Many parents believe their children are getting a reasonable amount of recreational screen time, Mark Tremblay said. But most U.S. and Canadian kids exceed the recommended two-hour maximum per day.
"We don't pay attention to the fact that it's half an hour here, half an hour there, an hour here, an hour there," Tremblay told Reuters Health. He is the director of Healthy Active Living and Obesity Research at the Children's Hospital of Eastern Ontario Research Institute in Ottawa, Canada, and wasn't involved in the new study.
Researchers used data from a long-term study of kids who took surveys every other year. The surveys included questions about their height and weight as well as how much time they spent watching TV and DVDs and playing computer and videogames.
Kids were between ages nine and 16 when the study started.
Out of about 4,300 girls in the study, 17 percent were overweight or obese. Twenty-four percent of the 3,500 boys were also above a healthy weight.
From one survey to the next, each one-hour increase in children's daily TV watching was tied to an increase of about 0.1 points on a body mass index (BMI) scale, which measures weight in relation to height. That's a difference of approximately half a pound per extra hour of TV.
Increases in total screen time between survey periods were linked with similar but smaller changes in BMI.
"The weight of the evidence is pretty strong that television viewing is related to unhealthy changes in weight among youth," Jennifer Falbe said.
But, she told Reuters Health, "It's important for parents to be aware of all the potentially obesogenic screens that they should really be limiting in their children's lives." Increases in DVD and video watching were tied to weight gain among girls, in particular.
Falbe led the study while at the Harvard School of Public Health in Boston. She is now at the University of California, Berkeley School of Public Health.
Image: Kids watching TV, via Shutterstock