After a 30-year, record-shattering rise, U.S. obesity rates appear to be stabilizing.

New statistics cited in two papers report only a slight uptick since 2005 – leaving public health experts tentatively optimistic that they may be gaining some ground in their efforts to slim down the nation.

Many obesity specialists say the new data, from the Centers for Disease Control and Prevention, are a sign that efforts to address the obesity problem – such as placing nutritional information on food packaging and revising school lunch menus – are beginning to have an effect in a country where two-thirds of adults and one-third of children and teens are overweight or obese.

“A good first step is to stop the increase, so I think this is very positive news,” said James O. Hill, director of the Center for Human Nutrition at the University of Colorado Health Sciences Center in Denver. “It may suggest our efforts are starting to make a difference. The bad news is we still have obesity rates that are just astronomical.”

Historically, there was little change in Americans’ sizes from 1960 through 1980. But obesity rates soared from the mid-1970s through the end of the century, for reasons that are still debated.

The new studies reflect 2009-10 data, the most recent available, from the government’s National Health and Nutrition Examination Survey, which examined 6,000 adults and 4,111 children, measuring their body mass index, among other items. Though a number of organizations measure obesity rates, the survey’s data are considered among the most accurate.

The statistics showed that more than 35 percent of U.S. adults (78 million people) are obese, defined as having a body mass index of 30 or greater. That is similar to the 2005-06 rate. Calculated as weight in kilograms divided by height in meters squared, the BMI is not a perfect measure of fatness but is still viewed as the gold standard in assessing populationwide trends.

An additional third of adults are overweight, the analysis found, also similar to the rates in 2005-06.

Likewise, data in children and teenagers from birth to age 19 reflect little change from the survey’s 2007-08 data, according to the reports, which were published online Tuesday in the Journal of the American Medical Association. Almost 17 percent are obese and 32 percent are overweight or obese.

But though obesity rates may be flattening overall, increases and disparities can still be found in specific racial and ethnic groups.

It’s not clear why obesity rates are still rising in some groups while stabilizing in others, said Cynthia Ogden, a co-author of the papers and a researcher at the CDC. “All we can do is to be happy that there are no increases.”

But the best bet of some leading obesity experts is that obesity prevention initiatives in some pockets of the country are paying off.

The Let’s Move! program founded by first lady Michelle Obama has raised national awareness through actions such as persuading Wal-Mart to stock more healthful foods and working with professional sports organizations to create public service announcements encouraging children to exercise.

Certain states, including California, have made obesity prevention a major health goal through measures to reduce access to sugary drinks and high-calorie, unhealthful snacks in schools.

A University of California, Los Angeles, study released in November showed obesity rates ticking down in some parts of the state between 2005 and 2010, including a decline of 2.5 percent in Los Angeles County.