Web surfing may not be as big a factor in teenage weight gain as how many excess pounds children are carrying around at the start of adolescence, a Swiss study suggests.

Researchers followed a group of 621 youths from age 14 to 16 and found the teens who were overweight at the start of the study were 20 times more likely to be overweight two years later than their peers who began at a healthy weight. For those who became overweight, excessive Internet use wasn't linked to the added pounds.

"Internet use could at most reinforce an already existing risk of being overweight," the study team concludes in the International Journal of Obesity.

"Nowadays Internet use is almost a necessity to survive in this world, as youths are asked to use this technology," lead study author Yara Barrense-Dias of the Institute of Social and Preventive Medicine at Lausanne University Hospital told Reuters Health by email.

"For this reason we must differentiate between screen time devoted to school or work and screen time devoted to leisure," Barrense-Dias added. "Parents should encourage their children to do other activities on the side, to eat healthily and do physical exercise regularly."

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Globally, roughly 1.9 billion adults are overweight or obese, as are about 42 million children under the age of 5, according the World Health Organization. Obesity increases the risk of heart disease, diabetes, joint disorders and certain cancers.

While many things - including activity levels and eating habits - can influence whether people become obese, scientists are increasingly looking at the role sedentary time spent in front of computers and television screens plays in weight gain.

To explore how web surfing influences weight gain, Barrense-Dias and colleagues surveyed teens to gather data on their height and weight, how much time they spent online and how much they devoted to exercise, as well as their eating habits, among other things.

At the start of the study, 13.5 percent of boys and 8.8 percent of girls were overweight. By the end, 19.4 percent of boys and 12.4 percent of girls were overweight, the study found.

Teen boys got more than three days a week of exercise at the start of the study, but less than three days by the end. Girls, meanwhile, started out with slightly less than three days a week of activity and ended with slightly more than two days.

Internet use didn't appear to impact whether the teens gained weight during the study period, though boys were more likely to be overweight than girls.

Limitations of the study include the reliance on teens to report on their own activities, height and weight, the researchers acknowledge. The study also excluded other screen time such as television or video games.

It's possible the study is too small, and the self-reported results too unreliable, to draw broad conclusions about the connection between teen Internet use and weight gain, said Dr. Paul Collings of the Bradford Institute for Health Research in the U.K.

"It has long been known that overweight and obesity track over time," Collings, who wasn't involved in the study, said by email. "From an energy balance perspective, active lifestyles and a balanced diet will help to prevent obesity."

From a biological standpoint, sedentary time can be associated with pediatric obesity because kids who sit around may burn fewer calories, snack more and sleep less, said Jonathan Mitchell, a pediatrics researcher at the Children's Hospital of Philadelphia.

"Sitting while using the Internet is one of many sedentary behaviors," Mitchell, who wasn't involved in the study, said by email. "Parents can certainly encourage their children to be more active and help them manage their sedentary time in the home during their free time."