ATLANTA -- For many years, doctors have been wringing their hands as more and more U.S. children grew fat. Now, that may be changing, with the first evidence of a national decline in childhood obesity.
In 18 states, there were at least slight drops in obesity for low-income preschoolers, health officials said Tuesday.
After decades on the rise, childhood obesity rates recently have essentially been flat. A few places -- Philadelphia, New York City and Mississippi -- reported improvements in the last couple of years. But the report from the Centers for Disease and Control Prevention shows signs of wider-ranging progress.
"Now, for the first time, we're seeing a significant decrease in childhood obesity" nationally, said Dr. Thomas Frieden, CDC director.
But rates are still too high, he added. One in 8 preschoolers is obese in the United States, and it's even more common in black and Hispanic kids.
"It's not like we're out of the woods," he said during a conference call with reporters Tuesday.
Obesity continues to be 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.
Some hope the report marks a turning point.
"I really do think this is a pivotal moment," said Sam Kass, executive director of a White House initiative to reduce childhood obesity.
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.
Tuesday's study used height and weight measurements from nearly 12 million low-income children in 40 states. The data was collected from 2008 through 2011.
Most of the children ages 2 to 4 were enrolled in the federal Women, Infants and Children (WIC) program, which provides food vouchers and other services.
It's harder to get national data on preschoolers of more affluent families, so it's not clear if the trend applies to all young children. But experts note that low-income kids tend to be heavier.
"If you're going to look at the problem of obesity early in childhood, the group at highest risk are low-income kids. That's what makes this data so valuable for understanding trends in this major public health problem," said Dr. Matthew Davis, a University of Michigan researcher who tracks health policy and children's health issues.
The biggest declines were in Florida, Georgia, Missouri, New Jersey and South Dakota. Each saw their obesity numbers fall at least 1 percentage point.
Other states showing improvement: California, Iowa, Idaho, Kansas, Massachusetts, Maryland, Michigan, Minnesota, Mississippi, Montana, New Hampshire, New Mexico and Washington. A substantial decline was also seen in the U.S. Virgin Islands.
"These signs of progress tell a clear story: we can reverse the childhood obesity epidemic. It isn't some kind of unstoppable force," said Dr. James S. Marks, in a statement. He's senior vice president at the Robert Wood Johnson Foundation, a philanthropy that supports programs to tackle obesity.
Despite the improvements, the numbers are still disappointing. Hawaii was the best, with about 9 percent of low-income preschoolers estimated to be obese in 2011. Even with some progress, California was worst, at nearly 17 percent.
Ten states were not included; some had changed how they track height and weight. One of the missing states is Texas, which has one of the largest populations of low-income children and is known to have a significant problem with childhood obesity.
Of the remaining 40 states, 18 showed at least slight improvement and 19 states and Puerto Rico had no significant change. Three states -- Colorado, Pennsylvania and Tennessee -- increased.
The last CDC study to look at childhood obesity data this way found very different results. From 2003 to 2008, significant declines in preschooler obesity were seen in only nine states and increases were seen in 24 states.
"We're seeing great progress," said the CDC's Ashleigh May, lead author of the new study.
The report didn't answer why some states improved while most others held steady, and Davis said there's a pressing need to do more research and understand how some states were able to scale back.
CDC officials said a change in WIC policies probably played a major role. The changes -- instituted in 2009 -- eliminated juice from infant food packages, provided less saturated fat, and made it easier to buy fruits and vegetables. Breast-feeding rates have been increasing, and kid's raised on mother's milk tend to have lower obesity rates, experts said.
Childhood obesity has been a focus of the Obama White House, with first lady Michelle Obama leading a campaign called "Let's Move!" Kass, the program's chief administrator, said thousands of preschools and day care centers across the country have pledged to increase physical playtime and serve healthier foods.
"While this announcement reflects important progress, we also know that there is tremendous work still to be done to support healthy futures for all our children," Michelle Obama said in a statement.