But more recently, studies have been coming in suggesting that this may not be not the case, Sutherland said.
For this latest study, Sutherland and colleagues looked at body mass index (BMI) and treatment data on 1,256 individuals with mild-to-moderate asthma.
Overweight and obese participants fared slightly worse on standard measures of asthma severity, such as lung function and predisposition to airway constriction, Sutherland said, but not enough to result in any real clinical difference.
"Having a high BMI doesn't really appear to be associated with more severe forms of asthma," he added.
Horovitz did point out that heavier people tend to have more shortness of breath. "We might have assumed it was worse asthma," he said.
Response to medications, however, took a different tilt.
When analyzing a subgroup of 183 people, lean people using inhaled corticosteroids showed a 55 percent lower level of one measure of airway inflammation.
The exact reasons for these findings are unclear, indicating the need for more studies, the researchers said.
The National Heart, Lung, and Blood Institute has more on asthma.
SOURCES: E. Rand Sutherland, M.D., associate professor, medicine, National Jewish Health and the University of Colorado, Denver; Len Horovitz, pulmonary specialist, Lenox Hill Hospital, New York City; April 2009, Journal of Allergy and Clinical Immunology
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