CHICAGO (AP) – Americans are not just getting fatter, they are ballooning to extremely obese proportions at an alarming rate.

The number of extremely obese American adults – those who are at least 100 pounds overweight – has quadrupled since the 1980s to about 4 million. That works out to about 1 in every 50 adults.

Extreme obesity once was thought to be a rare, distinct condition whose prevalence remained relatively steady over time. The new study contradicts that thinking and suggests that it is at least partly due to the same kinds of behavior – overeating and under-activity – that have contributed to the epidemic number of Americans with less severe weight problems.

In fact, the findings by a RAND Corp. researcher show that the number of extremely obese adults has surged twice as fast as the number of less severely obese adults.

On the scale of obesity, “as the whole population shifts to the right, the extreme categories grow the fastest,” said RAND economist Roland Sturm. He added: “These people have the highest health care costs.”

Sturm said health problems associated with obesity – including diabetes, heart disease, high blood pressure and arthritis – probably affect the extremely obese disproportionately and at young ages.

Sturm analyzed annual telephone surveys conducted nationwide by the Centers for Disease Control and Prevention. His report covers surveys from 1986 through 2000. The findings appear in Monday’s Archives of Internal Medicine.

In 1986, 1 in 200 adults reported height and weight measurements reflecting extreme obesity, or a body-mass index of at least 40.

By 2000 that had jumped to 1 in 50, Sturm found.

The prevalence of the most extreme obesity – people with a BMI of at least 50 – grew fivefold from 1 in 2,000 to 1 in 400, Sturm said.

By contrast, ordinary obesity – a BMI of 30 to 35 – doubled, from about 1 in 10 to 1 in 5, based on the same surveys.

Body-mass index is a ratio of height to weight.

Americans tend to understate their weight, and a recent study based on actual measurements found an obesity rate of nearly 1 in 3, or almost 59 million people. Sturm said his findings probably understate the problem for the same reason.

The average man with a BMI of 40 in Sturm’s study was 5-foot-10 and 300 pounds, while the average woman was 5-foot-4 and 250 pounds.

Dr. Mary Vernon, a trustee of the American Society of Bariatric Physicians, said the study reflects what doctors who specialize in treating obesity are seeing in their offices. Vernon said the number of her patients weighing 300 to 350 pounds or so has doubled in the past several years.

She said thinking has evolved from a generation ago, when many doctors believed extreme obesity was due to hormonal abnormalities or other distinct conditions.

Now many believe it is a combination of lifestyle factors and genetics, as well as a propensity for some people’s bodies to be hyper-efficient at storing calories. This tendency would benefit people in societies where starvation is rampant but is a huge problem in developed countries where food is plentiful and lifestyles are increasingly sedentary, Vernon said.

Vernon said the biggest challenge in treating severely obese people, who typically have tried mightily to lose weight, “is giving them enough hope that it’s worth trying again.”

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On the Net:

Archives of Internal Medicine: http://www.archinternmed.com

American Society of Bariatric Physicians: http://www.asbp.org

AP-ES-10-13-03 1434EDT