Doctors have long known that obesity increases the risk for developing asthma, but new research suggests the opposite also may be true.

Scientists in Spain found that people with asthma are at greater risk for obesity, particularly those who develop the condition as adults and those diagnosed with asthma without allergies.

"We already know that obesity can be a trigger for asthma, perhaps via a physiological, metabolic or inflammatory change. Until now, there has been very little research on whether the reverse is true, whether asthma can lead to obesity," said Dr. Subhabrata Moitra. He is a European Respiratory Society research fellow at the Barcelona Institute for Global Health.

"In this study, we have enough people and we have followed them for long enough to observe the relationship between these two conditions," Moitra said in a society news release.

The researchers analyzed data on more than 8,600 people from 12 countries included in the European Community Respiratory Health Survey. None of the participants was obese when the survey began.

Those who ever had an asthma attack, woke up due to shortness of breath or were taking asthma medications were classified as having asthma. The researchers followed up with each participant after 10 years and again at 20 years.

After taking other risk factors into consideration — such as age, gender, and level of physical activity — the investigators found that about 10 percent of those who had asthma when the study began were obese 10 years later. The same was true for only about 8 percent of those who didn't initially have asthma.

"Our findings suggest the relationship between the two conditions is more complicated than we previously realized," Moitra said. "It's important that we do more work to pick this apart. For example, we do not know why having asthma increases the risk of developing obesity or whether different asthma treatments have any effect on this risk."

According to Guy Brusselle, chair of the European Respiratory Society Science Council, "This research is an important step in helping us untangle the relationship between obesity and asthma, but it also raises new questions about why the two are linked and what can be done to help patients."

The findings are scheduled for presentation Sunday at the European Respiratory Society International Congress in Paris. Research presented at such meetings should be considered preliminary until published in a peer-reviewed journal.