South Carolina ranks seventh in the country in the number of adults who report they are obese, according to a new government survey.

Mississippi, Alabama and Tennessee lead the nation when it comes to obesity. Colorado was the least obese, with about 19 percent fitting the category.

An estimated 28.4 percent of South Carolina adults reported being obese in 2007, compared with the national average of 25.6 percent.

The findings reported Thursday were derived from the Centers for Disease Control and Prevention's Behavioral Risk Factor Surveillance System, a state-based telephone survey. Nationally, more than 350,000 adults are interviewed annually.

Self-reported studies offer conservative estimates because men commonly overstate their height and women often lowball their weight, health experts say.

While the numbers change little year to year, a dramatic change is visible during the past 10 to 20 years, said Patrick M. O'Neil, director of Medical University of South Carolina's Weight Management Center. In 1997, only 16.9 percent of South Carolinians reported being obese.

Obesity is defined as a body mass index of 30 or above. BMI is calculated using height and weight. For example, a 5-foot-9-inch adult who weighs 203 pounds would have a BMI of 30, putting that person into the obese category.

Said O'Neil: "It's not just that a few more people each year gain a couple of pounds and edge into the obese category. The entire group of people who qualify for obesity is growing numerically and so is the extent to which people are obese."

Southern states have clustered at the top of the list for years, and some experts blame the traditional Southern diet — high in fat and fried food.

"Let's not forget sweet tea," O'Neil said. The regional drink can contain as much sugar as a soft drink.

Where the South leads, the rest of the United States is likely to follow, O'Neil said. "I don't think we're unique in diet that's high fat. We eat the same fast food," he said.

Diet might 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, Dietz said.

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.

Nationally, the proportion of adults who report they are obese increased nearly 2 percent between 2005 and 2007.

The Associated Press contributed to this report. Reach Jill Coley at 937-5719 or jcoley@postandcourier.com.