FRIDAY, Dec. 10 (HealthDay News) -- Most people probably find
drinking a milkshake a pleasurable experience, sometimes highly so.
But apparently that's less apt to be the case among those who are
overweight or obese.
Overeating, it seems, dims the neurological response to the
consumption of yummy foods such as milkshakes, a new study
suggests. That response is generated in the caudate nucleus of the
brain, a region involved with reward.
Researchers using functional magnetic resonance imaging (fMRI)
found that that overweight and obese people showed less activity in
this brain region when drinking a milkshake than did normal-weight
"The higher your BMI [body mass index], the lower your caudate response when you eat a milkshake," said study lead author Dana Small, an associate professor of psychiatry at Yale and an associate fellow at the university's John B. Pierce Laboratory.
The effect was especially strong in adults who had a particular
variant of the taqIA A1 gene, which has been linked to a heightened
risk of obesity. In them, Small said, the decreased brain response
to the milkshake was very pronounced. About a third of Americans
have the variant.
The findings were to have been presented earlier this week at an
American College of Neuropsychopharmacology meeting in Miami.
Just what this says about why people overeat or why dieters say
it's so hard to ignore highly rewarding foods is not entirely
clear. But the researchers have some theories.
When asked how pleasurable they found the milkshake, overweight
and obese participants in the study responded in ways that did not
differ much from those of normal-weight participants, suggesting
that the explanation is
not that obese people don't enjoy milkshakes any more or
And when they did brain scans in children at risk for obesity
because both parents were obese, the researchers found the opposite
of what they found in overweight adults.
Children at risk of obesity actually had an increased caudate
response to milkshake consumption, compared with kids not
considered at risk for obesity because they had lean parents.
What that suggests, the researchers said, is that the caudate
response decreases as a
result of overeating through the lifespan.
"The decrease in caudate response doesn't precede weight gain, it follows it," Small said. "That suggests the decreased caudate response is a consequence, rather than a cause, of overeating."
Studies in rats have had similar results, said Paul Kenny, an
associate professor in the behavioral and molecular neuroscience
lab at the Scripps Research Institute in Jupiter, Fla.
When rats were given access to highly palatable, highly
rewarding food for extended periods, they became obese. The fatter
they got, the more the response in their brain reward centers
"Over time, the reward systems began to slow down," Kenny said. "They were not functioning properly. We think something similar may be going on in humans."
"As you go through your life and continue to eat these highly palatable foods, you are overstimulating your brain reward center," he explained. "Over time, the system fights back, and it tones itself down -- which is why the higher the BMI, the less activity you see in the reward area."
Among other things, the brain's caudate nucleus is involved with
regulating impulsivity, which is related to self control, and
addictive behaviors, Small noted.
"The caudate is a region of the brain that receives dopamine," she said. "What this brain response could mean is that overeating causes adaptations in the dopamine system, which could confer further risk of overeating."
The question for dieters, then, is whether the caudate response
can be restored to normal if they lose weight. The researchers said
they didn't know but planned to test that.
Research in people with other addictions suggests that, over
time, there may be some return to normalcy in the brain's reward
processing but perhaps never a complete return to where you
started, Kenny said.
A second study to be presented at the meeting found that that
the brains of obese people responded differently than the brains of
normal weight people to anticipated food or monetary rewards and
It found that obese individuals showed greater brain sensitivity
to anticipated reward and less sensitivity to anticipated negative
consequences than normal-weight people. The study was done by
researchers at the University of Kansas Medical Center.
Because the findings from both studies were to be presented at a
medical meeting, they should be viewed as preliminary until they
are published in a peer-reviewed journal.
About 30 percent of the U.S. population is classified as obese,
and the medical consequences of that cost more than $100 billion
annually, said Dr. Nora Volkow, director of the U.S. National
Institute on Drug Abuse and an expert on the neurobiology of
One of the primary culprits behind obesity, she said, is the
constant availability of "excessively rewarding food" that, when
eaten often, may alter the brain's reward system.
"It's increasingly being recognized that the brain itself plays a fundamental role in obesity and overeating," Volkow said.
The U.S. Centers for Disease Control and Prevention has more on
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