The Centers for Disease Control and Prevention said 34.3% of adults -- or more than 72 million people -- were obese in 2005 and 2006. The figures were essentially unchanged from the previous two-year period for the first time since 1980.
But in comparison to a quarter-century of expanding waistlines that doubled the proportion of Americans considered obese, some experts called the study a tentative milestone.
"It is great that the problem is not increasing," said Meir Stampfer, a professor of nutrition and epidemiology at the Harvard School of Public Health. But he added: "This does not mean that the problem is over."
The report didn't weigh in on why Americans seem to be holding steady. Experts surmised that the government's incessant calls for people to slim down may be finally paying off. They added that membership in fitness clubs is on the rise, and prohibitions against artery-clogging trans fats in restaurants are gaining ground in the name of public health.
Others raised a more fundamental explanation.
Glenn Gaesser, an exercise physiologist at the University of Virginia and author of the iconoclastic book "Big Fat Lies," said there just may be metabolic limit to body weight.
Americans may have finally reached that point. "You can only get so fat," he said
Obesity is defined as having a body-mass index -- a mathematical formula of the ratio of weight and height -- of 30 or more. A woman who is 5 feet tall would be considered obese if she weighed at least 154 pounds. A 6-foot-tall man would be obese if his weight topped 220 pounds.
Turning back the remarkable rise of obesity has been one of the great public health challenges of recent times.
Health officials have long nagged the public over its bad habits: bigger portion sizes, eating more meals in restaurants, sedentary lifestyles, and an increasing proclivity for automobiles over walking. Not only have more adults become obese, but those who are weigh more than ever.
The problem is not just aesthetic. People who are obese are more likely to suffer from diabetes, cardiovascular disease, high blood pressure and some types of cancer. Obesity is also associated with an increased risk of overall disability and of death from any cause. The more one weighs, the higher the risk.
In 2004, the CDC issued an alarming report that said the nation logs 400,000 deaths a year due to obesity, just slightly below the 435,000 who die from smoking.
Federal health guidelines set by the Department of Health and Human Services call for the nationwide obesity rate to drop to 15% by 2010 -- a level not seen since the early 1970s.
The latest report is based on data from a CDC project called the National Health and Nutrition Examination Survey, which combines interviews with physical examinations and is conducted every two years.
The latest round of data found that the obesity rate remains slightly higher among women, at 35.3%, but that figure has stabilized since 1999, said Cynthia Ogden, the CDC epidemiologist in Washington who led the study. The rate for men was lower at 33.3%, but it is not yet clear whether they have reached a plateau as well, she said.
Middle-age adults are still packing on the most pounds. The study found that 53% of non-Latino black women and 51% of Mexican American women between the ages of 40 and 59 were classified as obese, compared to 39% of non-Latino white women.
After age 60, the obesity rate for black women jumped to 61%, though it declined to 37% for Mexican American women and to 32% among non-Latino white women, according to the report.