Atlanta — Nearly one-third of the residents of Mississippi, Alabama and Tennessee are obese, giving the three states the distinction of topping the list of U.S. states with overweight people.

A new government report on Thursday also said that the South remains the nation's fattest region, led by Mississippi.

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Colorado was the least obese, with about 19 percent fitting that category based on a random telephone survey last year by the Centers for Disease Control and Prevention.

The 2007 findings are similar to results from the same survey the three previous years.

Why is the South so heavy? The traditional Southern diet — high in fat and fried food — may be part of the answer, said Dr. William Dietz, who heads CDC's nutrition, physical activity and obesity division.

The South also has a large concentration of rural residents and black women — two groups that tend to have higher obesity rates, he added.

Colorado, meanwhile, is a state with a reputation for exercise. It has plentiful biking and hiking trails, and an elevation that causes the body to labor a bit more, Dietz said. Obesity is based on body mass index — a calculation using height and weight. An adult who is 5 feet 9 inches (175.26 centimeters) tall who weighs 203 pounds (92 kilograms) would have a BMI of 30, which is considered the threshold for obesity.

CDC officials believe the telephone survey of 350,000 adults offers conservative estimates of obesity rates, because it's based on what respondents said about their height and weight. Men commonly overstate their height and women often lowball their weight, health experts say.

"The heavier you are, the more you underestimate your weight, probably because you don't weigh yourself as often," Dietz said.

Overall, about 26 percent of the respondents were obese, according to the study, published this week in CDC's Morbidity and Mortality Weekly Report.

A different CDC survey — a gold-standard project in which researchers actually weigh and measure survey respondents — put the adult obesity rate at 34 percent in 2005 and 2006, the most recent years for which there are data.