The body’s sensitivity to a particular brain chemical may help separate those of us who can’t sit still from those who can’t seem to get off the couch, a study in rats suggests.

Because spontaneous activity throughout the day is a major factor in calorie burning, researchers say this brain response might play a role in obesity risk.

In their study, lean rats were more sensitive to a brain chemical called orexin A, and tended to fidget and move around through much of the day. Obesity-prone rats, on the other hand, were more likely to resist orexin A’s “get moving” signal and take it easy instead.

What’s more, the lean rats maintained their physiques despite eating as much as their heavier counterparts - pointing up the importance of their high activity levels.

The findings are published in the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology.

While going to the gym or taking a jog may be important for weight control, so too is the incidental physical activity people get during the day. Standing, casually walking or moving in any way all burn more calories than sitting in front of the TV.

Previous research has found that normal-weight people move around significantly more often over the course of a day than overweight individuals do.

It’s possible that sensitivity to orexin A plays a role in a person’s tendency to be a mover or a couch potato, thereby contributing to obesity, according to the study’s senior author, Dr. Catherine M. Kotz of the University of Minnesota and the VA Medical Center in Minneapolis.

“There could be something at work that makes some people more sedentary,” she told Reuters Health.

Kotz and her colleagues focused their research on orexin A because it’s known to spur appetite, as well as wakefulness and movement. They wanted to see whether “obesity-resistant” rats differed from obesity-prone rodents in their response to the brain chemical.

The obesity-resistant rats were created by breeding lean rats with lean rats, while the obesity-prone animals were bred from two obese parents.

From early on, the researchers found, the obesity-resistant rats were naturally more active, and over time, they stayed lean, while the obesity-prone animals packed on the pounds.

When the researchers infused the animals’ brains with orexin A, they found that the lean mice became even more active, while their heavier counterparts showed little response to the chemical.

Though it’s not clear that this is true of humans, the findings support the notion that some people “have to fight their biology” to get moving, Kotz said.

A pill that would boost orexin’s powers is not out of the question, though that would take some time, according to Kotz. More practically, she said, the findings highlight the importance of “low-level” activity throughout the day.

For people who are significantly overweight and sedentary, Kotz said, simply standing and moving more is a good first step toward becoming active.

SOURCE: American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, online August 14, 2006.