While some behaviours 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 behaviours raised the risk of obesity for both boys and girls, the study found. Eating school lunch regularly increased the risk of obesity by 29% for boys and 27% for girls. Watching two or more hours of television a day boosted the odds of obesity by 19% for both genders.
The study, which found links but not cause and effect, is published online on 12 August 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% were obese, according to a 2010 report.
Earlier this month, a new report from the US 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% of boys and about 31% of girls were overweight or obese.
The obese boys and girls had lower HDL ("good") cholesterol, higher blood pressure and other indicators of heart disease risk than the normal or underweight students.
When Jackson looked at the habits, she found the "predictors" of obesity.
Sports-team activity reduced the risk of obesity in boys by 23%. Milk drinking reduced the risk of obesity in girls by 19%. Jackson said it's possible that those who drank milk may be less likely to drink sugary beverages, which are linked with weight gain.
The link between TV viewing and weight issues is well known. The risk of obesity linked with eating school lunches regularly, she said, may be related to the fact that children who often eat school lunches (sometimes subsidised) may be from lower-income families, and lower socioeconomic status has been linked with a higher likelihood of obesity.
Jackson said sports may not have shown up as a risk-reduction behaviour for girls because they may have underreported. For instance, they may not have considered dancing or cheerleading as sports activities.
"There are no big surprises really in the major findings, all have been previously reported," said Michael Goran, a professor of preventative medicine and director of the Childhood Obesity Research Center at the University of Southern California Keck School of Medicine. He reviewed the findings.
Strategies to reduce the risk of obesity vary by age, Goran said. For middle-school children, he suggests that parents reduce the intake of sugary beverages, set limits on television and other media time, limit desserts and other treats. "Establish patterns as early as possible for healthy eating and active living," he said.
"Shop, cook and eat together and include children in the decisions and planning around meals, treats and activity," he said.
To that, study author Jackson added that schools and parents could also encourage girls to participate more in sports.
To learn more about fighting childhood obesity, visit Let's Move!