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A new U.S. study has a good news-bad news take on the "childhood obesity epidemic." Better-off teens in the United States appear, over all, to be getting thinner. But their less-affluent counterparts are still gaining pounds.

The links between poverty and obesity are already well documented in many countries, including Canada. But according to a new study, analyzing data from two major U.S. longitudinal surveys, Harvard University researchers found that from 1999 to 2002, obesity increased at similar rates among all adolescents, regardless of socioeconomic status. Other research has recently found that increase in obesity appears to have stalled.

But the new study, published in the Proceedings of the National Academy of Sciences, suggest that rates have declined among higher-income teens but continued to rise among their poorer peers. The authors found that from 1999 to 2010, as education campaigns about healthy eating and exercise expanded, higher income families reported eating fewer calories and exercising more.

As co-author Kaisa Snellman explained in an e-mail interview with The Globe and Mail, the study found that one in five kids from less-educated, low-income families could be categorized as physically inactive, which means they get no moderate-to-vigorous activity at all. (Health experts typically recommend 60 minutes a day for kids.) By comparison, only one in 10 teens with college-educated parents were getting no physical activity.

"The fact that wealthy and poor kids differ so much in their exercise is very troubling," says Snellman, who is now a professor for INSEAD in France. "If the current trend continues, the health divide among American kids is going to become even more pronounced."

One consequence, she points out, is that an increased rate of obesity means poor teenagers will be more likely to have serious health problems as adults.

The authors, which include Carl Frederick and Robert Putnam, suggest in the paper that public education campaigns need to better recognize the rising class factors that contribute to obesity and to target lower income families.

It also means schools need to play a bigger role in creating equal access to physical education and sports, Snellman says. One suggestion she makes is extending recess. A recent study found grade school students tend to burn more calories during these break times than during traditional gym class. "This is one reason why the increase in 'pay to play' programs is troubling," she says.

But aside from better messaging about healthy eating habits and longer playground time – and in light of 2014 being the 50th anniversary of former U.S. president Lyndon B. Johnson's War on Poverty campaign – advocates may point to a more broad-based strategy: to make better progress reducing the number of children growing up in poverty in the first place.

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