Americans will keep growing fatter until 42 per cent of the nation is considered obese, and having fat friends is part of the problem, researchers said on Thursday.
The prediction by a team of researchers at Harvard University contradicts other experts who say the nation's obesity rate has peaked at 34 per cent of the U.S. population.
The finding is from the same group, led by Nicholas Christakis, that reported in 2007 that if someone's friend becomes obese, that person's chances of becoming obese increase by more than half.
They now think this same phenomenon is driving the obesity epidemic, which will climb slowly but steadily for the next 40 years.
Alison Hill, a graduate student at Harvard and the Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology, said the study is based on the idea that obesity can spread like an infectious disease and people can catch it from their friends.
For the study, she and colleagues applied a mathematical model to four decades of data from the long-running Framingham study -- a study of the health and habits of nearly an entire town in Massachusetts.
"We looked at the probability of becoming obese and what that was influenced by," Hill said in a telephone interview.
"We found there is some baseline risk of becoming obese based on the friends you have," Hill said.
Hill said that based on their calculations and looking at the influence of social interactions on obesity in the Framingham study, they think the U.S. obesity rates will top out at 42 per cent of the population.
Over the long-running study, the rate of weight gain caused by social interaction -- a person's contact with friends who are obese -- has grown quite rapidly since 1971, Hill said.
"It looks like obesity is becoming more infectious," said Hill. The findings are reported in the Public Library of Science journal PLoS Computational Biology.
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