The relation of body mass index to asthma, chronic bronchitis, and emphysema

Background: Recent studies have suggested a relationship between asthma and obesity. Despite these reports, the effect of being underweight or overweight as a risk factor for airway obstructive diseases (AODs) is not clear. Objectives: To determine whether a relation of body mass index (BMI) to asthma, chronic bronchitis (CB), or emphysema exists (analysis 1), and, if so, whether the association between obesity and asthma is modified by gender (analysis 2). Design: Nested case-control study from the longitudinal cohort of the Tucson Epidemiologic Study of Airways Obstructive Diseases. Patients: Analysis 1: physician-confirmed incident cases of asthma (n = 102), CB (n = 299), or emphysema (n = 72) who denied any prior AODs. Analysis 2: all 169 incident cases of asthma, regardless of any previous AODs, stratified by gender and by other potential effect modifiers. In both analyses, we selected only subjects at least 20 years old who had weight and height measured during the study. Measurements: BMI and other risk factors were assessed prior to the onset of the AOD (cases) or prior to the last completed survey (control subjects). Results: A diagnosis of emphysema was significantly associated with a BMI < 18.5 (odds ratio [OR], 2.97; 95% confidence interval [CI], 1.33 to 6.68, when compared to healthy control subjects). A BMI ≥ 28 increased the risk of receiving a diagnosis of asthma (OR, 2.10; 95% CI, 1.31 to 3.36) and CB (OR, 1.80; 95% CI, 1.32 to 2.46). About 30% of the patients with asthma and 25% of the patients with CB (vs 16% of the control subjects, p < 0.001) were preobese or obese, regardless whether BMI was assessed before the diagnosis or before the onset of respiratory symptoms. The relation of elevated BMI to asthma was significant only among women. Conclusions: Patients with emphysema are more likely to be underweight, and patients with CB are more likely to be obese. However, the temporal relationship between abnormal BMI and the onset of COPD is uncertain. Preobese and obese women are at increased risk of acquiring asthma. This relation, particularly if it is causal, has potentially relevant public health implications.