The number of U.S. residents who are severely obese shot up by 70 percent in the past decade or so, though the increase has slowed in more recent years, according to a U.S. study.
Between 2000 and 2010, the proportion of U.S. residents who were severely obese - at least 45 kilograms (100 lbs) overweight - rose from 4 percent to almost 7 percent, said researchers whose findings appeared in the International Journal of Obesity.
The increase showed signs of slowing after 2005, they added. But the bad news is that the severely obese remain the fastest-growing segment of obese Americans, said study leader Roland Sturm, a senior economist at the non-profit research institute RAND Corporation.
"Everybody's talking about obesity leveling off," Sturm said. But what tends to get lost in the discussion is the fact that severe obesity, with a body mass index (BMI) of 40 or higher - is still rising fast.
That's important, Sturm said, because those are the people who have the highest healthcare costs, about double those of normal-weight people.
More than one-third of U.S. adults are obese, which means having a BMI of 30 or higher. BMI is a measure of weight relative to height.
Recent studies have found that the nation's obesity rate among adults and children may be leveling off, but most of those folks are moderately obese.
The findings for the current study were based on data from an annual government health survey of U.S. adults. BMI estimates were made based on people's self-reported weight and height.
Moderate obesity, the study found, rose relatively slowly after 2000 and seemed to level off from 2005 on. In contrast, the proportion of Americans with a BMI of 40 climbed by more than 70 percent - translating to about 15 million U.S. adults.
The rate of severe obesity was 50 percent higher among women than men, and twice as high among black Americans as among white and Hispanic adults.
The increases were bigger among people under 40.
Severely obese people are at high risk of conditions like diabetes, severe arthritis and heart disease - and could also be candidates for obesity surgery.
But Sturm said there are other costs besides the healthcare price tag, such as the human cost of living with obesity.
"There's the disability and inability to work. People may be basically forced into retirement because they can't work," he said.
Sturm said that doctors once thought of severe obesity as a problem that affected a small and stable percentage of people who were genetically vulnerable to huge weight gain.
"That thinking has been proven wrong. This is something that can happen to a surprisingly large percentage of the population," he added.
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