Researchers say they may have worked out why the obese are more prone to asthma than those of normal weight.
The protein may increase appetite as well as inflaming the lungs
The link between the two conditions is well-established, but the relationship is ill-understood.
Now scientists at King's College London say they have pinned down a protein which contributes to inflammation of the lungs as well as increasing hunger.
The study in the Proceedings of the National Academy of Sciences said further research was now needed.
The researchers investigated molecules produced by Th2 cells - specialised cells belonging to the immune system which can inflame the lungs and contribute to the development of asthma.
But these cells also produce a protein known as PMCH which is known to increase appetite.
"These findings may provide a mechanistic link between allergic inflammation, asthma and obesity," the researchers wrote.
Several European and American studies have found a link between obesity and asthma which cannot be explained by weight gain brought on by the inactivity asthma encourages. In many cases, the obesity precedes the asthma.
One study of 330,000 patients published earlier this year found that for every normal weight person with asthma, there were 1.5 who were overweight or obese.
The latter category effectively ran a 50% greater risk of developing the condition.
However, people with asthma are not always obese, so the lead researcher of this latest study, Dr David Cousins, said further investigation was needed into possible genetic variations of PMCH, the gene known to boost appetite.
Dr David Haslam of the National Obesity Forum said that from the obesity perspective, the research was interesting although as yet there were no therapeutic implications.
"Working out the mechanisms, the links between diseases is important, and it adds to the growing body of evidence which gives obesity some form of genetic basis."