Asthmatic children are more than 50 per cent more likely to become obese within ten years of being diagnosed, according to new research.
Scientists believe children may be at greater risk of gaining excess weight because difficulty breathing causes them to take less exercise.
However, the ten-year study of more than 2,000 children also revealed that asthmatics can reduce their chance of becoming obese by 43 per cent by using reliever inhalers during asthma attacks.
A side effect of many asthma medications is weight gain, researchers from the the University of Southern California also found.
One in five British children aged 10 and 11 are obese, while one in 11 are prescribed inhalers and other asthmatic medicines.
Scientists behind the study said they could not be sure of the exact causal dynamic between asthma and unhealthy weight gain.
“Asthma and obesity often occur together in children, but it is unclear whether children with asthma are at higher risk for onset of obesity or whether obese children develop asthma, or both,” said Dr Zhanghua Chen, who led the research.
His team examined the medical records of 2,171 five to seven-year-olds who were not obese when they joined the Southern California Children's Health Study (CHS).
At the beginning of the trial, over an eighth, or 13.5 per cent, had asthma. Over a ten-year follow-up, researchers found a sixth of all the children had become obese.
The findings were confirmed using a different group of nine to ten year olds who were followed until they left school.
Senior author Professor Frank Gilliland said the fact reliever but not preventer inhaler medications reduced obesity was a surprise.
He said: "Early diagnosis and treatment of asthma may help prevent the childhood obesity epidemic.
"Part of the problem may be a vicious cycle where asthma and obesity negatively affect each other.
Having asthma and being obese may also contribute to the development of other metabolic diseases, including prediabetes and Type 2 diabetes in later life.
Dr Erika Kennington, Head of Research at Asthma UK: “Asthma is a complex condition that affects 5.4 million people in the UK, yet years of research underfunding means it remains a relative mystery.
"Our research tells us that children who are obese are more likely to get a diagnosis of asthma, and that if you have asthma and you are obese, your symptoms are likely to be more severe.
"However, we do not yet fully understand the link between the two.
“This study is interesting in that it suggests that childhood asthma contributes to the development of childhood obesity, but more research is needed to confirm this and explain the reasons why.
"In the meantime, getting an early diagnosis and ensuring that asthma is well-managed is crucial in minimising asthma symptoms and the risk of a potentially life-threatening asthma attack."
The University of Southern California study was published in American Journal of Respiratory and Critical Care Medicine.