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RAND study finds people who are severly overweight grow faster than other obese Americans

April 10, 2007

The proportion of Americans who are severely obese — about 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 Corporation study issued today.

"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," said Roland Sturm, author of the report and an economist at RAND, a nonprofit research organization.

The findings will be published later this year in the journal Public Health.

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 overweight.

Sturm 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 RAND Health study found that based on self-reported height and weight, which tends to understate actual BMI, 3 percent of Americans are already severely obese. Since that is the fastest growing group of obese Americans, widely published trends for obesity underestimate the consequences of the obesity epidemic because illness and service use are much higher among severely obese individuals.

Among middle-aged adults, people with a BMI over 40 are expected to have health costs that are double those experienced by normal weight peers, while moderate obesity (a BMI of 30-35) is associated with only a 25 percent increase.

The prevalence of severe obesity continues to surge despite a rapid increase in the use of bariatric procedures, which are surgeries that limit the amount of food patients can eat. The number of bariatric surgeries increased from an estimated 13,000 in 1998 to more than 100,000 in 2003. Experts estimate that as many as 200,000 of the procedures were performed in 2006.

"The explosion in the use of bariatric surgery has made no noticeable dent in the trend of morbid obesity," Sturm said.

Sturm said the latest findings challenge a common belief held by physicians that people who are obese are a fixed proportion of the population and are not affected by changes in eating and physical activity patterns in the general population.

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 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.

RAND Corporation