The deadly effects of obesity are well known. And now you can add another negative effect - the condition can severely worsen the impact of asthma.

What's more, it may also mask its severity in standard tests, according to researchers in New Zealand.

The researchers who studied lung function in asthmatic women with a range of body mass indexes (BMIs) found significant differences in the changes in respiratory function that occur with asthmatic bronchoconstriction in relation to obesity.

This is the first prospective study to reveal a significant comparative difference in how the airways and lungs respond to a simulated asthma attack in obese and non-obese individuals.

It establishes a direct link between obesity and the development of a phenomenon known as "dynamic hyperinflation"-when air breathed into the lungs cannot be exhaled.

This often occurs with acute asthma, but is more frequent in obese individuals.

"We have demonstrated significant differences in the changes in respiratory function that occur with asthmatic bronchoconstriction in relation to obesity," said principal investigator, D. Robin Taylor, M.D., of the University of Otago in New Zealand.

In the study, the researchers recruited 30 asthmatic women and divided them into three groups by BMI: normal weight, overweight and obese.

Each woman breathed nebulized methacholine to artificially induce an asthma-like attack, and was then assessed for changes in lung function on several measures, including how much air remained in her lungs after exhalation (functional residual capacity, or FRC) and how much air she could breathe in on her next breath (inspiratory capacity, or IC).

"After the methacholine challenge, the amount of bronchoconstriction was identical for each of the three groups, but the changes in FRC and IC were greatest in the obese group. This indicated to us that greater dynamic hyperinflation was occurring among obese individuals," said Dr. Taylor.

With increasing BMI, FRC was higher, whereas IC was significantly decreased.

"This means that among women with greater BMI, an asthma-like episode has the potential to cause greater breathing difficulties than in non-obese women. The greater dynamic hyperinflation means that obese individuals lose the ability to inhale as deeply or exhale as fully as normal weight individuals," said Dr. Taylor.

Curiously, the group of obese individuals with asthma differed from their non-asthmatic counterparts in having a lower FRC before the methacholine challenge than the non-obese group, yet still recorded a greater increase in FRC after the methacholine challenge.

"This is the surprising finding in our study. It is quite counterintuitive. You would expect individuals with a heavier chest wall not to develop hyperinflation quite so readily as those who are lighter. But that is not what happened," said Dr. Taylor.

The study also showed that simple spirometry was inadequate to determine the level of pulmonary dysfunction which was occurring in obese individuals.

The study is published in the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.