USA Today:

The calories that people burn in everyday activities, such as standing, pacing and cleaning, are an even more important factor in obesity than scientists had realised. But some people do not seem hard-wired to move around much. A study recently published states that overweight couch potatoes sit about two-and-half hours more a day than normal-weight couch potatoes. The heavier people studied were not sitting more just because of their weight, the researchers say. “Their movements were really fixed, like it was biologically driven,” says endocrinologist James Levine of the Mayo Clinic in Rochester, Minn. “Many of us are programmed not to move much. When you offer some people a comfy chair, they’ll take it.” Researchers recruited 20 sedentary persons, 10 obese and 10 normal weight. They wore movement monitors for 10 days. The obese people sat for an average of 164 minutes more a day. The leaner people moved around doing just basic activities, such as puttering about the house, for 152 more minutes a day. The leaner people burned off about 350 more calories a day. Initially the researchers speculated that if the heavy people lost weight, they might move more. If the lean ones gained weight, they might be more sedentary. So in a calorie-control phase, the obese people lost an average of 15 pounds; lean people gained 10 pounds. But both groups spent the same time sitting or moving. There may be “a brain chemical difference between people because of their genes,” Levine says.

A genetic trait may be to blame, agrees Eric Ravussin, professor of human physiology at the Pennington Biomedical Research Centre.