The percentage of U.S. adults who are obese grew by nearly 2 percent between 2005 and 2007, from just under 24 percent to 25.6 percent, the Centers for Disease Control and Prevention reported Thursday.

Alabama, Mississippi, and Tennessee had the worst rates, with 30 percent of adults reporting weights that made them medically obese.

Colorado had the slimmest population, with 18.7 percent of people reporting weights that put them in the obese category.

Obesity is defined as having a body mass index or BMI of 30 or above. BMI is calculated using height and weight. For example, a 5-foot, 9-inch tall adult who weighs 203 pounds would have a BMI of 30.

BMI takes into account variations in build for all but the most heavily muscled athletes.

“The epidemic of adult obesity continues to rise in the United States indicating that we need to step up our efforts at the national, state and local levels,” said Dr. William Dietz, director of the CDC's Division of Nutrition, Physical Activity, and Obesity.

“We need to encourage people to eat more fruits and vegetables, engage in more physical activity and reduce the consumption of high calorie foods and sugar sweetened beverages in order to maintain a healthy weight,” Dietz said.

CDC researchers used data from the Behavioral Risk Factor Surveillance System, an annual telephone survey of more than 350,000 adults.

Southerners are the heaviest Americans, on average, with 27 percent obese. Just over 25 percent of adults in the Midwest, 23 percent in the Northeast, and 22 percent in the West were obese.

In May, the CDC reported that the childhood obesity epidemic had leveled off after surging for about 20 years, with 16 percent of young people obese.

World health officials have agreed on a BMI of 30 as an easily remembered cutoff point for obesity. People are considered overweight when they have a BMI of 25, and the health effects of obesity and overweight, such as diabetes, heart disease and cancer, increase steadily as weight goes up.

A government BMI table is available at www.nhlbi.nih.gov/guidelines/obesity/bmi-tbl.htm

(Reporting by Maggie Fox; editing by Julie Steenhuysen and Vicki Allen)