Harvard researchers say a study of thousands of nurses has provided the first strong evidence that obesity greatly increases the risk of developing asthma.
Even a little bit of fat increases the risk, and obese people _ those who are roughly 35 percent overweight _ are at least three times as likely to develop the respiratory ailment that afflicts millions of Americans, the researchers say.
For years, doctors have been aware of a connection between obesity and asthma. But the general assumption was that asthma comes first, that patients were prone to putting on weight because breathing problems limited exercise.
The new study was designed to test that assumption, and the results "guarantee that obesity preceded the diagnosis of asthma," said research leader, Dr. Carlos A. Camargo Jr. of Harvard Medical School in Boston.
One outside expert, however, said more study is needed.
The results will be presented this week at the international conference of the American Thoracic Society and the American Lung Association.
Both obesity and asthma are on the rise in developed nations. From 1982 to 1994, the rate of asthma rose 61 percent in this country, where health officials say one in three people is obese. An estimated 15-million Americans have asthma, which kills some 5,000 people a year.
Camargo's study used data from women in the Nurses Health Study II. Of the 89,061 nurses tracked in the asthma study, 1,652 developed the ailment from 1991 to 1995.
Their weight and height were known at the start of the survey, which was controlled for such factors as age, race, smoking and physical activity.
How obesity increases risk is unknown, Camargo said, but he speculated that excess weight compresses the airways, making them smaller and therefore more reactive to cold and other asthma triggers.
"This is certainly a very useful study if true, and it seems plausible," said Dr. Ronald M. Ferdman, an asthma specialist at Childrens Hospital Los Angeles. "But the explanation about compressing airways seems too simplistic."
Dr. Richard Honsinger, a clinical professor at the University of New Mexico, said he wasn't convinced the causal effect of obesity has been proven.
He wondered if any genetic link exists between asthma andobesity and said more data is needed on the exercise routine of asthma sufferers.
"How many people have a little bit of asthma and don't exercise, and so get fat?" he asked. "Like every good preliminary study, it (Camargo's research) raises more questions than it answers."
In another study to be presented at the conference, British researchers studied the relationship of asthma to birth weight, and to weight and height at age 26.
The heaviest adults among the 8,000 people studied were 80 percent more likely to have asthma than the thinnest ones.