Though not completely surprising, that finding could be important as parents, after-school teachers and camp counsellors try to encourage youngsters to move more and head-off obesity before it starts, researchers said.

The results are also in line with research that’s been done in teens and adults, who tend to look like the rest of their friend group in terms of weight and fitness level.

“This is more evidence that peers and social networks do influence health behaviours,” said Dr Pooja Tandon, a childhood obesity researcher from the University of Washington in Seattle who wasn’t involved in the new study.

“The next steps will be (understanding) how to harness the power of social networks to promote health behaviours,” such as physical activity in kids, she added.

Researchers from Vanderbilt University in Nashville studied 81 racially diverse public school students, ages five to 12, who went to after-school programmes at one of two different sites.

To see how the kids’ friendships affected their physical activity — and vice versa — paediatrics researcher Sabina Gesell and her colleagues spent time with the students during three week-long periods over the spring of 2010.

Over the course of the study, Gesell and her colleagues found that kids didn’t necessarily make or break friendships based on how active they were compared to other students.

Instead, when kids made new friends who were more or less active, they tended to change their own activity level accordingly, the researchers reported Monday in the journal Pediatrics.

The research is still a few steps away from leading to changes in how after-school programmes are run in the real world, Gesell said. But in the future, counsellors could shake up sedentary friend groups and encourage a couple of less-active kids to join those that go straight to the gym or the playground, she added.

“The after-school programmes have had this long history of keeping kids safe and keeping them off the street,” Gesell said. “Now the thought is, what if we use this ideal arena to improve health?”