(CNN) -- Boys with ADHD may be at risk for obesity later in life, according to a new study - which, if confirmed in larger studies, may have implications for more than 4 million kids in the United States living with the disorder.
Researchers at NYU's Langone Medical Center have been following more than 200 kids for four decades. They found those who had ADHD in their early years were twice as likely to be obese at age 41.
"This study was started by Dr. Rachel Klein in 1970, and it involved a number of waves of evaluation, during which the results of having hyperactivity in childhood were assessed," said Dr. F. Xavier Castellanos, a professor of child and adolescent psychiatry at NYU and one of the study authors.
"We brought back individuals who were 41 years of age, and examined a number of measures, including brain imaging analyses. But during those brain imaging analyses, we noted that men who had been hyperactive children had a greater difficulty sitting in the scanner - they were too large for the research scanner."
That's when the idea took shape to look at all of the subjects' height and weight. Castellanos and his team instantly noticed the high levels of obesity -- twice as high as those adults who never suffered from ADHD.
"This was not the first time this has been noted, so in that sense it is a confirmation," said Castellanos. "But other studies have not been able to be as definitive. Other studies have found a general tendency towards increased weight, but this is the first study that puts this in terms of clear clinical obesity."
However, there is no clear reason as to why ADHD may lead to obesity.
"The most reasonable explanation is that the characteristics of ADHD which involve being impulsive - having a difficult time selecting between (things) that maybe immediately gratifying but in the long run are not such a good idea - that that translates to the choices that are made at lunchtime and dinner and snacking," said Castellanos, though he said there was no direct evidence of that being the case, only speculation.
Other experts say while this correlation appears to be strong, more research needs to be done.
"The sample size was relatively small, and they only looked at white men," said CNN.com expert Dr. Jennifer Shu, a spokeswoman for the American Academy of Pediatrics. "That said, their conclusion summed it up nicely: people need to be aware that having childhood ADHD may put them at risk for later obesity."
Shu also suggested another possible explanation for the link - current treatments are largely centered around stimulant medications, which tend to reduce appetite. If the medication is stopped, appetite increases and patients may start gaining weight.
The bottom line, says Castellanos?
"It's very difficult across the board for people to lose weight and keep it off, so it's one of those things that is really best prevented," he said. "That's the major importance of alerting the public - we can look into the future and say, 'This is coming up, so it's better to not ignore this potential risk and wait for it to become a problem.'"