WASHINGTON—The proportion of Americans who are severely obese (100 pounds or more overweight) increased by 50 percent from 2000 to 2005, twice as fast as the growth seen in moderate obesity, according to a Rand Corp. study issued last week.

“The proportion of people at the high end of the weight scale continues to increase at a brisk rate despite increased public attention on the risks of obesity and the increased use of drastic weight loss strategies such as bariatric surgery,” says Roland Sturm, author of the report and an economist at Rand, a nonprofit research organization.

People with a body mass index (BMI) of 40 or higher are classified as severely obese. Researchers found that from 2000 to 2005, the proportion of Americans with a BMI of 30 or more increased by 24 percent, the proportion of people with a BMI of 40 or more increased by 50 percent and the proportion of Americans with a BMI of 50 or more increased by 75 percent. The heaviest groups have been increasing at the fastest rates for the past 20 years.

The study found that 3 percent of Americans are severely obese. Among middle-aged adults, people with a BMI over 40 are expected to have health costs that are double those of normal weight peers while moderate obesity (BMI of 30-35) is associated with a 25 percent increase.

The prevalence of severe obesity continues despite an increase in bariatric procedures, which are surgeries that limit the amount a food a person can eat. Bariatric surgeries increased from 13,000 in 1998 to more than 100,000 in 2003. Estimates say as many as 200,000 bariatric procedures occurred in 2006.

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.