Vitamin D for the treatment of respiratory diseases: is it the end or just the beginning?

A large number of human, animal and in vitro studies have suggested that vitamin D3 (VD3) plays a critical role in inflammatory airway diseases such as asthma, chronic rhinosinusitis, and allergic rhinitis. VD3 acts upon a broad range of immune cells involved in the pathogenesis of these diseases including T-cells, dendritic cells (DCs), macrophages, and B-cells. In addition, VD3 can also regulate the functions of a number of non-immune cells including epithelial cells, fibroblasts, and smooth muscle cells. Given that VD3 has known effects on the immune system, it seems logical that supplementation with VD3 would prove efficacious in the treatment of these three diseases. While many studies, most of which are observational, have suggested that VD3 deficiency is associated with more severe disease, VD3 supplementation trials in humans have resulted in varied outcomes in terms of efficacy. In this review article we will discuss the role of VD3 in these three commonly associated respiratory diseases. We will explore the literature describing associations of VD3 deficiency with patient outcomes, cells in the respiratory microenvironment susceptible to VD3 regulation, conflicting results of VD3 supplementation trials, and potential gaps in our knowledge that may be limiting the widespread use of VD3 for the treatment of respiratory diseases such asthma, chronic rhinosinusitis and allergic rhinitis. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.