While some behaviors increase the risk of obesity for both boys and girls, new research shows there are gender differences.

For instance, although being on a sports team reduced the risk of obesity for middle school-aged boys; it did not for girls, said study author Dr. Elizabeth Jackson, an assistant professor of medicine at the University of Michigan School of Medicine.

On the other hand, “Girls who drank milk seemed to have more protection [against obesity],” she said.

Meanwhile, certain behaviors raised the risk of obesity for both boys and girls, the study found. Eating school lunch regularly increased the risk of obesity by 29 percent for boys and 27 percent for girls. Watching two or more hours of television a day boosted the odds of obesity by 19 percent for both genders. The study, which found links but not cause and effect, is published online Aug. 12 and in the September print issue of Pediatrics.

Childhood obesity is a major public health concern. During the past 30 years, obesity has increased dramatically among children and teens. Among middle-school children, for instance, nearly 20 percent were obese, according to a 2010 report.

Earlier this month, a new report from the U.S. Centers for Disease Control and Prevention held a glimmer of hope: Obesity rates among low-income preschoolers had dropped slightly in at least 19 states. However, there is still a long way to go, experts agreed.

In the new study, Jackson looked at data obtained from more than 1,700 sixth-grade students from 20 schools in Michigan between 2004 and 2011. The researchers had information on body mass index (a measure of body fat), blood pressure, cholesterol, blood sugar and other measures of health, along with habits such as TV viewing.

More than 37 percent of boys and about 31 percent of girls were overweight or obese.