The finding comes from a government study considered a gold-standard gauge of trends in the public's health. The researchers found that obesity among children ages 2 to 5 decreased - to 8 percent, from 14 percent a decade ago. That would represent a 43 percent drop.
But the only decline was seen in preschoolers, not in older children. And some experts note that even the improvement in toddlers wasn't a steady decline, and say it's hard to know yet whether preschooler weight figures are permanently curving down or merely jumping around.
It is enough of a decline to be optimistic, said Cynthia Ogden, one of the study's authors.
"There's a glimmer of hope," said Ogden, an epidemiologist with the Centers for Disease Control and Prevention.
The report was published online Tuesday in the Journal of the American Medical Association.
Health officials have long been hoping for more substantial evidence that they've turned a corner in the fight against childhood obesity.
Obesity is seen as one of the nation's leading public health problems - health officials call it a longstanding epidemic. A third of U.S. children and teens and more than two-thirds of adults are obese or overweight.
Officials are particularly worried about the problem in young children. Preschoolers who are overweight or obese are five times more likely than other children to be heavy as adults, which means greater risks of high cholesterol, high blood sugar, asthma and even mental health problems.
After decades on the rise, childhood obesity rates recently have been flat. But a few places - including New York City and Mississippi - reported improvements in the last couple of years. Seattle joined that list last week, with a report of recently declining obesity in older school children in low-income school districts.
More broadly, health officials last year reported at least slight drops in obesity for low-income preschoolers in 18 states. But they mainly were children enrolled in the federal Women, Infants and Children (WIC) program, which provides food vouchers and other services. Experts attributed the improvement to WIC policy changes in 2009 that eliminated juice from infant food packages, provided less saturated fat, and made it easier to buy fruits and vegetables.
The new study is a national survey of about 9,100 people - including nearly 600 infants and toddlers - in 2011-2012, in which participants were not only interviewed but weighed and measured. The results were compared to four similar surveys that stretched back to 2003.
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