Females tend to have asthma later than males. (CBC)

Young boys are more apt to have childhood asthma than girls, but they're also more likely to grow out of it, say researchers who studied more than 1,000 children.

Children aged five to 12 with mild to moderate asthma were followed over nine years with tests of airway responsiveness, commonly called twitchy airways.

In people with asthma, the airways are extra sensitive and narrow in response to triggers such as pet dander or cold air. Chronic inflammation in the lungs leads to wheezing.

Researchers have noticed sex differences in asthma, with a higher incidence among boys of early school age and a higher incidence in girls during adolescence and adulthood.

Dr. Kelan Tantisira of Brigham and Women's Hospital in Boston and her team said their study is the first to examine this difference over time, by measuring airway responsiveness to methacholine powder.

"While our results were not unexpected, they do point to intriguing potential mechanisms to explain the gender differences in asthma incidence and severity," Tantisira said.

"Especially intriguing is that the differences in gender begin at the time of transition into early puberty."

Hormones a possible link

While girls showed almost no change over the years, boys seemed to improve as they got older, with the difference beginning to appear as they entered puberty.

By the age of 16, it took more than twice as much powder to provoke a 20 per cent narrowing in the boys' airways on average as it did with the girls, the team reports in the August issue of the American Journal of Respiratory and Critical Care Medicine.

By age 18, 27 per cent of boys showed an improvement in airways functioning, compared with 14 per cent of girls.

The researchers speculate that hormonal factors may play a role, although they did not study this.

Progesterone, which increases during puberty in girls, is known to make airways less likely to open. Conversely, testosterone, which increases in boys during puberty, is tied to relaxation of smooth muscle in animal studies, the researchers said.

The team hopes the findings will lead to a better understanding of how asthma arises, which may aid in the development of better therapies and preventive methods.