Associations between Vitamin D, Pulmonary Function, Asthma Control and Asthma Outcome: The Role of Vitamin D Receptor and Its Binding Protein

Background: The mechanisms linking vitamin D and asthma are unknown. Distinct roles played by vitamin D binding protein (VDBP) and its receptor (VDR) may provide an explanation. Objectives: To study the link among the level of Vitamin D, VDR, and VDBP with the pulmonary function, control of asthma and outcome in asthmatic Saudi females. Methods: randomized case control study included 161 Saudi pre-menopausal women aged 25 to 45 years, 81 were asthmatics and 80 were age and weight matched controls normal. Data were collected using a structured questionnaire and baseline spirometry. Assessment of asthma control test (ACT) were done according to GINA guidelines. The 25-hydroxy vitamin D (25(OH)D), VDBP, and VDR were analyzed using ELISA. Results: asthmatic women especially at risk of exacerbation, had significantly low levels of 25(OH)D. Uncontrolled asthmatics had significantly lower 25(OH)D and higher VDR than those with better control. The 25(OH)D correlated negatively with VDR but not with VDBP. The 25(OH)D predicted asthmatics (OR 0.924, 95% CI: 0.900-0.949), (OR=3.86, 95% CI: 1.27-11.76) and ACT (B=0.192, 95% CI: 0.173-0.211). VDR predicted ACT (B= -0.011 (95% CI:-0.014-0.007). Conclusions: Vitamin D deficiency are frequent in asthmatic women and are associated with poor control and outcome. Serum levels of VDR are linked to 25(OH) and behave in its opposite direction. The role of VDBP in clinical asthma parameters may be independent to the level of 25(OH)D and needs further studies.