Sin DD, Jones RL, Man SF. Arch Intern Med. 2002;162:1477–1481
Purpose of the Study.
Previous research suggests that obesity is an important risk factor for asthma. However, because obesity can cause dyspnea through mechanisms other than airflow obstruction, diagnostic misclassification of asthma could partially account for this association. The purpose of the study was to determine if there is a relationship between obesity and airflow obstruction.
Study Population and Methods.
A total of 16 171 participants (17 years or older) from the Third National Health and Nutrition Examination Survey (NHANES III) were divided into 5 quintiles based on their body mass index (BMI) to determine the association between BMI quintile and risk of self-reported asthma, bronchodilator use, exercise performance, and airflow obstruction. Significant airflow obstruction was defined as a ratio <80% the predicted value of forced expiratory volume in 1 second to forced vital capacity adjusted for age, sex, and race.
The highest BMI quintile (ie, the most obese participants) had the greatest risk of self-reported asthma (odds ratio [OR]: 1.50; 95% confidence interval [CI]: 1.24–1.81), bronchodilator use (OR: 1.94; 95% CI: 1.38–2.72), and dyspnea with exertion (OR: 2.66; 95% CI: 2.35–3.00). Paradoxically, the highest BMI quintile had the lowest risk for significant airflow obstruction (P = .001).
This study demonstrates that while obesity is a risk factor for self-reported asthma, obese participant are at a lower risk for (objective) airflow obstruction. Many more obese than nonobese participants were using bronchodilators despite a lack of objective evidence for airflow obstruction. These data suggest that mechanisms other than airflow obstruction are responsible for dyspnea genesis in obesity and that asthma might be overdiagnosed in the obese population.
The obvious problem with this study is that a single measure of airflow cannot be considered diagnostic for asthma. Nonetheless, it is unlikely that this study population was gathered to answer specific questions about asthma. My guess is that the authors gathered a lot of information about their obese population (including baseline spirometry) and later went back to see if they could make inferences regarding asthma. It’s a lesson on how this is not the best way to conduct clinical research. On the other hand, I suppose it points out that not all dyspnea (even that which is reported to improve with bronchodilators) represents asthma.