American children consumed fewer calories in 2010 than they did a decade before, a new federal analysis shows. Health experts said the findings offered an encouraging sign that the epidemic of obesity might be easing, but cautioned that the magnitude of the decline was too small to move the needle much.

And while energy intake has not changed considerably for adults in recent years, fewer of their calories are coming from fast food, researchers said. Obesity rates for adults have plateaued after years of increases. A third of adults are obese.

The results of the research on childhood consumption patterns, the only federal analysis of calorie trends among children in recent years, came as a surprise to researchers. For boys, calorie consumption declined by about 7 percent to 2,100 calories a day over the period of the analysis, from 1999 through 2010. For girls, it dropped by 4 percent to 1,755 calories a day.

“To reverse the current prevalence of obesity, these numbers have to be a lot bigger,” said Marion Nestle, professor of nutrition, food studies and public health at New York University. “But they are trending in the right direction, and that’s good news.”

National obesity rates for children have been flat in recent years, but some cities have reported modest declines. The new evidence of a lower calorie intake for children may also foreshadow a broader national shift, experts said.

“A harbinger of change is a good phrase,” said R. Bethene Ervin, a researcher at the Centers for Disease Control and Prevention and one of the authors of the report. “But to see if it’s really a real trend we would obviously need more years of data.”

A drop in carbohydrate consumption drove the decline, a point of particular interest for those who study childhood obesity. Sugars are carbohydrates, and many argue that those added to food like cereal and soda during processing are at the heart of the childhood obesity epidemic. Dr. Ervin said it was not clear whether such added sugars alone were behind the carbohydrate decline.

Over all, calories from fat remained stable, while those from protein increased and those from carbohydrates fell. The calorie decline was most pronounced among boys ages 2 to 11, and among teenage girls.

Carbohydrate consumption declined among white and black boys, but not among Hispanic boys. Among girls, whites were the only group that consumed fewer calories from carbohydrates.

Another surprise, researchers said, was the decline in calories coming from fast food among American adults. Those calories fell to 11.3 percent of adults’ total daily intake in 2010, down from 12.8 percent in 2006. The decline was sharpest among 40- to 59-year-olds, said Cynthia L. Ogden, a C.D.C. researcher who oversaw the research, which comprised two studies, one on caloric intake for children and the other on fast-food consumption among adults. For the analysis, called the National Health and Nutrition Examination Survey, people were interviewed in their homes and at mobile examination centers around the country.

Americans eat about a third of their calories outside the home, according to federal data, and some demographic groups still get a lot of calories from fast food. Blacks between the ages of 20 and 39 consumed more than a fifth of their calories from fast food, the highest share for any group. The lowest rate was among older people, ages 60 and above, who got 6 percent of their daily caloric intake from fast food.

Obese people also consumed more fast food, researchers found.

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By SABRINA TAVERNISE