"People with obesity are tremendously efficient," 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 carefully 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 risque-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.

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 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," he 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."

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

Ravussin said it might be possible to help people stay lean by making their environments less conducive to sitting, though that would take major societal changes like rebuilding neighborhoods in which people can walk to markets instead of "the remote shopping mall with 10,000 parking spots and everybody is fighting for the handicapped one."

A participant in the study, Othelmo da Silva, 41, an academic adviser at Rochester Community and Technical College in Minnesota, said he was overweight and felt encouraged by the study and the idea that people could lose weight by moving around more and did not necessarily have to join a gym.