According to government data from the years 2007-08 published Wednesday, the obesity rate has held steady for about five years, reflecting earlier signs it had stalled after steadily climbing.
Dr. William Dietz, an obesity expert with the Centers for Disease Control and Prevention, cautiously called the results promising. "We're at the corner; we haven't turned the corner," he said.
Not only are the vast majority of adults overweight, 34 percent are obese; and 17 percent of children are obese. Even the youngest Americans are affected - 10 percent of babies and toddlers are precariously heavy.
The CDC data were contained in two reports published online in the Journal of the American Medical Association.
"Even though this finding is certainly good news, the statistics are still staggering," said Dr. J. Michael Gaziano, a contributing editor at the journal.
The new data are based on health surveys involving height and weight measurements of 5,700 adults and 4,000 children, surveys the CDC does every two years.
The results in adults, showing 68 percent are too heavy, have been virtually the same in the last three surveys.
In most age groups, black adults had the highest rates of obesity, followed by Mexican-Americans and whites.
Among children ages 2 to 19, 32 percent were too heavy - a rate that was mostly unchanged. But disturbingly, most obese kids were extremely obese. And the percentage of extremely obese boys ages 6 to 19 has steadily increased, to 15 percent from about 9 percent in 1999-2000.
CDC researcher and study author Cynthia Ogden said it was disappointing to see no decline, and troubling that the heaviest boys seem to be getting even heavier. The study didn't examine the causes, but Ogden cited the usual reasons - soft drinks, video games and inactivity - as possible explanations.
"We shouldn't be complacent. We still have a problem," Ogden said.
Gaziano, a cardiologist at Boston's Veterans Affairs hospital and Brigham and Women's Hospital, said getting the nation to turn the corner and reduce obesity requires changing many unhealthy behaviors, and getting restaurants, schools, food manufacturers and communities to support the fight.
That's starting to happen, from efforts to pull soda from school vending machines to campaigns by groups like the NFL to encourage physical activity, he noted.
The epidemic is also high on the White House agenda. President Barack Obama has pushed to make obesity prevention part of health care reform. Overhaul measures pending in Congress include encouraging employer-based wellness programs and requiring large restaurant chains to list calories. And Michelle Obama on Wednesday said fighting childhood obesity will be a top priority this coming year.
In a round-table conversation with reporters about her first year in office and upcoming goals, Mrs. Obama said the new CDC reports suggest a generation of children will be destined for increased rates of heart disease, high blood pressure and strokes "if we don't get a handle on this issue."
"We have a chance to change the fate of the next generation if we get on it," she said.
People like Darrell Pender are paying attention.
Obesity "is constantly in the news," said Pender, a 42-year-old New York City computer technician who decided to get serious about fighting fat after being diagnosed with diabetes three years ago.
Pender was tempted by a TV ad for obesity surgery, but chose a less drastic option - a nutrition support group that he credits with helping him make healthier food choices. So far, he's lost 50 pounds over several months. At 350 pounds, he's still very obese, but his diabetes is under control and he feels healthier.
Karen Congro, Pender's nutritionist at the Brooklyn Hospital Center, said obese patients in recent years seem more willing to try lifestyle changes rather than quick fixes doomed to fail.
Fifteen years ago, "I would have said this seems almost hopeless. Patients would say, 'I had an overweight uncle who lived to 99,'" Congro said. "Now I almost never hear that."