Background
We recently reported that obese and non-obese patients with asthma have similar airflow limitation and bronchodilator responsiveness, but obese patients have more symptoms overall. There is limited information on the effect of obesity on asthmatics of varying severity measured by objective physiological parameters. Understanding how obesity affects asthmatics of differing severity can provide insights into the pathogenesis of asthma in the obese and a rationale for the therapeutic approach to such patients.
Methods
Participants with asthma from two American Lung Association-Asthma Clinical Research Center studies were grouped by tertiles of airflow obstruction (FEV1% predicted, FEV1/FVC) and methacholine reactivity (PC20FEV1). Within each tertile we examined the independent effect of body mass index (BMI), divided into normal weight, overweight and obese categories, on lung function, airway reactivity and symptoms.
Results
Overall, both FEV1 and FVC decreased and symptoms worsened with increasing BMI; airway reactivity was unchanged. When stratified by the degree of airflow obstruction, higher BMI was not associated with greater airway reactivity to methacholine. Higher BMI was associated with more asthma symptoms only in the least obstructed FEV1/FVC tertile. When stratified by degree of airway reactivity, BMI was inversely associated with FVC in all PC20FEV1 tertiles. BMI was directly associated with asthma symptoms only in those with the least airway reactivity.
Conclusions
Obesity does not influence airway reactivity in patients with asthma and it is associated with more symptoms only in those with less severe disease.