In the United States, about 25 million children are either overweight or obese.

A new study explored how being overweight or obese might affect brain development in children.

A report on the study appeared this month in JAMA Pediatrics, a publication of the American Medical Association. The writers suggested a link between body mass index, brain development and "executive functions, such as working memory."

An editorial published with the report called the study an important addition to growing evidence of a link between weight, brain structure and mental function. It also warned against misinterpreting the findings.

The study

The study involved 3,190 U.S. boys and girls, aged 9 and 10. Researchers had height and weight measurements and MRI scans of their brains. MRI is short for magnetic resonance imaging, a method used to take pictures of organs and tissues within the body.

The children took computer-based tests of mental function, including memory, language, reasoning and impulse control. Nearly 1,000 of the kids — almost 1 in 3 — were overweight or obese, similar to levels nationwide.

Researchers found differences in the brain scan images of the heaviest children. They observed slightly less volume -- the amount of space -- in the brain behind the forehead. This area controls what are known as "executive function" tasks. Such tasks include things like planning, controlling impulses and dealing with two or more activities at the same time.

Compared with normal-weight children, the differences were small, noted Scott Mackey, a neuroscientist at the University of Vermont.

The heaviest children also had slightly worse results on the computer-based tests of executive function. But it is unknown whether any of the differences had much of an effect on children's behavior or performance in school. Those are the opinions of Mackey and Jennifer Laurent, a University of Vermont researcher. Laurent was the lead author of the report on the study.

It is also unclear how the differences relate to weight. Mackey said other factors not measured in the study, including physical activity and good nutrition, are likely important.

Research in adults has linked obesity with low-level inflammation throughout the body. The condition can damage blood vessels and may increase risks for heart disease and loss of mental ability.

Some studies have also found less brain volume in obese adults. Researchers suspect that the decreased brain volume could be from inflammation.

The new study raises the possibility that inflammatory changes affecting weight, brain structure and brain function might begin in childhood.

Be careful interpreting the results

Eliana Perrin is a doctor and specialist of children's health at Duke University in North Carolina. She co-wrote the editorial published in JAMA Pediatrics. She says that people should be careful about how they interpret the study.

"We don't know which direction these relationships go nor do they suggest that people with obesity are not as smart as people at a healthy weight," she said.

The latest research confirms results of earlier studies in children and adults, but it leaves many questions unanswered, said Marci Gluck. Gluck is with the U.S. National Institute of Diabetes and Digestive and Kidney Diseases. She was not part of the research, and warned people against making a link between executive function and intelligence.

Natasha Schvey is an obesity researcher with the Uniformed Services University of the Health Sciences. She called the study impressive, but noted that many factors influence people's diets and obesity.

"We know from a lot of really good research that obesity is not as much in an individual's control as we think it is. People talk about willpower — that's a very small part of the equation," she said. "There are much bigger contributors to our weight and a lot of it is genetic."

I'm John Russell.

Lindsey Tanner reported on this story for The Associated Press. John Russell adapted the AP story for Learning English. George Grow was the editor.

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obese – adj. very fat; fat in a way that is unhealthy

body mass index – n. a measurement that shows the amount of fat in your body and that is based on your weight and height

function – n. the special purpose or activity for which a thing exists or is used

misinterpret – v. to misunderstand or be mistaken; to get the wrong idea

editorial – n. a published report that gives the opinions of the editors or publishers of a newspaper or magazine

impulse – n. a sudden strong desire to do something

factor – n. a fact or influence that influences a result

inflammation – n. a physical condition in which part of the body becomes reddened, hot and often painful

impressive – adj. moving or touching