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September 29, 2003
Now, scientists have developed a model for
a biological link between stress and the drive to eat: Food
with lots of sugar, fat and calories appears literally to calm
down the body's response to chronic stress.
In addition, research indicates that stress hormones encourage
formation of fat cells, particularly the kind that are the
most dangerous to health. That may be at least one reason why
obesity rates are skyrocketing in the United States and many
other modern societies.
"In highly industrialized countries, people do apparently
seem to feel more stressed -- more under the gun," said Mary
F. Dallman, a professor of physiology at the University of
California at San Francisco, who outlined her theory in a paper
to be published in an upcoming issue of the Proceedings of
the National Academy of Sciences. "And they certainly
are eating a lot more."
The new theory has been drawing praise from other scientists
since it was posted on the Internet earlier this month.
"It's an important new model," said Alan G. Watts, a professor
of neuroscience at the University of Southern California. "She's
brought together under one roof two parallel processes. This
is the first time anybody's been able to put together a united
theory on stress and energy metabolism. It presents a new way
of thinking about this."
While the relationship between stress and eating is driven
by a complex mixture of emotional, psychological, social and
physiological factors, the new research does appear to explain
puzzles that have long baffled researchers, said Elissa S.
Epel, an expert on stress, eating and fat at the University
of California at San Francisco.
"There's a multitude of behavioral, attitudinal and
psychosocial factors that determine whether humans engage in
comfort eating.
So it would be a real distortion to say it's all driven by
the stress response," Epel said. "But this explains
mysteries that stress researchers have been unable to resolve
for a long time."
Scientists have long known that during times of stress, parts
of the brain emit a chemical signal called corticotropin-releasing
factor (CRF), which in turn causes the adrenal gland to pump
out large amounts of hormones known as adrenal corticosteroids,
including cortisol. These "stress hormones" flood
the body, producing a wide array of effects designed to get
ready to flee or fight: The immune system gets damped down.
Alertness increases. Heart rate quickens. Activity jumps.
During acute stress -- a car accident, an argument -- a feedback
system kicks in and shuts down this response fairly quickly.
But during chronic stress, the system keeps going, caught in
a vicious cycle.
To examine the relationship between chronic stress and food,
Dallman and her colleagues conducted a series of experiments
with rats, which are considered good models for how the same
systems work in people. The researchers studied levels of stress
hormones, brain activity and chemical signals, as well as fat
distribution in the rats' bodies, comparing animals experiencing
acute and chronic stress induced by exposure to cold or being
restrained. The researchers also manipulated some animals'
stress hormone levels by removing their adrenal glands, administering
stress hormones or injecting them with the chemical signals
that produce stress hormones.
When the rats were under chronic stress and had high levels
of stress hormones coursing through their bodies, they became
very active. They ingested large amounts of high-calorie lard,
eschewing their normal feed, and drank prodigious amounts of
sugar water. They ignored water containing saccharin, even
though it tasted equally sweet. This, in turn, tended to make
the rats develop deposits of fat cells in their abdominal areas.
In humans, fat that gathers around the waist tends to increase
the risks for various health problems.
"When you've got animals in the wild, or people in
underdeveloped countries facing, say, a drought, they will
turn on their adrenal
corticosteroid system. That will make them run to get food
and then they get food and eat and create stores of fat, which
they need to do," Dallman said.
"It works beautifully when there isn't plenty of food
around," she
said. "But when there is plenty of food around, like in
our society, where there's a McDonald's on every corner, it
gets us into deep doo-doo, because this is the kind of fat
that if it stays on is very bad for you. It's associated with
diabetes and heart disease and stroke."
The fat cells, in turn, appear to send signals back to the
brain, shutting down the production of stress hormones, which
makes animals -- and people -- feel better and relax until
they burn off those fat deposits. After ingesting high-sugar,
high-fat diets, and developing fat deposits, the levels of
CRF in the laboratory rats dropped.
But losing weight apparently reactivates the stress response
system, starting the whole process again, said Norman Pecoraro,
who works with Dallman.
"You're losing that metabolic signal to the brain
that's calming things down. So you're removing that yourself
by dieting. So
one thing that's going to happen is that you'll feel more anxious
and won't feel as good and you'll mount this compulsion system
to go get the goodies," Pecoraro said.
Other researchers said that the work needs to be followed up
with additional studies in animals and people.
"I think it's a fascinating new insight into this
thing we refer to as comfort foods," said Bruce S. McEwen, a professor
of neuroendocrinology at the Rockefeller University in New
York. "Often what we call old wives' tales turn out to
have a scientific basis, and this seems to be the beginning
of understanding this old wives' tale."
If scientists can identify some of the chemical signals involved
in the feedback loop of eating, fat and stress, and then design
drugs to block them, that could lead to new treatments for
obesity, McEwen said. "One might be able to find a drug
that helps to calm this system down, so to speak."
Dallman hopes the new understanding might help people control
their appetite without drugs.
"It seems to me that when I know there's a reason
for something that's happening," she said, "then maybe I can have
more control over it."
© 2003 The Washington
Post Company
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