Its the first broad decline among 2- to 5-year-olds in an epidemic that often leads to lifelong struggles with weight and higher risks of cancer, heart disease and stroke.
New evidence has shown that obesity takes hold young: Children who are overweight or obese between 3 and 5 are five times as likely to be overweight or obese as adults.
A few states have reported modest progress in reducing childhood obesity in recent years, and last year federal authorities noted a slight decline in the obesity rate among low-income children. But the figures on Tuesday showed a sharp fall in obesity rates among all young children, offering the first evidence that America might have turned a corner in the obesity struggle.
Only about 8 percent of 2- to 5-year-olds were obese in 2012.
This is the first time weve seen any indication of any significant decrease in any group, said Cynthia Ogden, a researcher for the Centers for Disease Control and Prevention, and the author of the report, which will be published in The Journal of the American Medical Association on Wednesday. It was exciting.
Ogden cautioned that these very young children make up a tiny fraction of the American population and that the figures for the broader society have remained flat.
And for women older than 60, the numbers increased.
A third of adults and 17 percent of children are obese, the federal survey found. Still, the lower obesity rates in the very young bode well, researchers said.
There was little consensus on why the decline might be happening, but many theories and a lot of rejoicing.
Progress among the youngest children is especially important because we know that preventing obesity at an early age helps young people maintain a healthy weight into adulthood, said Dr. Risa Lavizzo-Mourey, president of the Robert Wood Johnson Foundation, which often underwrites obesity research.
Children now consume fewer calories from sugary beverages than they did in 1999. And more women are breast-feeding, which can lead to a healthier range of weight gain.
Federal researchers have also chronicled a drop in overall calories for children in the past decade, down by 7 percent for boys and 4 percent for girls, but health experts said those declines were too small to make much difference.
Barry Popkin, a researcher at the University of North Carolina who has tracked American food purchases in a large data project, said families with children have been buying foods that contain fewer calories over the past decade, a pattern he said was unrelated to the economic downturn.
He credited those habits, as well as changes in a federally funded nutrition program the Special Supplemental Nutrition Program for Women, Infants and Children for the decline.
The program, which subsidizes food for low-income women, reduced funding for fruit juices, cheese and eggs and increased it for whole fruits and vegetables.
PUSHES FROM THE TOP
Another possible explanation is that some combination of state, local and federal policies aimed at reducing obesity is starting to make a difference. First lady Michelle Obama has led a push to change young childrens eating and exercise habits, and 10,000 child-care centers across the country have signed on.
Under Mayor Michael Bloomberg, New York City also made a major push to combat obesity. The city told restaurants to stop using artificial trans fats and required chain restaurants to display calorie information on their menus.
Many scientists doubt that anti-obesity programs actually work, but proponents of the programs say a broad set of policies applied systematically can affect behavior positively.
The obesity rate for preschoolers has fluctuated over the years, but Ogden said the pattern became clear with a decades worth of data. About 1 in 12 children in this age group were obese in 2012. Rates for blacks (1 in 9) and Hispanics (1 in 6) were higher.