The prevalence of American adults who are 100 or more pounds over a healthy weight has risen dramatically since 2000, a study released Monday shows.

About 3% of people, or 6.8 million adults, were morbidly obese in 2005, up from 2% or 4.2 million people in 2000, says Roland Sturm, an economist with the RAND Corp., a non-profit think tank.

The evidence of such a significant increase in the number of Americans who are extremely heavy "is mind-boggling," he says.

Sturm analyzed government data on about 1.5 million people who reported their own weights and heights. Participants were categorized as severely or morbidly obese if they had a body mass index (a height-weight ratio) of 40 or higher.

According to government data, about 66% of people in the USA are now either overweight or obese, which is defined as 30 or more pounds over a healthy weight. Obesity increases a person's risk of contracting numerous diseases, including diabetes, heart diseases and cancer.

Sturm's study, which was released Monday on the website of the journal Public Health, shows that 24.6% of people were obese in 2005, up from 20% in 2000. That's an increase of 24%.

People usually under report their weight, so the percentage of people who are morbidly obese is actually higher than 3%, Sturm says. A large government survey in which people are actually weighed and measured suggests that about 5% of U.S. adults are morbidly obese and a third are obese, Sturm says.

He says his analysis highlights the dramatic increase in the number of morbidly obese people over a relatively short period of time.

"Even though we've had an explosion of bariatric surgery in that time, it doesn't seem to have made a dent in these numbers," he says. Bariatric surgery often reduces the size of the stomach.

For years, some experts believed that severe obesity was a rare condition that affected a fixed percentage of the population that might be more predisposed to weight gain for genetic or metabolic reasons, Sturm says.

"But these numbers show the trend is really paralleling what is going on in our society," he says. To help reverse the numbers, "we need to move to a healthier environment with friendlier staircases and more walkable environments," Sturm says.

George Blackburn, associate director of nutrition at Harvard Medical School, calls the increase in the percent of severely obese people a catastrophe.

"It is an emergency because the disability, the discrimination and the health care costs for this population are enormous," he says.