UPDATE 1-American adults getting fatter, Mississippi leads

* Mississippi has fattest residents; Colorado has thinnest

* Obesity rates climbing in 23 states

* No state saw improvement in past year

* Childhood obesity rates flat, but alarmingly high (Recasts throughout, adds comments, details)

By Julie Steenhuysen

CHICAGO, July 1 (Reuters) - Obesity rates continued to climb in the past year with 23 U.S. states reporting adults in their states are fatter now than they were a year ago, two advocacy groups said on Wednesday.

Obesity rates did not decrease in a single state last year, and the groups warned that the U.S. obesity epidemic must be addressed as lawmakers reform the nation's health system.

"Our health care costs have grown along with our waistlines," said Jeff Levi, executive director of Trust for America's health, which released the report along with the Robert Wood Johnson Foundation.

He said the obesity epidemic is contributing to skyrocketing health costs, and said the problem has to be addressed at the highest levels of government.

Being overweight or obese raises the risk of heart disease, diabetes, some cancers, arthritis and other conditions.

The annual ranking of obesity rates in U.S. states found Mississippi continues to be the state with the fattest residents, with nearly a third of adults -- 32.5 percent -- considered obese. The state has topped the list for the past five years.

Three other states -- West Virginia, Alabama, and Tennessee -- now have obesity rates above 30 percent, they found.

Colorado has the thinnest residents, with an obesity rate of 18.9 -- the only state under 20 percent. Massachusetts is next, at 21.2 percent, followed by Connecticut, at 21.3.

Two-thirds of American adults are either obese or overweight, as defined by their body mass index or BMI.

BMI is equal to weight in kilograms divided by height in meters squared. Someone with a BMI of 25 to 29 is classified as overweight; 30 and over is considered obese.

A person 5 feet-5 inches (165 cm) tall becomes overweight at 150 pounds (68 kg) and obese at 180 pounds (82 kg).

IMPACT OF ECONOMIC CRISIS

Although still rising, fewer states saw increases in obesity rates this year compared to last, in which 37 states saw gains.

"We're still getting fatter, but maybe a little more slowly than before," Dr. James Marks of the Robert Wood Johnson Foundation said on a conference call.

"We are hopeful that policy changes some of the states are putting into place in their schools and communities are beginning to make a difference."

The groups warned that the economic crisis could exacerbate the obesity epidemic as rising food prices and shrinking family budgets make it more difficult to eat healthy foods.

Among U.S. children, obesity rates held steady, but at still alarmingly high levels, with 30 states reporting the percentage of obese or overweight children at above 30 percent.

A report last year found the U.S. childhood obesity epidemic leveled off this decade after surging for about 20 years, but a worrisome number of young people remain obese, risking serious health problems.

Obese children are more likely to be saddled with risk factors associated with heart disease and type 2 diabetes. They also are much more likely to be obese in adulthood.

The U.S. National Institutes of Health has an online BMI calculator at www.nhlbisupport.com/bmi/.

(Editing by Jackie Frank)