The prevalence of obstructive airway disease (OAD), including asthma, is increasing in the United States. Because obesity is also on the rise, associations have been made between obesity and OAD. This concept has been supported by several studies. One study found an increased risk for asthma among persons who gained 25 kg (55 lb). Conversely, weight reduction improves asthma symptoms. It is uncertain whether OAD is directly responsible for the increased symptoms of dyspnea and wheezing noted in obese persons. Perhaps other aspects of the breathing mechanism, such as respiratory muscle function, lung volumes, and the energy cost of breathing, are negatively affected by obesity, resulting in asthma-like wheezing. Sin and associates used participant data from the National Health and Nutrition Examination Survey (NHANES III) to review the presumed association between obesity, OAD (represented by real spirometric changes), and self-reported asthma.

Study participants were selected using a stratified, multistage, probability sample. Among persons in this large study population, self-reported asthma, increased use of bronchodilators, and exercise limitation were noted among the group with the highest body mass index (BMI). With spirometric testing, however, the highest prevalence of significant airflow obstruction was found to be among participants in the lowest BMI group, while the lowest prevalence was found among the highest BMI group. Surprisingly, bronchodilators were more often used in the group with the highest BMI, even among participants without evidence of airflow obstruction.

The high rate of asthma diagnosis among the highest BMI group may be related to increased complaints of dyspnea and exercise limitations in obese persons. This would suggest that bronchodilators are being used by obese persons for conditions for which they are not indicated. Obesity may be related to dyspnea by adverse effects on gas exchange, decreased lung volumes, increased breathing work, and decreased end-expiratory lung volumes, factors which cause flow limitations even when the airways are normal.

The authors conclude that obesity is associated with increased use of bronchodilators and increased diagnosis of asthma, but significant airway obstruction is actually less prevalent in obese persons. This finding indicates that OAD might be overdiagnosed in this population. Further study is needed to identify more clearly the effect of obesity on the diagnosis and treatment of asthma.