Substantial evidence now indicates that vitamin D may enhance the innate immune response and protect against respiratory infection. The human cathelicidin hCAP- 18 appears to have a particularly important role in the vitamin D–mediated mechanism against infection. Additionally, early evidence suggests that vitamin D modulates regulatory T-cell activity and IL-10 production, which are critical to the Th1–Th2 balance. These findings may be particularly important in the pathogenesis, control, and severity of asthma. However, much of these data are in cell culture or observational clinical studies, and the observed inverse associations may be due to residual confound- ing, such as from outdoor physical activity. Future laboratory and clinical studies, with a special focus on children and adults with asthma, will help to clarify the vitamin D–asthma association.
…it is premature to recommend vitamin D for either the prevention or treatment of viral respiratory infections. It is not, however, too early to recommend that health-care providers aggressively diagnose and adequately treat vitamin D deficiency.
At this time, there is a considerable amount of evidence that implicates vitamin D as a factor associated with various chronic lung diseases. The question remains whether vita- min D deficiency contributes to the etiology of lung disease or if vitamin D deficiency is simply a manifestation of the lung disease and/or its treatment. As research in this field unfolds, vitamin D supplementation will need to be evalu- ated in larger trials focused on specific respiratory diseases.
High-dose vitamin D supplementation in a sample of patients with COPD did not reduce the incidence of exacerbations. In participants with severe vitamin D deficiency at baseline, supplementation may reduce exacerbations.
There was no benefit of vitamin D3 supplementation in decreasing the incidence or severity of symptomatic URIs during winter. Further studies are needed to determine the role of vitamin D in infection.
WHO 19 Jan 2012:
Some studies have suggested that vitamin D supplementation can decrease the frequency and severity of respiratory infections among children. There is a need for further research before recommendations can be made.