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NEW YORK (Reuters Health) – Overweight children and teenagers may eat more when they have a snack with an overweight friend rather than a thinner peer, a new study suggests.

In a study of 9- to 15-year-olds, researchers found that all kids, regardless of their weight, tended to eat more when they had the chance to snack with a friend than when they were with a peer they did not know.

But the biggest calorie intakes were seen when an overweight child snacked with an overweight friend.

The findings, reported in the American Journal of Clinical Nutrition, highlight the role of friends' influence in how much kids eat -- and, possibly, in their weight control.

It's not surprising that children eat more when they are with friends instead of strangers, according to lead researcher Dr. Sarah-Jean Salvy, an assistant professor of pediatrics at the State University of New York at Buffalo.

The same pattern has been found in adults, Salvy told Reuters Health in an email. This, she explained, may be partly because people are more self-conscious around strangers, and partly because friends act as "permission-givers."

"They set the norm for what is appropriate to do, or in this case eat," Salvy said.

For the study, Salvy and her colleagues had 23 overweight and 42 normal-weight children and teens spend 45 minutes with either a friend or an unfamiliar peer. Each pair was given games, puzzles and books for entertainment, along with bowls of chips, cookies, carrots and grapes.

Overall, the researchers found, pairs of friends downed more calories than did unacquainted pairs. And overweight friends consumed the most -- 738 calories, on average, versus 444 calories when an overweight child was paired with normal-weight friend.

Normal-weight kids consumed an average of about 500 calories when paired with a friend, regardless of the friend's weight.

Salvy noted that a recent study of adults found that people were more likely to gain weight over three decades if their same-sex friends were overweight or obese -- suggesting a role for "social influence" in body weight.

When it comes to children and teens, it's known that many follow their friends' lead in deciding whether to smoke or drink. The current findings, Salvy said, suggest that kids' eating habits are also "largely determined by their social network."

The good side of that, according to Salvy, is that helping one child make healthy changes may end up influencing his or her friends as well. She said her research interest now is to see whether there is in fact such a "contagion effect" on friends' eating habits.

SOURCE: American Journal of Clinical Nutrition, August 2009.