WASHINGTON — More than a quarter of Americans self-report that they're obese, and in three states — Alabama, Mississippi and Tennessee — more than 30 percent do, the Centers for Disease Control and Prevention reported yesterday.

Colorado had the lowest reported obesity rate: 18.7 percent.

Hawai'i was among the bottom five states, tied with Rhode Island at 21.4 percent.

The 2007 national average of 25.6 percent compares with a 23.9 percent obesity rate in 2005, the CDC said. Actual obesity is likely to be greater because the figures are based on self-estimated height and weight from telephone surveys of 350,000 people.

Obesity is defined as a body mass index of 30 or above. It is calculated using height and weight. An adult who is 5 feet 9 and weighs 203 pounds, for example, would have a BMI of 30.

That the South tips the scales as the nation's fattest region comes as no surprise to experts.

In part, they blame Southern eating habits, poverty and demographic groups that have higher obesity rates.

The South has had high death rates from heart disease and stroke, health risks that have been linked to obesity, some experts noted.

The CDC study only surveyed adults, but results for kids are similar, said Dr. Miriam Vos, assistant professor of pediatrics at Atlanta's Emory School of Medicine.

"Most of the studies of obesity and children show the South has the highest rates as well," Vos said.

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 said.

The study found that about 36 percent of black survey participants were obese, while 28.5 percent of Hispanics and 24.5 percent of whites were.

High poverty rates in the South probably are another factor, said Naa Oyo Kwate, assistant professor of sociomedical sciences at Columbia University's Mailman School of Public Health.

In today's America, poor people tend to be obese: The cheapest foods tend to be calorie-heavy, and stores offering healthier, and more expensive, food choices are not often found in poor neighborhoods, she said.

And why is Colorado so thin? It's 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.

Health experts believe the telephone survey offers conservative estimates of obesity rates because men commonly overstate their height and women often lowball their weight.

"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 Associated Press and McClatchy News Service contributed to this report.