Among those inclined to be sedentary, lean ‘couch potatoes’ apparently move and stand more during the day - and sit for around two hours less - than their obese counterparts, according to new findings released Thursday.

Moreover, when researchers asked the self-described couch potatoes to either gain or lose weight, their sitting and standing habits didn’t change, suggesting that their baseline level of activity is “biologically determined,” the researchers write in the journal Science.

However, this doesn’t mean that obese people who pass the hours at a desk or in front of a TV are stuck with their extra pounds, cautioned study author Dr. James A. Levine of the Mayo Clinic in Rochester, Minnesota.

“What’s key is that obesity rates were negligible 50 years ago, whereas our biology is unchanged,” he said. This indicates that people have little inclination to add activity to today’s largely sedentary lifestyle, but it’s possible to break the cycle, Levine explained.

For instance, people who sit most of the day can squeeze in an extra few minutes of activity every hour, perhaps parking further away in a parking lot, taking the stairs for one or two flights, or visiting a co-worker with a message instead of shooting off an e-mail, he said.

“It must be possible for obese people to get that extra two hours (of non-sitting time), because lean people have it in the same environment,” he said. “It’s not that difficult, but it does take a tremendous commitment.”

In an interview with Reuters Health, Levine explained that he and his team were inspired to conduct the study after one of their earlier reports showed that when people overeat, lean eaters tend to subconsciously burn more calories in daily activities - such as talking, sitting and standing - than obese people do.

To investigate whether this unplanned daily activity plays a role in obesity, Levine and his team asked 10 lean and 10 mildly obese people, all self-proclaimed “couch potatoes,” to wear underwear that included sensors that measured their activity for 10 days.

They found that obese participants typically sat for 2 hours longer each day than lean participants. Thin volunteers were also upright for almost 2 hours longer each day than obese people, and the more people moved their bodies, the lower their fat mass.

The researchers calculated that if obese participants were as active as lean participants, they would burn an extra 350 calories per day.

To determine whether these differences in activity are a cause or consequence of extra body weight, Levine and his team asked lean participants to overeat for 8 weeks and obese volunteers to lose weight over the same period. They saw that activity levels stayed the same, suggesting that this activity may be set at a biological level.

SOURCE: Science, January 28, 2005.