It was a small difference overall, Chakravorty said. And for children who were normal weight, there was no major effect.
"It's not making normal-weight children obese," Chakravorty noted.
But there was a clearer impact on kids who were overweight before surgery. Of those children, 52 percent had become officially obese seven months after surgery, compared with 21 percent of overweight children in the watchful-waiting group, the study found.
There are a few possible explanations for the post-surgery weight gain, according to Katz's team. Calorie-burning may dip when children are no longer laboring to breathe during sleep. And some kids may burn fewer calories during the day because they become less active after their sleep apnea improves. (Paradoxically, poor sleep often causes children to be "hyperactive," rather than drowsy, Katz explained.)
Sleep apnea itself also causes metabolic changes, Katz said. Growth hormone is released at night, and the sleep disorder can interfere with that. So the body may adapt metabolism in an effort to maintain a child's growth.
"When the sleep apnea is relieved, they're set up for rapid weight gain," Katz said.
But these days, with childhood obesity on the rise, many kids with sleep apnea are already overweight or obese. If they rapidly put on pounds, Katz said, their sleep apnea might return in six months to a year.
"After surgery, parents are often very satisfied," Katz noted. "Their kids are sleeping better, and they may be better behaved and doing better in school. But there's this insidious issue of weight gain."
Chakravorty agreed that diet and exercise are key for children with sleep apnea -- but she said that's true regardless of the treatment approach. And, for heavy kids, weight loss alone might clear up the sleep apnea.
But if children ultimately do need surgery, parents shouldn't skip it due to worries about weight gain, according to Katz. He noted that if improved sleep apnea symptoms explain the post-surgery pounds, then other treatments -- including medication or continuous positive airway pressure devices -- could also spur excess weight gain.
"I think there are implications beyond surgical treatment," Katz said.