Effects of Vitamin D Supplementation and Baseline Vitamin D Status on Acute Respiratory Infections and Cathelicidin: A Randomized Controlled Trial

Background: Vitamin D supplementation may lower the risk of acute respiratory infection (ARI), and the effects may be mediated through the induction of cathelicidin production. Objective: To study the effect of vitamin D supplementation on ARI and cathelicidin concentration in a randomized controlled trial (RCT) and to study the associations between baseline serum 25 hydroxyvitamin D (25(OH)D) and ARIs and cathelicidin concentrations in a 14-week follow-up study. Methods: In the RCT study, the participants were randomized into 2 groups to receive either 20μg of vitamin D₃ or an identical placebo daily. Blood samples were obtained 3 times, at the beginning (study week 0), mid-term (study week 6), and at the end of the study period (study week 14). The follow-up study had 412 voluntary young men from 2 different locations and seasons (January and July). The primary outcomes were the number of ARIs diagnosed and the number of days off because of ARI. Results: In the RCT, vitamin D supplementation had no effect on ARI or days off because of ARI. However, regardless of the group, vitamin D insufficiency (<50mol/L) was associated with increased ARI. In the 14-week follow-up study, insufficient serum 25(OH)D at baseline was also associated with increased risk of ARI (odds ratio [OR], 2.1; 95% confidence interval [CI], 1.2-3.7) and also days-off duty (OR, 2.3; 95% CI, 1.3-4.0) and was inversely associated with cathelicidin concentration (OR, 0.49; 95% CI,. 24-.99). Conclusions: Sufficient serum 25(OH)D may be preventive against acute respiratory infection and the preventive effect could be mediated through the induction of cathelicidin production.