One in five Americans, or 19.8 percent, had considered themselves obese in a 2000 survey based on people's own assessments of their girth.
The new 1999-2000 survey puts the real number at 31 percent — a doubling over the past two decades. The new number is considered more reliable since people consistently underestimate their weight.
"The problem keeps getting worse," said Health and Human Services Secretary Tommy Thompson. "This has profound health implications."
In addition, a measurement-based survey of young people found that 15 percent of youngsters ages 6 to 19 were seriously overweight. That is nearly 9 million youths and triple the number in a similar assessment from 1980.
"One of the most significant concerns from a public health perspective is that we know a lot of children who are overweight grow up to be overweight or obese adults, and thus at greater risk for some major health problems such as heart disease and diabetes," said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention.
The findings appear in Wednesday's Journal of the American Medical Association.
They come from nationally representative surveys of 4,115 adults ages 20 through 74 and 4,722 children from birth through age 19.
The National Health and Nutrition Examination Surveys have been conducted periodically for several years. Twenty-three percent of adults were obese in 1994 and 15 percent in 1980.
Obesity increases the risk for a number of serious ailments, including diabetes, heart disease, strokes, high blood pressure and some types of cancer.
Obesity is defined as having a body-mass index of 30 or above. The index is a measure of weight relative to height.
The latest survey also found that nearly two-thirds of U.S. adults were overweight, or had a body-mass index of between 25 and 30.
In the youth survey, even toddlers were affected, with more than 10 percent of children ages 2 through 5 seriously overweight, compared with 7.2 percent in 1994.
"The numbers are pretty shocking," said Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest.
She said talk from the Bush administration about encouraging Americans to eat more healthfully and be more active is "too low a dose of treatment to try and cure this health problem."
"They need to put into place real policy," such as offering more healthful foods in government meal programs and requiring fast-food restaurants to list calories on menus, she said.
The trend has paradoxically occurred while health clubs, home exercise equipment and heavily promoted diet plans have proliferated.
In an accompanying editorial, Dr. Phil Fontanarosa, JAMA's executive deputy director, questioned whether dietary guidelines are adequate and whether doctors have been ineffective in counseling overweight patients.