(CNN) -- Even a little bit of television viewing goes a long way to potentially hurt a child's health, according to new research.
Kindergartners and first-graders who watched even an hour of television a day were more likely to be overweight or obese, according to new research presented at the Pediatric Academic Societies annual meeting in San Diego.
The research builds on existing studies that have shown a direct link between sedentary behavior and obesity for children and adults.
Earlier studies have shown the more TV that people watch, the more likely they are to gain weight. Children who have televisions in their room are also more likely to be overweight or obese, research suggests. TV habits made early on can lead to a lifetime of weight problems.
That's in large part why parents are encouraged to create "screen-free" zones in their home and limit screen time to no more than two hours a day, according to the American Academy of Pediatrics, and children under the age of 2 should not watch any television.
This study, from Dr. Mark DeBoer, an associate professor of pediatrics with the Division of Pediatric Endocrinology and Diabetes at the University of Virginia, suggests that even two hours may be too much.
On average, kindergartners watch about 3.3 hours of television, according to this study. But those who watched even as little as an hour of television were 50% to 60% more likely to be overweight and 58% to 73% more likely to be obese, compared with those watching less than an hour. And children who watched an hour or more of TV daily were 39% more likely to become overweight and 86% more likely to become obese between kindergarten and first grade.
"And it's important to point out this is even after taking into account parental income and education and other factors that have an impact on obesity," DeBoer said.
The study looked at data from over 11,000 children enrolled in the Early Childhood Longitudinal Study in the 2011-2012 school year. A year later, researchers interviewed the majority of the children's parents about their children's screen time. They also measured the children's height and weight.
It was television viewing that seemed to make the most difference in a child's weight gain. Scientists saw no similar results with kindergartners who were using computers.
What is it about television that may lead to obesity in children and adults? Since this was an observational study, researchers don't know exactly, but other studies with more controls have shown that it may have something to do with television being a passive activity.
Sedentary behavior can replace the time children would normally spend on physical activity. Other research has looked at the influence commercials have on people's eating behaviors. Commercials can encourage children to eat food or consume drinks that are rich in calories. People tend to snack more when watching TV and sleep less, which is also a risk factor for obesity.
As a pediatric endocrinologist, DeBoer often has to have the difficult conversation with parents about their child's weight. "I see a lot of children with weight problems, and the impact that has on their overall health after the course of a lifetime is significant and can contribute to many ill health effects," he said.
Childhood obesity in the United States has more than doubled in the past 30 years, according to the Centers for Disease Control and Prevention. In 2012, more than a third of children and teens were overweight or obese. Children who are overweight are more likely to have problems with cardiovascular disease. They are also more likely to have prediabetes. They are more at risk for many kinds of cancer and are more likely to be bullied than thinner children.
The good news is that when parents do set TV-watching rules, children do watch less, other research shows. Parents who also encourage children to play or engage in some other physical activity like dance are typically successful in curbing a child's chance at gaining weight.
DeBoer thinks it is never too early create an environment that will help a child maintain a healthy weight. "It's hard to turn around weight gain later on," he said. "So we need to be action-minded and prevention-minded with this issue."
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