of vitamin D were stronger in subjects with a baseline serum 25-hydroxyvitamin D concentration of <25 nmol/L (corresponding to <10 ng/mL, which are serum levels indicative of severe vitamin D deficiency) compared to those with a baseline 25-hydroxyvitamin D concentration of ≥25 nmol/L (≥10 ng/mL)