The proportion of Americans who are severely obese -- those people 100 pounds or more overweight -- continues to increase rapidly and much faster than those with moderate obesity, but the rate of growth has slowed, according to a new RAND Corporation study.

The RAND study found that from 2000 to 2010, the proportion of Americans who were severely obese rose from 3.9 percent of the population to 6.6 percent -- an increase of about 70 percent.

The findings mean that more than 15 million adult Americans are morbidly obese with a body mass index of 40 or more. The good news is that beginning in 2005, the near-exponential growth of the severely obese group began to flatten out.

"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," said Roland Sturm, lead author of the report and a senior economist at RAND, a nonprofit research organization. "But for the first time in the past 20 years there is evidence the trend is slowing."

The study suggests that clinically severe obesity, instead of being a rare pathological condition among genetically vulnerable individuals, is an integral part of the population's weight distribution. As the whole population becomes heavier, the extreme category -- the severely obese -- increases the fastest.

The findings were published online by the International Journal of Obesity.

The trend of severe obesity varies by gender and ethnicity, although the trend remained upward among all groups. The prevalence of severe obesity was about 50 percent higher among women than among men, and about twice as high among blacks when compared to Hispanics or whites. For all levels of obesity, the increases over time were faster among age groups younger than 40.

To be classified as severely obese, a person must have a body mass index (a ratio of weight to height) of 40 or higher -- roughly 100 pounds or more overweight for an average adult man. The typical severely obese man weighs 300 pounds at a height of 5 feet 10 inches tall, while the typical severely obese woman weighs 250 pounds at a height of 5 feet 4 inches.

People with a BMI of 25 to 29 are considered overweight, while a BMI of 30 or more classifies a person as being obese. For a 5-foot-10 inch male, a BMI of 30 translates into being 35 pounds too heavy.

The body mass index allows researchers to define obesity and severe obesity over a population of people with varied heights and weights. The index is defined as weight in kilograms divided by the square of height in meters. The standard cut-off point for obesity is a body mass index of 30 or more, corresponding to a person 5 feet 4 inches tall and weighing 174 pounds, or 5 feet 10 inches tall and weighing 209 pounds or more.

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The RAND study is based on the Behavioral Risk Factor Surveillance Survey (BRFSS), an annual survey conducted by the U.S. Centers for Disease Control and Prevention. The BRFSS, the world's largest annual telephone survey, tracks health risks in the United States. Height and weight is based on self-reporting. More than 3 million respondents were included in the analysis for the last decade.

Support for the study was provided by the National Institute of Child & Human Development and the federal Agency for Healthcare Research and Quality. Aiko Hattori, a post-doctoral fellow at UCLA and RAND, co-authored the study.

RAND Health, a division of the RAND Corporation, is the nation's largest independent health policy research program, with a broad research portfolio that focuses on health care costs, quality and public health preparedness, among other topics.