SANTA MONICA, Calif. – A study conducted by the RAND Corp. found that from 2000 to 2010, the proportion of Americans who are morbidly obese rose from 3.9 percent to 6.6 percent. The findings mean more than 15 million adult Americans are morbidly obese with a body mass index of 40 or more. The research firm said one piece of good news from the study is that beginning in 2005 the rising number of morbidly obese adults began to flatten.

“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. “But for the first time in the past 20 years there is evidence the trend is slowing.”

The study is based on the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance Survey, an annual telephone survey that tracks health risks in the United States. Height and weight included in the survey is self-reported, and more than 3 million respondents were included in the analysis for the past decade, according to RAND.

The study suggested 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 increases the fastest.

The trend varies by gender and ethnicity, although it 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 lbs. or more overweight for an average adult man. The typical severely obese man weighs 300 lbs. at a height of 5 feet 10 inches tall, while the typical severely obese woman weighs 250 lbs. 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 lbs. too heavy.

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