Federal health authorities on Tuesday reported a 43 percent drop in theobesity rate among 2- to 5-year-old children over the past decade, the first broad decline in an epidemic that often leads to lifelong struggles with weight and higher risks for cancer, heart disease and stroke.

The drop emerged from a major federal health survey that experts say is the gold standard for evidence on what Americans weigh. The trend came as a welcome surprise to researchers. New evidence has shown that obesitytakes hold young: Children who are overweight or obese at 3 to 5 years old are five times as likely to be overweight or obese as adults.

A smattering of states have reported modest progress in reducing childhood obesity in recent years, and last year the 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 2- to 5-year-olds, offering the first clear evidence that America’s youngest children have turned a corner in the obesity epidemic. About 8 percent of 2- to 5-year-olds were obese in 2012, down from 14 percent in 2004.

“This is the first time we’ve seen any indication of any significant decrease in any group,” said Cynthia L. Ogden, a researcher for the Centers for Disease Control and Prevention, and the lead author of the report, which will be published in JAMA, The Journal of the American Medical Association, on Wednesday. “It was exciting.”

She cautioned that these very young children make up a tiny fraction of the American population and that the figures for the broader society had remained flat, and had even increased for women over 60. A third of adults and 17 percent of youths are obese, the federal survey found. Still, the lower obesity rates in the very young bode well for the future, researchers said.

There was little consensus on why the decline might be happening, but many theories.

Children now consume fewer calories from sugary beverages than they did in 1999. More women are breast-feeding, which can lead to a healthier range of weight gain for young children. Federal researchers have alsochronicled 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 M. Popkin, a researcher at the University of North Carolina at Chapel Hill who has tracked American food purchases in a large data project, said families with children had been buying lower-calorie foods over the past decade, a pattern he said was unrelated to the economic downturn.

He credited those habits, and changes in the federally funded Special Supplemental Nutrition Program for Women, Infants and Children, for the decline in obesity among young children. 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.

Another possible explanation is that some combination of state, local and federal policies aimed at reducing obesity is starting to make a difference.Michelle Obama, the first lady, has led a push to change young children’s eating and exercise habits and 10,000 child care centers across the country have signed on. The news announcement from the C.D.C. included a remark from Mrs. Obama: “I am thrilled at the progress we’ve made over the last few years in obesity rates among our youngest Americans.”

New York City under Mayor Michael R. Bloomberg also made a major push to combat obesity. The city told restaurants to stop using artificial trans fats in cooking 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 over a period of time can affect behavior.

The obesity rate for preschoolers — 2- to 5-year-olds — has fluctuated over the years, but Dr. Ogden said the pattern became clear with a decade’s worth of data. About one in 12 children in this age group was obese in 2012. Rates for blacks (one in nine) and Hispanics (one in six) were much higher.

Researchers welcomed the drop but cautioned that only time will tell if the progress will be sustained.

“This is great news, but I’m cautious,” said Ruth Loos, a professor ofpreventive medicine at the Icahn School of Medicine at Mount Sinai hospital in New York. “The picture will be clearer when we have a few more years of data.”

Still, she added that the 2- to 5-year-olds “might be riding a new wave,” in which changes in habits and environment over many years are finally sinking in. She noted that people who are now 60 years old caught the beginning of what she called the obesity wave that carried the next generation with it.

“Once the obesity epidemic emerged in the 1980s, it took us a while to realize that something bad was happening,” Dr. Loos said. “We’ve been trying to educate parents and families about healthy lifestyles, and maybe it’s finally having an effect.”

Tom Baranowski, a professor of pediatrics at Baylor College of Medicine, said there was not enough data to determine whether the decline would spread to older children. Since 2003, the rate for youths over all — ages 2 to 19 — has remained flat, said Dr. Ogden, author of the C.D.C. report.

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But 2- to 5-year-olds are perhaps the most significant age group, as it is in those years that obesity — and all the disease risk that comes with it — becomes established, and it is later very difficult to shake, said Dr. Jeffrey P. Koplan, a professor of medicine and public health at Emory University in Atlanta.

“You have to say maybe some real progress is taking place at the very time it can have the most impact,” Dr. Koplan said. He said he believed the decline was real, as the finding followed several studies that detected patterns of decline among young children, including one by researchers in Massachusetts and the large study by the C.D.C. of low-income children.

“The weight of evidence is becoming more marked,” he said. Still, he cautioned that the age group was only a small slice of American society: “One blossom doesn’t make a spring.”