FRIDAY, Oct. 5 (HealthDay News) -- Severely obese people -- those with at least 100 pounds of excess weight -- are the fastest-growing group of overweight Americans, according to new research.

Analysts examined data from the U.S. Centers for Disease Control and Prevention and found that from 2000 to 2010, the proportion of Americans who were severely obese rose from 3.9 percent to 6.6 percent, an increase of about 70 percent.

The findings mean that more than 15 million U.S. adults are severely obese, with a body-mass index of 40 or more. BMI is a measure of body fat based on height and weight.

But there was some good news. Beginning in 2005, the increasing rate of severely obese people began to slow, according to the investigators at the RAND Corporation, a nonprofit research organization.

"The proportion of people at the high end of the weight scale continues to increase faster than any other group of obese people, despite increased public attention on the risks of obesity," study lead author Roland Sturm, a senior economist at RAND, said in a corporation news release. "But for the first time in the past 20 years, there is evidence the trend is slowing."

Rising rates of severe obesity varied by gender and ethnicity but were present in all groups. The prevalence of severe obesity was about 50 percent higher among women than among men, and about twice as high among blacks than among Hispanics and whites.

For all levels of obesity, the increases over time were highest among people younger than 40.

The study was published online in the International Journal of Obesity.

More information

The U.S. National Heart, Lung, and Blood Institute has more about overweight and obesity.