Contact:
Christine Guilfoy
Office: (301) 634-7253
cguilfoy@the-aps.org
Obesity: Is It In Your Head?
Being a couch potato may
have a biological basis in the brain
Bethesda,
MD (August 14, 2006) –Some brains may be wired to encourage fidgeting and
other restless behaviors that consume calories and help control weight,
according to new research published by The American Physiological
Society.
The study found that the brains of rats bred to be lean
are more sensitive to a chemical produced in the brain, orexin A, which
stimulates appetite and spontaneous physical activity such as fidgeting and
other unconscious movements. Compared to rats bred to be obese, the lean
rats had a far greater expression of orexin receptors in the hypothalamus.
“The greater expression of orexin receptors suggests
the lean rats’ brains were more sensitive to the orexin the brain produces,”
said Catherine M. Kotz, the study’s senior researcher. “The results point to
a biological basis for being a couch potato.”
This line of research suggests that frequent minor
unconscious movements such as fidgeting and other behaviors associated with
restlessness burn calories and help control weight, Kotz said. Further, it
suggests a strategy to reduce weight gain and could lead to the development
of a drug to stimulate minor activity.
The study “Elevated hypothalamic orexin signaling,
sensitivity to orexin A and spontaneous physical activity in obesity
resistant rats,” appears in the online edition of the American Journal of
Physiology-Regulatory, Integrative and Comparative Physiology published
by The American Physiological Society. The study was done by Jennifer
A. Teske and Allen S. Levine of the University of Minnesota and the
Minnesota Obesity Center, St. Paul; Michael Kuskowski, VA Medical Center,
Minneapolis; James A. Levine, Mayo Clinic, Rochester, Minnesota; and
Catherine M. Kotz, the VA Medical Center, University of Minnesota, and the
Minnesota Obesity Center.
Study looks at obese
versus lean rats
“Many people focus on diet, but it may be more feasible
for some people to stand or move more throughout the day” as a way to
control their weight, Kotz said. Contrary to common belief, metabolism rates
don’t vary greatly from person to person and weight gain usually results
from eating too much, burning too few calories, or both, she said.
The researchers drew their conclusions after performing
a series of experiments with obesity-prone and obesity-resistant rats. The
obesity-prone strain was developed for obesity research by breeding obese
rats with other obese rats. The obesity-resistant rats were developed by
breeding lean rats with lean rats, Kotz noted. The study also employed a
control group of normal laboratory rats.
Each rat consumed the same number of calories each day.
The researchers took baseline measurements of each rat’s activity using
sensors to measure even minor movements, such as grooming and standing.
They found that the lean group moved significantly more
during this baseline period than the obese group, Kotz said. This was true
even though the rats were young and both groups weighed the same --
eliminating the obesity itself as the cause of the decreased movement. After
the baseline data gathering, the researchers moved to the experimental part
of the study.
Lean rats have elevated
expression of orexin receptors
“We knew from previous studies that orexin stimulated
physical activity, and so we wanted to find out whether it enhances activity
more in lean rats than in obese rats, Kotz explained. The researchers
injected orexin into the lateral hypothalamus area of the brains of both
groups and found that the lean rats became even more active, while the obese
rats didn’t respond much at all. “Not only do the lean rats have a higher
base activity rate but they respond more to orexin,” she said.
Orexin must bind to receptors in the brain to produce
increased activity, so the researchers reasoned that the lean rats must have
more orexin receptors. When they did a blind analysis of the brains of obese
and lean rats of various ages, they found that the lean rats had double the
gene expression level of orexin receptors compared to the obese rats, Kotz
explained.
The greater gene expression of orexin receptors does
not conclusively prove that there are more orexin receptors, but it is
highly suggestive of that finding. Kotz and her fellow researchers are now
looking to see if the lean rats have a greater number of orexin receptors in
their brains.
Activity level important
to weight control
Because the rats in this study ate the same amount of
food, the researchers concluded that the weight gain of the obese rats comes
more from expending too few calories than from consuming too many. Other
studies have shown that disabling the orexin system of lean rats causes them
to eat less and move less, which leads them to become obese, Kotz said. When
the orexin system is working optimally, the increase in eating which orexin
causes is believed to be offset by increased physical activity, she said.
It would be impossible to do a similar study of the
brain in humans. But one of the researchers, James Levine, found in a
previous study with humans that lean individuals move about two hours per
day more than obese individuals. What does this mean for those who are
overweight?
“If we can get obese individuals to a slightly higher
level of activity, that would be very beneficial,” Kotz concluded.
Future studies will focus on:
Funding
Funding was provided by the Minnesota Department of
Employment and Economic Development (Minnesota Partnership for
Biotechnology and Medical Genomics), the U.S. Department of
Veterans Affairs and the National Institute of Diabetes and Digestive
and Kidney Diseases of the National Institutes of Health.
Editor’s note: The media may obtain a copy of
Teske et al. by contacting
Christine Guilfoy, American Physiological Society, (301) 634-7253 or
cguilfoy@the-aps.org.
* * *
The
American Physiological Society was founded in 1887 to foster basic and
applied bioscience. The Bethesda, Maryland-based society has 10,500 members
and publishes 14 peer-reviewed journals containing almost 4,000 articles
annually.
* * *
APS
provides a wide range of research, educational and career support and
programming to further the contributions of physiology to understanding the
mechanisms of diseased and healthy states. In 2004, APS received
the Presidential Award for Excellence in Science,
Mathematics and Engineering Mentoring.
