CHICAGO -- Americans are not just getting fatter, they are ballooning into extremely obese proportions at a rate that alarms health experts.

Roland Sturm, of the RAND Corp., analzyed data on a telephone survey of adults conducted between 1986 and 2000. He found that the number of extremely obese American adults -- those who are at least 100 pounds overweight -- has quadrupled since the 1980s. There are about 4 million extremely obese adults, which works out to about one in every 50.

Sturm suggests the problem is at least partly due to overeating and under-activity. That's what created an epidemic number of Americans with less severe weight problems.

A researcher said the number of extremely obese adults has surged twice as fast as the number of less severely obese adults.

The study is published in Monday's issue of the Archives of Internal Medicine.