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North American schools have been busy banishing chocolate bars, potato chips and pop from cafeterias and vending machines (and even fundraising drives ) – replacing them with salads and veggie wraps to help kids stay thin as the population keeps getting fatter.

But the soda strike may be a waste of time, a new U.S. study says. (And not just because students are skipping off to McDonald's for lunch.) Turns out, school junk food isn't causing obesity in today's youth, at least not in middle school.

Using data from a national survey to track 19,500 American students who attended school in the same county in both Grade 5 and Grade 8, researchers at Pennsylvania State University found that while there had been a "significant increase" in the percentage of schools selling junk food in that time frame, there was no corresponding rise in the percentage of students who were overweight or obese. Instead, the percentage of overweight students dropped between those grades, to 35 per cent from 39 per cent.

The results were so surprising, explains lead author Jennifer Van Hook, a sociology professor at Pennsylvania State University, that researchers held off publishing the results for two years to make sure they had it right.

"There has been a great deal of focus in the media on how schools make a lot of money from the sale of junk food to students, and on how schools have the ability to help reduce childhood obesity," Prof Van Hook said in a press release. "In that light, we expected to find a definitive connection between the sale of junk food in middle schools and weight gain among children between fifth and eighth grades. But, our study suggests that – when it comes to weight issues – we need to be looking far beyond schools and, more specifically, junk food sales in schools, to make a difference."

Her finding calls into question the effectiveness – and cost – of several provincial initiatives to restrict junk food in schools. Quebec, for example, has prohibited the sale of deep-fried foods and soft drinks on high-school grounds. Ontario, British Columbia, Nova Scotia and New Brunswick now have mandatory food and beverage policies for schools.

Prof. Van Hook says that public policy might achieve better results by focusing on home life and neighbourhoods, as well as working to create healthy eating habits even before children attend school. By middle school, her research suggests, the kind of food sitting in a school vending machine isn't the problem.

Another idea: Schools could spend the money they make on chocolate-covered almonds to fund extracurricular sports.

Should schools ban junk food even if it isn't proven to combat obesity?

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