New American research has found a strong link between teenagers' own weight and that of their closest peers.
The journal Economics and Human Biology further justifies the notion of imitative obesity - aping of friends who gain weight.
It came to the conclusion after looking at data on nearly 5,000 teenagers, many of whom were later followed up after two-year interval.
After analyses, researchers found friendships between the adolescents tended to cluster according to weight, meaning overweight children tended to hang out together.
When they looked at weight changes over time, they found having a fat friend could lead to weight gain for a child.
The study authors from the University of Hawaii say they cannot tell from their work whether overweight teens influence their friends to become overweight or whether obese adolescents simply choose to flock together.
A spokeswoman from Weight Concern said: "We do learn from our peers and eat with our friends, so these children may be picking up unhealthy habits.
"But I would not assume that the overweight teenagers are necessarily the ones with the bad habits. Most teenagers have unhealthy diets, but not all of them are obese.
"And most of the food consumed is still at home with the family."
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