People who literally cannot sit still may have inborn behavior that keeps them slim even if they overeat a little, researchers in the United States said.

Tests on slim and overweight people who all described themselves as 'couch potatoes' showed the main difference between the two groups was how long they spent sitting still.

"Our study shows that the calories that people burn in their everyday activities are far, far more important in obesity than we previously imagined," said Dr. James Levine of the Mayo Clinic in Rochester, Minnesota, who helped lead the study.

His team recruited 10 normal-weight and 10 obese men and women for their study, persuading them to wear special underwear with sensors that logged every move, however small.

They found the obese people spent, on average, more than two extra hours a day sitting still compared with the lean volunteers. That did not include sleeping time, which was the same between the two groups.

The difference in activity accounted for about 350 calories a day -- enough to add 10 pounds a year.

Then they tested the idea that maybe heavier people were forced to sit more.

"You might think the reason that people with obesity are seated 164 minutes more per day is because they are heavier and fall into their chairs, so to speak," Levine said in a telephone interview.

"If that were the case, then you'd think if obese people lost the weight, they would actually get up and walk around more. That wasn't the case."

They put their obese volunteers on a 1,000 calorie-a-day diet for two months and they lost, on average, 18 pounds. But their activity levels did not change.

"And how about if lean people gained weight?" Levine asked. "We took lean people and we overfed them and they gained a lot of excess weight and they remained get-up-and-goers."

'GET-UP-TO-GO' GENE?

The tendency to fidget may be genetic or it may be learned at a very early age, Levine said.

"The idea is there is either a 'get-up-to-go' gene or there is a gene that sends you into your chair," Levine said. "I am actually of the belief that what happens in childhood is absolutely key."

Either way, the answer may be to encourage plenty of physical activity early on in life. With two-thirds of the U.S. population overweight or obese and other countries quickly catching up, someone clearly needs to figure something out, Levine said.

"Perhaps we need to think about how schools are run and the fact that kids always want to run and we tell them not to," he said. "Kids don't get out and play at lunch any more."

Levine, who has hooked up a laptop computer to a treadmill, was speaking as he exercised.

"I have converted my sedentary job to a completely active one," Levine said, the sound of the treadmill audible in the background.

"And I don't get home tired. I get home energized."

Levine said his group got $2 million in U.S. National Institutes of Health funding for the study, which began after his group discovered in 1999 what they call non-exercise activity thermogenesis, or NEAT, underlies the difference between people who can get away with snacking and those who cannot.

They used sensors designed for controlling fighter planes and implanted them in specially designed underwear while keeping their volunteers on carefully controlled diets.

"If you are going to attach sensors to people that they are going to wear all day everyday, you have to put these things where they don't show," he said.