CHICAGO - Obesity-related diseases account for nearly 10 percent of all U.S. medical spending or an estimated $147 billion a year, researchers said on Monday.

They said U.S. obesity rates rose 37 percent between 1998 and 2006, driving an 89 percent increase in spending on treatments for obesity-related diseases such as diabetes, heart disease, arthritis and other conditions.

Obese people spent an extra $1,429 per year or 42 percent more for medical care in 2006 than did normal weight people, with most of that spent on prescription drugs, the researchers said.

The study, published in the journal Health Affairs, was released at the U.S. Centers for Disease Control and Prevention's Weight of the Nation conference in Washington, where the CDC issued 24 new recommendations on how communities can fight back.

It is critical that we take effective steps to contain and reduce the enormous burden of obesity on our nation, CDC Director Dr. Thomas Frieden said in a statement.

For the study, Eric Finkelstein of the non-profit research institute RTI International and researchers at the CDC and the Agency for Healthcare Research and Quality analyzed national medical cost data from 1998 and 2006.

More than 26 percent of Americans are obese, which means they have a body mass index of 30 or higher. BMI is equal to weight in kilograms divided by height in meters squared. A person 5 feet 5 inches tall becomes obese at 180 pounds (82 kg).

Finkelstein's team found obesity accounts for 9.1 percent of all medical spending in the United States, up from 6.5 percent in 1998.

What we found was the total cost of obesity increased from $74 billion to maybe as high as $147 billion today, so roughly double over that time period, Finkelstein said in a telephone interview.

Obesity accounts for 8.5 percent of health costs among people on Medicare, the federal program for the elderly and disabled, and 11.8 percent of costs from Medicaid, the joint state-federal program for the poor.

An obese Medicare beneficiary spends an $600 more per year on drug costs than a normal weight person on Medicare, the team found.

One of the big drivers is diabetes, which has been estimated to cost as much as $180 billion per year. There is evidence that most of the diabetes in the U.S. is caused by excess weight, Finkelstein said.

To address these issues, the CDC has devised 24 obesity prevention strategies being tested in Minnesota and Massachusetts.

They aim to address issues in the environment -- lack of access to healthy food in poor neighborhoods and sedentary lifestyles -- that contribute to the nation's weight problem.

All of these factors just make it easier to be overweight and harder to be thin, Finkelstein said.

(Editing by Maggie Fox and Vicki Allen)