Overweight people have a tendency to sit, while lean ones have trouble holding still and spend two hours more a day on their feet, pacing around and fidgeting, researchers are reporting in recent findings published.

The difference translates into about 350 calories a day, enough to produce a weight loss of 30 to 40 pounds in one year without trips to the gym - if only heavy people could act more restless, like thin ones.

The difference in activity levels may be biological and inborn, the researchers say, the result of genetically determined levels of brain chemicals that govern a person's tendency to move around. It is the predisposition to be inactive that leads to obesity, and not the other way around, they suggest.

The findings, being published today in the journal Science, are from a study in which researchers at the Mayo Clinic outfitted 10 lean men and women and 10 slightly obese ones - all of whom described themselves as "couch potatoes" - with underwear carrying sensors that measured their body postures and movements every half second for 10 days on several occasions. By the end of the study, which required a staff of 150, the researchers had collected 25 million pieces of data on each participant.

One thing that convinced the scientists that the activity levels were innate, and not the product of a person's being overweight or underweight, was that the levels did not change when the subjects were forced to gain or lose weight in different phases of the study. To make sure they knew exactly how many calories the subjects were eating, the researchers cooked all their meals for weeks at a time, and had them pledge not to cheat. A total of 20,000 meals were prepared.

The director of the study, Dr. James Levine, an endocrinologist and nutritionist at the Mayo Clinic, said the findings offered hope to overweight people, suggesting that relatively simple and painless changes in their daily behavior, like making an effort to walk more and ride less, could help control weight. He said increases in obesity in recent decades could be traced more to declines in daily exercise - more time spent in cars, behind desks and in front of computers and televisions - than to increases in eating.

In an environment that allows people to be sedentary, those with a biological predisposition to sit still will do so, he said. In contrast, the restless ones will still find ways to burn off calories, even if it means walking around their desks.

"People with obesity are tremendously efficient," Dr. Levine said. "Any opportunity not to waste energy, they take. If you think about it that way, it all makes sense. As soon as they have an opportunity to sit down and not waste those calories, they do."

Participants in the study went through three 11-week phases over a year or so in which their diets were controlled to maintain, increase or decrease their weight. They were paid $2,000 at the end of each phase, for a total of $6,000.

Each phase included a 10-day period during which they had to wear the underwear with the sensors around the clock, taking it off for only about 15 minutes a day to shower and get a fresh set from the researchers.

The top was either an undershirt or a sports bra made of Lycra, and the bottom was a risqué-looking pair of shorts with openings at the crotch and backside so the garment would not have to be lowered during the day, which would have disturbed the sensors.

Dr. Levine said he had designed the outfit with a colleague.

"We had to be very creative," he said. "And you have to test them for comfort. I would put them on top of my suit. Mayo has a very strict dress code. Nothing gave me more pleasure than to wander around with this bizarre underwear over my suit."

Dr. Eric Ravussin, an obesity researcher at the Pennington Biomedical Research Center in Baton Rouge, La., who wrote an essay in Science about Dr. Levine's study, said that because the tendency to sit still seemed to be biological, it might not be easy for obese people to change their ways. "The bad news," Dr. Ravussin said, "is that you cannot tell people, 'Why don't you sit less and be a little more fidgety,' because they may do it for a couple of hours but won't sustain it for days and weeks and months and years."

But Dr. Rudolph Leibel, an obesity researcher at Columbia University Medical Center, said, "People can be taught and motivated to change their behavior in service of their health."

Dr. Leibel also noted that although it was plausible that the tendency to be inactive was biologically determined, it had not been proved.