Optimal vitamin D levels have been shown to help maintain a healthy immune system. Mechanisms of this action are via activation of macrophages (a type of white blood cell), stimulation of anti-microbial peptides, modulation of TH17 cells and reduction of inflammatory cytokines in the lungs. (1,2,3) Optimal vitamin D has been shown to reduce the severity of respiratory infections, with a vitamin D deficiency increasing chances of respiratory infection. (4,5) Recent studies suggest low vitamin D status was associated with more severe respiratory infection. (6,7,8)

1: Hewison M. Vitamin D and immune function: an overview. Proc Nutr Soc. 2012;71(1):50-61. doi:10.1017/S0029665111001650

2: Zdrenghea MT, Makrinioti H, Bagacean C, Bush A, Johnston SL, Stanciu LA. Vitamin D modulation of innate immune responses to respiratory viral infections. Rev Med Virol. 2017;27(1). doi:10.1002/rmv.1909

3: Fawaz L, Mrad MF, Kazan JM, Sayegh S, Akika R, Khoury SJ. Comparative effect of 25(OH)D3 and 1,25(OH)2D3 on Th17 cell differentiation. Clin Immunol. 2016;166-167:59-71. doi:10.1016/j.clim.2016.02.011

4: Martineau AR, Jolliffe DA, Greenberg L, et al. Vitamin D supplementation to prevent acute respiratory infections: individual participant data meta-analysis. Health Technol Assess. 2019;23(2):1-44. doi:10.3310/hta23020

5: Zhou J, Du J, Huang L, Wang Y, Shi Y, Lin H. Preventive effects of vitamin D on seasonal influenza A in infants: multicenter, randomized, open, controlled clinical trial. Pediatr Infect Dis J. 2018;37(8):749-754. doi:10.1097/INF.0000000000001890

6: Merzon E, Tworowski D, Gorohovski A, et al. Low plasma 25(OH) vitamin D level is associated with increased risk of C0VlD-19 infection: an Israeli population-based study [published online ahead of print, 2020 Jul 23]. FEBS J. 2020;10.1111/febs.15495. doi:10.1111/febs.15495

7: Jain SK, Parsanathan R. Can Vitamin D and L-Cysteine Co-Supplementation Reduce 25(OH)-Vitamin D Deficiency and the Mortality Associated with C0VlD-19 in African Americans? [published online ahead of print, 2020 Jul 13]. J Am Coll Nutr. 2020;1-6. doi:10.1080/07315724.2020.1789518

 8: Panagiotou G, Tee SA, Ihsan Y, et al. Low serum 25-hydroxyvitamin D (25[OH]D) levels in patients hospitalised with C0VlD-19 are associated with greater disease severity [published online ahead of print, 2020 Jul 3]. Clin Endocrinol (Oxf). 2020;10.1111/cen.14276. doi:10.1111/cen.14276