[summary]

How much we weigh as adults is more influenced by the people around us during childhood than those we spend time with as adults, a new study suggests.

The study compared the weights of 236 pairs of adolescent siblings who lived together, and 838 pairs of adult siblings who did not live together. In both groups, the body mass indices (BMIs) of siblings became more similar over time. Body mass index is a ratio of weight to height, and is considered an indicator of body fatness.

The researchers found that family factors, such as heredity and a similar upbringing, explained similarities in BMI in both the siblings who lived together and those siblings who didn't. This suggests that behaviors formed in childhood are carried into adulthood, said study researcher Heather Brown, of Newcastle University in the United Kingdom.

The findings demonstrate the importance of early interventions to prevent obesity, Brown said.

The results also suggest the effect of our adult social networks on our weight may have been overestimated, the researchers said. In the study, factors that changed over time, such as friends and opportunities for exercise, explained similarities in BMI only for adolescent siblings.

That's not to say that our larger social circles aren't important, Brown said. But it may be that the people we interact with intimately, such as those in our household, are the most important people in terms of influencing our health-related behaviors, Brown said.

As we grow up, our siblings may have less of an impact on our weight, because we move to different households. Indeed, although siblings were similar in BMI in both childhood and adulthood, they were less similar in adulthood.

The findings do not overturn the idea that obesity can "spread" between friendswho are grown adults, said Daniel Hruschka, of the Arizona State University School of Human Evolution and Social Change, who was not involved in the study. The study can only speak to changes in social interactions between siblings, and suggests that as we grow up, obesity may not "spread" between siblings as easily as when we're young.

But our social networks outside our immediate siblings may still contribute to obesity spread, Hruschka said.

The study is published in the journal Obesity.

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