One-third of adults and 17 percent of children and teens are obese, said CDC researchers who focused on more than 9,000 adults and children in 2011-2012 and compared them to five previous obesity analyses dating back to 2003-04.
"We found overall that there was no change in youth or adults," said study author and epidemiologist Cynthia Ogden.
But within specific age groups, weight shifts were apparent. More older women are obese, but very young children seem to be slimming down.
One specialist in childhood obesity was pleased with the overall findings.
"I tend to be an optimist. The fact that we are seeing a leveling off is actually a good thing," said Dr. Sara Lappe, a pediatrician at Cleveland Clinic Children's who specializes in childhood obesity.
Obesity in adults is defined as a body mass index (BMI) of 30 or above. BMI is a calculation of body fat based on height and weight. A 5-foot 9-inch adult who weighs 203 pounds has a BMI of 30 and is considered obese, for example.
Obesity in kids is defined as a child who has a BMI at or above the 95th percentile for children of the same age and sex.
Ogden said the results for preschool-age children are a bright spot in the findings.
"We found among preschoolers, 2- to 5-year-olds, there was a significant decrease in obesity," Ogden said. Prevalence of obesity in children that age dipped from 14 percent in 2003-2004 to about 8 percent in 2011-2012, she noted.
Cleveland Clinic's Lappe said: "I think this piece of the study is actually good. There are a lot of early intervention programs in Head Start and preschools, and education directly to parents that may be starting to pay off."
"Hopefully," Lappe added, "as they [the children] get older, we'll see the numbers come down."
The authors of the study, published in the Feb. 26 issue of the Journal of the American Medical Association, report that many preventive health programs and efforts have been launched by the government in recent years to combat the obesity epidemic in the United States.
These include new food labeling measures by the U.S. Department of Agriculture, as well as state and community programs sponsored by the CDC, and First Lady Michelle Obama's Let's Move program.
Even so, the overall numbers haven't inched down. In fact, obesity prevalence ticked up in women 60 and older, from less than 32 percent in 2003-2004 to more than 38 percent in 2011-2012.
Overall, more than two-thirds of adults are either overweight or obese, and more than 6 percent are extremely obese.
There hasn't been a big impact on prevalence in the last eight years, but at least there's a leveling off, said obesity expert Dr. William Yancy, an associate professor of medicine at Duke University Medical Center in Durham, N.C.
"There are a bunch of competing factors that make it hard for people to manage their weight," Yancy said. "Genetics are involved, chemicals in foods and the environment may be involved. Clearly, the food environment stimulates us to eat more and more higher-calorie foods, and our environment also encourages us not to be active."
Those factors make it "really difficult" to maintain a healthy weight, he said. "I liken it to how difficult it is to get people to stop smoking," said Yancy. As with smoking, he said, it may take bigger policy changes to bring prevalence down, such as taxes and restrictions, but it's a complicated matter.
"People have to eat but they don't have to smoke, and there's a lot of controversy about what's a healthy food and what's not," Yancy said.
Ogden agreed there's no simple solution. "Obesity is obviously a multifactorial problem. It's very complex," he said, adding that surveillance of obesity in the United States will continue.
To learn about First Lady Michelle Obama's efforts to reduce obesity, visit the Let's Move program.