Scientists have discovered a link between obesity and asthma which may explain why overweight people are up to twice as likely to develop the respiratory condition.
The finding suggests there is a common underlying process behind many cases of combined obesity and asthma, leading doctors to believe it could pave the way for new drugs to treat both conditions.
The rapid rise in obesity in recent decades has been mirrored by a steep increase in asthma, but this is the first study to find evidence that the two could be linked.
Asthma is caused by an overreaction of the immune system, where cells that evolved in humans long ago to fight parasites such as flukes and worms instead react to grass pollen, house dust mites and pet allergens. In the lungs, these cells, known as Th2 cells, cause inflammation which triggers asthma.
Researchers at the Centre in Allergic Mechanisms of Asthma at King's College London extracted Th2 immune cells from the blood of obese asthmatics and found that as well as causing asthma, they secreted high levels of a hormone, promelanin. The hormone is usually found in the brain, where it plays a crucial role in governing appetite.
David Cousins, an immunologist at the centre, said the discovery may explain why obesity and asthma often go hand in hand.
"We know that women who are obese are twice as likely to be diagnosed with asthma as women who aren't overweight, and that if obese people with asthma manage to lose weight, their asthma usually improves."
Previously, some doctors had suspected that the two conditions were linked simply because people with asthma may put on weight because of difficulties in exercising.
Tak Lee, director of the MRC-Asthma UK Centre, said: "The implication from this research is that both asthma and obesity are serious health problems and this could open a door to tackle both problems." The study appears in the journal Proceedings of the National Academy of Sciences.
The scientists do not believe the new link can account for all cases of asthma and obesity. But Dr Cousins said that in some people, it may lead to a vicious circle whereby an obese person develops asthma, and then becomes more obese because their immune cells churn out more promelanin.
"It can't be the only factor. There are still lots of fat people who aren't asthmatic and asthmatics who aren't large, but we are now interested to know whether there's a genetic factor here," added Professor Lee.
Other teams are exploring whether promelanin can be targeted to suppress appetite.
"What we do not know is whether this protein has other important effects which would be affected if you suppressed it," Prof Lee said.
Since 1980, obesity rates in England have risen dramatically, from 6% in men and 8% in women to 23% in both sexes in 2004.
Asthma now affects 5.2 million people in the UK, 2.6 million severely. According to the patients' group Asthma UK, childhood asthmas have more than doubled since the 1960s.
Jenny Versnel, director of research and policy at Asthma UK, said: "No one has known whether people become obese because they have asthma and can't exercise.
"This paper suggests a reason for that link. We know that people who are obese often find it harder to manage their asthma symptoms and may even respond less well to their asthma treatments. This research is important because it could potentially help some people with asthma to gain more effective control of their condition."
