The Case for Vitamin D Supplementation to Improve Protection against Respiratory Tract Infections

Background: An adequate vitamin D status is essential for normal immune function. Vitamin D sub-nutrition is widespread, especially in low sunlight regions and in certain high-risk groups. There is growing evidence that vitamin D supplementation can reduce the risk of infection of the respiratory tract. Objective: The aim was to review the mechanisms whereby vitamin D supports the chemistry of optimal innate and adaptive immunity, and to evaluate critically the current evidence for the use of vitamin D supplements to help to minimize respiratory infection risk, particularly in individuals with sub-optimal vitamin D plasma levels. Method: PUBMED and MEDLINE were searched using the terms: vitamin D; cholecalciferol; calcitriol; calcifediol; respiratory infections; influenza; pneumonia; respiratory syncytial virus, respiratory tract infection; chronic obstructive pulmonary disease (COPD); immunisation; vaccination; innate immunity; adaptive immunity. Papers for citation were selected by the authors on the basis of quality and relevance. Findings: Vitamin D is needed for optimal function of innate and adaptive immune systems. The evidence for a protective effect of vitamin supplementation is the strongest for viral infection, particularly in people with plasma calcifediol levels below 25 nmol/L. However, the role of vitamin D in influenza risk and as an adjunct to influenza immunization is not clear. There is some evidence that a sufficient vitamin D status reduces the risk of infective exacerbations of COPD, and probably helps in the defence against some bacterial infections of the respiratory tract. Conclusion: Individuals with vitamin D deficiency have enhanced protection against some respiratory infections when given supplements. There are also other benefits including improved bone and muscle function, and mood. Further research is needed to identify sub-groups most likely to benefit from supplements.