Vitamin D and Bronchial Asthma: An Overview of Data From the Past 5 Years

Abstract

Purpose: Vitamin D is a potent immunomodulator capable of dampening inflammatory signals in several cell types involved in the asthmatic response. Its deficiency has been associated with increased inflammation, exacerbations, and overall poor outcomes in patients with asthma. Given the increase in the prevalence of asthma over the past few decades, there has been enormous interest in the use of vitamin D supplementation as a potential therapeutic option. Here, we critically reviewed the most recent findings from in vitro studies, animal models, and clinical trials regarding the role of vitamin D in treating bronchial asthma.

Methods: Using the key terms [Vitamin D, asthma, clinical trials, in vivo and in vitro studies], the [PubMed, Google Scholar] databases were searched for [clinical trials, original research articles, meta-analyses, and reviews], English-language articles published from [2012] to the present. Articles that were [Articles that did not meet these criteria were excluded] excluded from the analysis.

Findings: Several studies have found that low serum levels of vitamin D (< 20 ng/mL) are associated with increased exacerbations, increased airway inflammation, decreased lung function, and poor prognosis in asthmatic patients. Results from in vitro and in vivo studies in animals and humans have suggested that supplementation with vitamin D may ameliorate several hallmark features of asthma. However, the findings obtained from clinical trials are controversial and do not unequivocally support a beneficial role of vitamin D in asthma. Largely, interventional studies in children, pregnant women, and adults have primarily found little to no effect of vitamin D supplementation on improved asthma symptoms, onset, or progression of the disease. This could be related to the severity of the disease process and other confounding factors.

Implications: Despite the conflicting data obtained from clinical trials, vitamin D deficiency may influence the inflammatory response in the airways. Further studies are needed to determine the exact mechanisms by which vitamin D supplementation may induce antiinflammatory effects.

Keywords: asthma; clinical trials; immunomodulation; vitamin D.

Figure 1. Immunomodulatory effects of vitamin D on inflammatory cells in allergic asthma

Vitamin D, by binding and activating VDR, has been found to alleviate inflammation associated with allergic asthma. In ASM cells, vitamin D reduced proliferation, production of pro-inflammatory cytokines, MMP and mucus secretion. Vitamin D has been shown to decrease costimulatory molecules, CCR-7 expression, maturation and antigen presentation in DCs while promoting tolerogenic DCs with increased IL-expression. In T-lymphocytes, vitamin D has been reported to shift the balance from Th17 cells to Treg cells evidenced by decrease production of IL-17 and increased production of IL-10. The hormone inhibits differentiation and proliferation of B-cells to plasma cells and is believed to play a role in decreasing antibody production. In innate immune cells involved in asthma, vitamin D has been shown to inhibit differentiation, maturation, homing and cytokine secretion from mast cells, neutrophils and eosinophils. The overall effect of this immunomodulation is decreased airway hyper-responsive, inflammation and remodeling in asthma. AHR-airway hyper-responsiveness; Ag-antigen; IL-interleukin; Th-T helper cell; TNF-tumor necrosis factor; TGF-transforming growth factor; Treg-T regulatory cells; PGE₂-prostaglandin E2