Boys are more likely to wheeze - but are likely to grow out of it
Boys are more likely than girls to grow out of childhood asthma when they hit their teenage years, research suggests.
In the American Journal of Respiratory and Critical Care Medicine, scientists say it points to an unknown mechanism behind the condition's development.
The Harvard Medical School experts tested more than 1,000 children's lung function over an average of nine years.
Asthma UK experts said sex hormones might play a role in the symptoms of asthma and its severity.
One in 10 children in the UK has asthma but many never go on to be chronic asthma sufferers as adults, and the precise reason for this is not fully understood.
The latest study found that while boys were more likely to wheeze in childhood, they were also more likely to shed these symptoms once they became adolescents.
Dr Kelan Tantisira, who led the study, said it was the first time that the gender differences in asthma had been documented in this way.
She enrolled the children, aged between five and 12, all of whom had mild to moderate asthma.
In annual tests over next nine years, they were given a drug to cause airway narrowing, and scientists noted the dosage that was needed to do this.
While the amount of the drug needed in girls did not change much over time, in many of the boys, bigger and bigger doses were needed year on year, suggesting that the severity of their asthma was decreasing.
By the age of 18, only 14% of the girls did not react to the drug, compared with 27% of boys.
Dr Tantisira said: "While our results were not unexpected, they do point to intriguing potential mechanisms to explain the gender differences in asthma incidence and severity.
"Especially intriguing is that the differences in gender begin at the time of transition into early puberty."
The scientists now plan to follow the children in adulthood to see how their lung function changes over time.
Leanne Male, from Asthma UK said: "We know that asthma prevalence is higher among boys than girls but that the condition, later in life, is more prevalent in women than men.
"This study is particularly exciting as it provides an insight into the mechanisms involved in asthma development during the transition period between childhood and adulthood, demonstrating how we can influence these processes and develop new targeted therapies.
"Hormones can play a key role in influencing asthma symptoms and severity, suggesting that gender is an important factor in asthma development."