Heymsfield's subjects might be onto something, according to new research by James A. Levine, MD, a professor of medicine at the Mayo Clinic College of Medicine in Rochester, Minnesota—but not in the sense that their bodies don't burn food efficiently. Levine says some people have a biological drive to beach themselves in a Barcalounger while others constantly flit around like hummingbirds. He calls this nearly unconscious physical activity NEAT, for "nonexercise activity thermogenesis." NEAT encompasses everything from sitting up straight to tapping your foot to gesturing with your hands when you talk. A hundred or so years ago, he says, people typically burned 1,500 more calories a day than they do now. Even those who would rather be relaxing, thank you, had to plow the fields or walk to town or take the laundry to the creek and slap it on rocks. But in our age, people born with the urge to sit find that the world is one big, comfy couch—an inert way of life that, Levine believes, is enough to explain the obesity epidemic.

For a study published two years ago in Science, Levine took 20 self-described couch potatoes—10 lean and 10 mildly obese—and dressed them in high-tech underwear that recorded their bodily movements every half second for 10 days. He discovered that his leaner spuds burned about 350 more calories a day through NEAT—or 33 pounds a year.

In an earlier NEAT study, Levine recruited 16 volunteers and for two months had them eat 1,000 calories a day over what they needed to maintain their weight. You would expect they'd all put on weight—1,000 extra calories a day is a lot. But at the end of the study, the gain per individual ranged from less than a pound to more than 9 pounds. And all the variation, says Levine, could be explained by the amount of NEAT.

The good news is that if you're not a natural-born fidgeter, you can consciously work at overriding your biology. When Levine noticed his body starting to thicken as he hit middle age, he put a treadmill in the living room, and every night when he came home and watched The Simpsons (some have their wine, others de-stress with Homer), he did it while walking. He lost 15 pounds over a period of nine months without changing anything he ate.