Small Airway Dysfunction May Mediate the Association Between Body Mass Index and Severe Asthma Exacerbations

Background Obesity is associated with poorer asthma outcomes and an increased risk of exacerbations, but the underlying mechanisms remain incompletely understood. Small airway dysfunction (SAD) may represent a key mechanistic link between excess body weight and adverse asthma outcomes. Methods In this multicenter observational study, adult patients with asthma underwent clinical characterization, spirometry and impulse oscillometry (IOS). SAD was defined using a composite criterion based on peripheral airway resistance (R 5–20 ), reactance area (AX) and the ratio of peripheral to total airway resistance (R 5–20 )/R 5 . Associations between body mass index (BMI), SAD and severe asthma exacerbations were assessed using multivariable regression models. Non-linear relationships were explored using generalized additive models and mediation analyses quantified the contribution of SAD to the obesity–exacerbation association. Findings Among 1169 patients, IOS-defined SAD was significantly more prevalent in individuals with BMI≥30 kg·m −2 . Increasing BMI was associated with worse oscillometric parameters (p<0.0001), following a non-linear pattern with steeper deterioration beyond BMI values of approximately 28–30 kg·m −2 . SAD was independently associated with obesity (adjusted OR 2.11, 95% CI 1.56–2.86) and with severe asthma exacerbations in the previous year (adjusted OR 2.01, 95% CI 1.53–2.65, both p<0.0001). Mediation analyses showed that SAD accounted for 26%–41% of the association between obesity and exacerbation risk (p=0.004 and 0.03). Spirometric indices provided limited additional information. Interpretation Oscillometry-defined small airways dysfunction (SAD) represents a non-linear functional trait underlying the association between obesity and severe asthma exacerbations, supporting its clinical relevance in obese patients with asthma and identifying SAD as a potential treatable trait.