August 23, 2006

Brain chemical’s call to exercise may go unheeded

By Amy Norton

NEW YORK (Reuters Health) - 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

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

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.