Does vitamin D deficiency in asthma affect clinical and functional parameters? A Turkish multicenter study

Background: In recent years, interest in the effects of vitamin D on human health and the immune system has increased. Objective: This study aimed to investigate the relationship of vitamin D with asthma severity, attacks, and clinical and functional parameters in adult patients with asthma who were living in different geographic regions in Turkey. Methods: A total of 384 patients with stable asthma and 87 control subjects were included. A physical examination and a pulmonary function test were performed, and routine blood analyses and vitamin D levels were evaluated. Asthma Control Test was applied. The number of exacerbations in the previous year, asthma therapy, and medication adherence were recorded. Results: In our study, vitamin D levels were below the target values in both patients with asthma (median [minimum-maximum] 16.0 ng/mL [3.5-48 ng/ml]) and control subjects (median [minimum-maximum] 20.0 ng/mL [5.8-58.79 ng/mL]). However, it was lower in the patients with asthma than in the control subjects (p = 0.001). There was a negative relationship between the levels of vitamin D and the severity of asthma (Kendall τ = -0.146; p < 0.001). Furthermore, the patients with severe asthma were received The Global Initiative for Asthma (GINA) step 5 treatment showed significantly lower vitamin D compared with the patients who received GINA step 4 treatment (p = 0.037). Vitamin D levels correlated with forced vital capacity (FVC), forced expiratory volume in the first second of expiration (FEV₁), and peak expiratory flow (r, 0.221-0.236; p ≤ 0.001). In addition, a positive relationship was found between Asthma Control Test and vitamin D (r = 0.229; p = 0.001). However, body mass index (BMI), asthma exacerbation, and hospitalization were inversely related to vitamin D (r, 0.198-0.233; p = 0.001). Multivariable regression analysis revealed that FVC (p = 0.002), FEV₁ (p = 0.033), and BMI (p = 0.037) were independent determinants associated with vitamin D. Conclusion: This study suggested a high prevalence of vitamin D deficiency in adults with asthma living in different geographic areas in Turkey. Vitamin D deficiency is associated with asthma severity, poor control, and lower lung function.