Researchers followed nearly 7,000 children in Finland and found that the 9 percent who had symptoms of ADHD at age 8 were more likely to be physically inactive and obese at age 16.
The investigators also found that children who were less likely to be physically active at age 8 were more likely to have inattention when they were teens.
Moreover, they found that a condition called "conduct disorder," which the researchers said is related to ADHD, increased the risk of teen physical inactivity and obesity. Conduct disorder involves tendencies toward delinquency, rule breaking and violence.
The study was published online recently in the Journal of the American Academy of Child and Adolescent Psychiatry.
"Obesity is a growing problem that we need to watch out for in all children and young people, but these findings suggest that it's particularly important for children with ADHD," senior study author Alina Rodriguez, from the School of Public Health at Imperial College London, said in a college news release.
"It appears that lack of physical activity might be a key factor," Rodriguez said. "We think encouraging children with ADHD to be more physically active could improve their behavior problems as well as helping them to stay a healthy weight. Studies should be carried out to test this theory."
Although the study found an association between childhood ADHD and increased risk for teen inactivity and obesity, it did not establish a cause-and-effect relationship.
Obese children and teens are considered at increased risk for a number of short- and long-term health problems, such as type 2 diabetes, heart disease and mental-health disorders.
The U.S. National Institute of Mental Health has more about attention-deficit/hyperactivity disorder.