Fact: Vitamin D prevents upper respiratory tract infections.
Routine vitamin D3 supplementation has been shown to be safe and effective in preventing acute respiratory tract infection. This was concluded from a large meta-analysis of 25 high quality randomised, double-blind placebo-controlled trials including 10,933 subjects. Vitamin D supplementation reduced the incidence of respiratory infection in all participants, including those with ‘adequate’ levels of vitamin D. It also had a profound effect in those who were vitamin D deficient. It is important to know that daily and weekly supplementation is effective, but larger infrequent doses (e.g. once a month) are not effective.
Reference: Martineau A, Jolliffe D, Hooper R, Greenberg L, Aloia J, Bergman P et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ [Internet]. 2017:i6583. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5310969/