Vitamin D and Covid-19 State of Evidence: Literature Review

Following the occurrence of the global pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), some studies highlighted the possible importance of vitamin D against the disease. Several studies have supported the role of vitamin D in the regulation of the immune system. Vitamin D can suppress cytokine production simultaneously by stimulating the innate immune system, thus reducing viral load, and by decreasing the overactivation of the adaptive immune system to respond immediately to viral load. Vitamin D activity is mediated by its receptor (VDR), which acts as a transcription factor modulating the expression of genes triggering the response against viruses. Observational studies report an independent association between low serum concentrations of 25-hydroxyvitamin D and susceptibility of the respiratory tract to acute infections. Calcitriol (1,25-dihydroxyvitamin D3) is acting on the ACE-2/Ang (1-7)/ MasR axis leading to improved expression of the angiotensin converting enzyme (ACE-2). ACE-2 is a receptor used by SARS- CoV-2 to infect host cells. Previous studies identified an association between high levels of ACE2 and improvement in the general health status of covid-19 cases. In additional, ACE-2 has been shown to protect against acute respiratory lesions. We conducted a review of the literature to clarify the protective role of vitamin D against SARS-CoV-2 infection through a review of the recently published studies.