Having attention-deficit/hyperactivity disorder (ADHD) may put children at higher risk of becoming obese in adolescence, a new study suggests.

Although previous reports have linked ADHD and obesity, it hasn't been clear exactly what explains the relationship or whether the behavioral difficulties or extra weight tends to come first.

"In general, people think of children with hyperactivity as moving around a lot and therefore should be slim," so this connection seems counterintuitive, senior author Alina Rodriguez said. She worked on the study at Imperial College London in the UK.

But kids who have ADHD tend to be overactive in a fidgety way, Rodriguez said.

"Children with ADHD are not more likely to participate in physical activity, as we show in our report," she told Reuters Health in an email. Rather, her team's results suggest kids with behavioral difficulties are actually less likely to be active as they get older.

According to the American Psychiatric Association, about five percent of U.S. kids have ADHD, which usually involves having trouble paying attention and poor impulse control.

The condition can't be cured, but can be treated with a combination of medication and behavior therapy, according to the Centers for Disease Control and Prevention.

The new study included almost 7,000 Finnish children born in 1986. When the kids were seven or eight years old, teachers told the researchers about their symptoms of ADHD or conduct disorder, a psychological disorder involving antisocial behavior. Parents reported their children's weight and height and how much time they spent actively playing.

When the kids were 16, the researchers asked their parents about their ADHD symptoms and the teens themselves about their physical activity and how often they engaged in binge eating, or "devour(ing) large amounts of food." The researchers also got information on young people's height, weight and waist size from doctors' examinations.

Kids who had symptoms of ADHD at a young age were almost twice as likely to be obese as teens, according to results published in the Journal of the American Academy of Child and Adolescent Psychiatry. That was true even after the researchers took their childhood weight into account.

Childhood ADHD or conduct disorder symptoms were linked with too little physical activity in adolescence, but not with binge eating.

Kids who were less active in childhood were also more likely to have trouble paying attention as teens. That means the relationship between ADHD and lack of physical activity probably goes in both directions, the authors write.

"This question is important to address not only on theoretical terms because it tells something about the origins of each disorder, but it also gives us useful information for prevention," Rodriguez said.

Researchers have known about the link between ADHD and later obesity for perhaps a decade, according to Dr. L. Eugene Arnold, a child psychiatrist and professor emeritus at The Ohio State University in Columbus.

The resulting extra weight for teens probably also means extra weight in adulthood, but he noted that some teens gain weight early before a growth spurt and will end up slimmer.

"Anything regarding obesity is important, with a good third of the population struggling to keep weight down," Arnold said.

Although the kids in this study were not on medication for ADHD, most ADHD drugs suppress appetite at first, he told Reuters Health.

There is some evidence that kids with ADHD tend to eat more carbohydrates and less protein than other kids, he noted.

Parents of kids with ADHD should encourage active pursuits and join the kids in being active if necessary, limit screen time and keep healthy snacks on hand to help prevent teenage weight problems, Arnold said.

Parents of all children should encourage physical activity, Rodriguez added.

"It may be possible that we could ameliorate problems with both ADHD and obesity by encouraging children to be more physically active," she said.