Asthma is generally thought to influence sedentary behavior rather than vice versa: A wheezy child opts to watch TV rather than risk an exercise-induced asthma attack.
A study from Scotland published online by the journal Thorax found the opposite to be true.
The study tracked 3,065 children from 3.5 years of age to 11.5 years. None had signs of asthma at the beginning of the study. About 6% of the children had asthma by age 11.
Those who watched TV for more than two hours a day in early childhood were almost twice as likely to have been diagnosed with asthma by age 11 as those who watched less.
What the researchers were striving to measure was not the effects of TV content (which may make you fat or get you pregnant but not necessarily sicken you, at least not physically) but the effects of sedentary behavior on the development of asthma. Television viewing served as a proxy for being a couch potato.
By limiting the study to children who had no symptoms of asthma at age 3.5, the researchers minimized the possibility that wheezing led children to avoid exercise.
(The data came from a a study of the long-term health of 14,000 children and their parents and dates from the mid-1990s, when personal computers were not in widespread use. So children who were not parked in front of the tube could be presumed to be doing something more active than updating their MySpace page or playing Tap Tap Revenge on their iPod Touch.)
The relationship between physical activity, sedentary behavior and asthma is complicated, the researchers said. One hypothesis is that failure to stretch airway muscles by the kind of regular deep breathing that comes from exercise may contribute to the development of asthma.
The damage appears to occur early. By age 11, pretty much everybody was a couch potato, but asthma rates did not continue to rise in adolescence.
"There may be a critical window for lung development, but we have not pinpointed it," said Dr. Andrea Sherriff of the Faculty of Medicine at the University of Glasgow in Scotland, a co-author of the study, in an e-mail.
Future studies will include a randomized, controlled trial to see if physical activity provides protection from the disease, she said.
-- Mary Engel