Fat is found in the lungs of overweight and obese people.
Australian researchers analyzed lung samples from 52 people and found that the amount of fat increased in accordance with the body mass index.
They said that their findings could explain why being overweight or obese increased the risk of asthma.
Lung experts say it would be interesting to see if the effect can be altered by weight loss.
In a study published in the European Respiratory Journal, scientists looked at lung deaths donated for research.
Fifteen did not have asthma reports, 21
Scientists use colorants to perform detailed analyzes of nearly 1,400 airways from lung samples under a microscope.
Researchers found adipose tissue in all airways, with a greater presence in people with a higher body mass index,
And they say that the increase in fat seems to alter the normal structure of the airways and causes inflammation in the lungs – which could explain the increased risk of asthma in overweight or obese people.
Dr Peter Noble, an associate professor at the University of Western Australia in Perth who is working on the study, said: "Being overweight or obese is already associated with asthma or you have worse asthma symptoms.
"Researchers have suggested that the relationship may be explained by the direct pressure of overweight on the lungs or by the general increase in inflammation created by overweight."
But, he said, their study suggested "another mechanism also "
" We have found that excess fat builds up in the walls of the airways, where it takes place and seems to increase inflammation in the lungs, "said Dr. Noble.
" We believe it causes airway thickening limits this air flow in and out of the lungs and this may at least partly explain the increase in asthma symptoms. "
" Essential "
Prof. Thierry Troosters, President of the European Respiratory Society, said:" This is an important finding for the relationship between body weight and respiratory diseases because it shows how being overweight or obese can worsen the symptoms in people with asthma.
"This goes beyond the mere observation that obese patients should breathe more with activity and exercise.
"Observation points for real changes in the airways that are associated with obesity."
He said that more research was needed to determine whether this accumulation of fat could be recovered through weight loss, but patients with asthma should be assisted in achieving a healthy weight.
Dr. Elizabeth Sapi, a reader of respiratory medicine at the British Thoracic Society, stated that this was the first time that body weight had an effect on the structure of the airways in the lungs.
"Given the increasing incidence of obesity nationwide and worldwide, the study may be essential to help us understand why asthma remains a major health problem and identify new ways to improve treatment for obesity. asthma, "she said.
"This is just a small study, but we need to evaluate it in larger patient groups and other lung diseases," added Dr. Sapi.