Researchers have found that adults in the U.S. have a much higher rate of obesity than Canadians, according to the U.S. Centers for Disease Control and Prevention.

The CDC found that 34 percent of Americans are obese compared to just 24 percent of those from Canada when they analyzed data from surveys taken from 2007 to 2009.

The two countries have different racial demographics. More blacks and Hispanics live in the U.S. , and both have higher rates of obesity. But even looking solely at white people, there was still a big difference — a 26 percent obesity rate in Canada compared to 33 percent in the United States.

It's not clear why that is, said study-co-author Cynthia Ogden, an epidemiologist with the CDC's National Center for Health Statistics.

"We share this border and many other things. It's a question we need to investigate more," said Ogden, whose co-authors included a researcher at Statistics Canada.

Another mystery: In children there was little difference. The childhood obesity rate was 15.5 percent in the United States and 12 percent in Canada, but the difference was not statistically significant.

This study is the first time the CDC has compared American obesity rates with another country. The report adds to a growing pile of studies that show Americans are less healthy than people in other developed nations.

For example, a 2006 Harvard Medical School study found that Americans are 42 percent more likely than Canadians to have diabetes, 32 percent more likely to have high blood pressure, and 12 percent more likely to have arthritis.

That study, based on telephone interviews rather than actual measurements, found about 21 percent of Americans said they were obese, compared with 15 percent of Canadians. That finding suggests Canadians and Americans share one characteristic — they tend to understate their heft.