Children who are overweight or obese are at increased risk for asthma, researchers report.

A retrospective study, published in Pediatrics, included 507,496 children followed for an average of four years. None of the children had incidents of asthma before the start of the study.

The researchers divided the children into three groups: overweight, defined as the 85th to 94th percentile for weight; obese, the 95th percentile or higher; and normal weight, the 25th to 64th percentile.

The study controlled for age, race, health insurance, food allergies, medications and other factors, and found that compared with children of normal weight, the overweight were 17 percent more likely to have been given a diagnosis of asthma and to take an asthma medication. The obese were 30 percent more likely.

The researchers estimate that 23 to 27 percent of asthma cases in obese children can be attributed to the obesity itself. Among all the children, 10 to 13 percent of asthma cases were attributable to obesity.

“Asthma is the most common chronic disease of childhood, the most common reason kids lose time in school, and the most common reason for hospitalization,” said the lead author, Dr. Jason E. Lang, an associate professor of pediatrics at Duke University. “There aren’t many preventable risk factors for it, and this study suggests that obesity may be one of them. So it’s really important to keep kids at a healthy weight.”