Asthma May Be Over-Diagnosed in Obese Individuals

Source: Tufts University
September 18, 2002 (Reviewed: September 3, 2004)

It is commonly believed that being overweight is a risk factor for asthma. However, some of the symptoms that are seen in asthma may not always be caused by airway obstruction. For example, asthma may be over-diagnosed in people who are obese. Researchers investigated this issue and published their findings in the Archives of Internal Medicine.

Analysis of health profiles

Data from over 16,000 participants in the Third National Health and Nutrition Examination Survey (NHANES III), aged 17 and older were analyzed. Age, sex, race, body mass index (BMI), self-reports of asthma, smoking status, emergency room and physician visits, hospitalizations, exercise capacity (walking and jogging), and use of bronchodilators were noted. Lung function tests were performed according to 1987 American Thoracic Society recommendations and included forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and FEV1/FVC ratio.

Significant airflow obstruction was defined as an FEV1/FVC ratio of less than 80% of the predicted value, adjusted for age, race, and sex.

The participants were divided into five quintiles according to their BMI: (1)</=22.1; (2)>22.1-24.8; (3)>24.8-27.3; (4)>27.3-31.0; and (5)>31.0. The three outcome variables of interest were lung function, self-reported asthma (including drug use), and exercise performance.

Link between breathing difficulty and weight

Both self-reported asthma and bronchodilator use increased as BMI increased (adjusted odd ratios 1.50; CI, 1.24-1.81 and 1.94; CI, 1.38-2.72, respectively, comparing BMI quintile 5 to quintile 2). Obese participants also reported more physical limitations. Those most overweight were 2.66 times more likely to experience shortness of breath walking up a hill than those in the lowest BMI category.

Surprisingly, however, obese participants were actually less likely than non-obese participants to have significant airflow obstruction. The relationship between BMI and lung function measurements was an inverted U-shape. (There were more smokers and ex-smokers in the lower BMI groups, so they were excluded from this analysis.) The first and fifth quintiles had the lowest mean FEV1 and FVC, suggesting that lung function may be related to extremes in weight.

Same symptoms, different diagnosis

It is not well understood how obesity causes dyspnea, but it may affect respirator mechanics and gas exchange, decreasing respirator muscle function and lung volume and making breathing more difficult. Since overweight people are likely to experience exercise limitations, dyspnea may explain the increased prevalence of an asthma diagnosis and bronchodilator use despite the absence of evidence of airflow obstruction. This is problematic, because the overuse of bronchodilators can be harmful. Potential side effects include aggravation of high blood pressure, cardiac arrhythmia, and other cardiovascular symptoms.

The authors conclude that obstructive airway disease may be over- diagnosed in people who are obese. Thus more obese than non-obese study participants were using bronchodilators despite a lack of objective evidence for airflow obstruction. Clearly, further studies are needed to determine the role of weight on the diagnosis and treatment of asthma.

Source

• Obesity is a risk factor for dyspnea but not for airflow obstruction. DD. Sin, RL. Jones, SFP. Man, Arch Intern Med, 2002, vol. 162, pp. 1477--1481

Related Links
Obstructive Sleep Apnea
Obesity May Increase Pneumonia Risk
The Management of COPD
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