Background:  Asthma and obesity are associated disorders, but the contribution of obesity to difficult‐to‐treat asthma as well as the mechanisms responsible for this relationship are unclear. The aim of this study was to investigate the relationship between obesity (body mass index ≥ 30) and factors related with asthma severity in patients with difficult‐to‐treat asthma.

Methods:  One hundred and thirty‐six nonsmoking asthmatic adults with persistent symptoms despite high doses of inhaled or oral corticosteroids and long‐acting bronchodilators were studied [70% female, median (range) age 44.6 (18–75) years, 32% on daily oral corticosteroids]. The association between obesity, lung function parameters [forced expiratory volume in 1 s (FEV₁), functional residual capacity/total lung capacity (FRC/TLC)], inflammatory markers [blood eosinophils, sputum eosinophils and neutrophils, exhaled nitric oxide (FE_NO), airway hyperresponsiveness, C‐reactive protein (CRP)] and aggravating co‐morbid factors (severe chronic sinus disease, gastro‐esophageal reflux, recurrent respiratory infections, psychopathology and obstructive sleep apnea) was investigated.

Results:  Obese patients (n = 29) had a higher FEV₁%pred (P = 0.05) and a lower FRC/TLC%pred (P < 0.01) compared with nonobese patients (n = 107). Body mass index was inversely related with sputum eosinophils (r = −0.36, P < 0.01) and FE_NO (r = −0.30, P < 0.01). Obese patients had an increased risk for gastro‐esophageal reflux (OR = 2.3) and sleep apnea (OR = 3.1).

Conclusion:  Obesity in patients with difficult‐to‐treat asthma is inversely related with sputum eosinophils and FE_NO, and positively associated with the presence of co‐morbid factors and reduced lung volumes. This suggests that other factors than airway inflammation alone explain the relationship between obesity and asthma severity.