(CNN)If children in the United States and Canada faced off against each other in an international episode of \"The Biggest Loser,\" it would not look good for the U.S.

Rates of obesity among children and teenagers in the U.S. have increased substantially more than in Canada since the late 1970s, according to a new report by the Centers for Disease Control and Prevention.

Although the growing obesity epidemic in the U.S. is well known, this report shines a light on how we compare to our neighbors to the north, said Cynthia L. Ogden, an epidemiologist at the CDC's National Center for Health Statistics and lead author of the new study.

The report found that, whereas the obesity rate among children between 3 and 19 was about 5% in both the U.S. and Canada in the late 1970s, it rose to 17.5% in the U.S. by 2012 and only 13% in Canada by 2013. However in both countries, the rates have leveled off in the last 10 years.
In recent years, the difference was most stark among girls between 7 and 12, of whom 19% were obese in the U.S. (2012) compared to 9% in Canada (2013).

There did not seem to be a difference among children ages 3 to 6 or teenagers 13 to 19 between the two countries. However, the study authors pointed out that a lack of sufficient data may account for that part of the result.

The study did not look into what could be accounting for the large spread in obesity prevalence. \"Now I am hoping we will have a chance to make comparisons between dietary and physical factors between the two countries,\" Ogden said.

Why are Canadian children less obese?
A couple of changes happened in the U.S. in the 1970s that probably put the country on the fast track to being overweight, said Barry Popkin, distinguished professor of nutrition at the University of North Carolina. First and foremost, children here started to drink a lot of sugary soda. Although they have been cutting back in the last decade, they still down about twice as many sodas a day as Canadian children.

The second factor is that \"we are a snacking nation,\" Popkin said. One study found that young people in the U.S. ages 2 to 18 went from having a little more than one snack a day in 1977 to almost three in 2006. These snacks also add a lot of sugar to children's diet, Popkin said.

A big reason that U.S. children overtake their Canadian counterparts in soda consumption and snacking is marketing, Popkin said. \"Coke and Pepsi pushed a lot harder here,\" he said. And there are also areas in Canada where companies cannot market at all. Quebec province passed a law in 1980 banning marketing of fast food and beverages to children under 13.

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Part of the reason for the obesity gap between the two countries could also arise from differences among minority and poor groups in the U.S., said Adam Drewnowski, director of the Center for Public Health Nutrition at the University of Washington School of Public Health. The current study found that there was no difference in obesity prevalence between the two countries among non-Hispanic boys.

\"Minorities and the working poor in Canada benefit from a better network of social support,\" Drewnowski said. \"Obesity has everything to do with economic inequalities and economic insecurity,\" he said.

There is a hopeful note in this report in that rates of childhood obesity have plateaued in both the U.S. and Canada. But, it will take aggressive action to get the rates to start to come down, Popkin said. The rest of the U.S. could take a lesson from Berkeley, which put a tax on sugary drinks in the last year, he said.

An earlier study by Ogden and her colleagues found that the rate of obesity among adults was also higher in the U.S. (34%) than in Canada (24%) in 2008. There was also less of a difference when they looked only among non-Hispanic whites.