The nonstop rise in childhood obesity in the United States is showing the first signs of slackening since the trend took hold more than two decades ago, a new study finds. While promising, the findings also confirm the unsettling reality that one-third of U.S. children remain overweight or obese and face a heightened risk of medical problems as a result.

The Centers for Disease Control and Prevention made the new determination based on health exam data from 8,165 children ages 2 to 19. The researchers found no significant change in the percentage of overweight or obese kids during the years 1999 to 2006. The analysis appears in the May 28 Journal of the American Medical Association.

“We’ve had 25 years of overwhelmingly bad news about childhood obesity,” says David Ludwig, a pediatric endocrinologist at Harvard Medical School and Children’s Hospital in Boston. “This study provides the first glimmer of hope, but it’s too soon to tell whether the data present a true plateau of obesity rates or just a temporary lull.”

While experts have debated the definition of what constitutes an overweight or obese child, a panel of scientists that convened last year identified obese children as those in the 95th percentile or higher for weight when compared with the standard child’s growth chart, established in the 1960s. Kids in the 85th to 94th percentile are overweight, the panel decided. The new study finds no significant change in the percentage of children in these very ranges.

Meanwhile, the historical picture shows how far the obesity epidemic has gone.

Using the 95th percentile as a definition of obesity, such children included 5 percent of all kids in the 1960s. Now they account for 16 percent, says study coauthor Cynthia Ogden, an epidemiologist at the CDC’s National Center for Health Statistics in Hyattsville, Md. And while kids considered to be overweight — the 85th to 94th percentile group — accounted for 10 percent of all children in the 1960s, they now are now 16 percent of the whole, she says.

Ogden and her colleagues used body mass index, a calculation of weight in relationship to height, in making their weight measurements. While BMI can be inexact — it doesn’t measure actual body fat, after all — it is pretty good at spotting people who are clearly obese or overweight, she says.

While the findings don’t represent an improvement in the obesity rate, “this is certainly not bad news,” Ogden says. “We can be cautiously optimistic that it’s leveling off.”

Nevertheless, the problem seems to be lingering, says Terry Huang, a public health scientist at the National Institute of Child Health and Human Development in Rockville, Md. “What struck me is that the prevalence level has stayed consistently very high,” says Huang, an obesity researcher who was not involved with this study. “One-third of children — this is a large segment of the population.”

The effects of these added pounds show up in more high blood pressure and increased rates of type 2 diabetes in adolescents. Later, excess weight also increases the risk of heart disease and other problems.

“Without substantial declines in obesity prevalence,” Ludwig says, “the toll of this epidemic will continue to mount because it can take many years for those children to develop weight-related complications.”

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