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 findings published Friday.

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 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 that 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 find ways to burn off calories, even if it means walking around their desks.