Vitamin D and Acute Respiratory Tract Infection

Acute respiratory tract infections (ARTI) generate a major disease burden worldwide, disproportionately affecting the elderly and the very young. A growing body of evidence supports an important immunomodulatory function for the biologically active metabolite of vitamin D,1,25-dihydroxyvitamin D (1,25[OH]2D). Respiratory epithelial cells and leucocytes express both the vitamin D receptor and CYP27B1, the enzyme which converts 25-hydroxyvitamin D to 1,25(OH)2D. Observational and ecological studies report associations between inadequate vitamin D status and susceptibility to ARTI, and vitamin D deficiency has been proposed as the seasonal stimulus for influenza epidemics. In children, associations between profound vitamin D deficiency and susceptibility to lower respiratory tract infection have been reported in a variety of settings. In adults, particularly amongst those with asthma and COPD, inadequate vitamin D status has been reported to be associated with susceptibility to upper respiratory tract infection. Vitamin D supplementation trials for the prevention of ARTI report conflicting results. This may reflect variation in study populations and methodology, or factors such as genetic heterogeneity within the vitamin D metabolic or signalling pathways. Further trials of vitamin D supplementation in different settings, with measurement of participants' vitamin D status and evaluation of genetic factors which might modify the effect of vitamin D supplementation are needed.