Link found between obesity and brain receptors
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UPTON, New York (CNN) -- Brain scans show that obese people, just like drug addicts, have fewer receptors for dopamine, a neurotransmitter that helps produce feelings of satisfaction and pleasure.
The finding at the Brookhaven National Laboratory has prompted scientists to theorize that one reason people overeat is to stimulate the dopamine "pleasure" circuits in the brain, just as addicts do by taking drugs.
"This is the first scientific contribution that the addictive pathways are deficient in the obese and it may explain their cravings," said Dr. George Blackburn, an associate professor of nutrition at Harvard Medical School.
In the study, scientists measured the number of dopamine receptors in the brains of 10 severely obese individuals and in 10 people with normal weight, by giving each subject an injection containing a radiotracer -- a radioactive chemical "tag" designed to bind to dopamine receptors in the brain. Researchers then scanned the subjects' brains using a positron emission tomography (PET) camera, which picks up radioactive signals. The strength of the signal indicated the number of receptors.
The researchers found that study subjects with the highest Body Mass Index had the fewest number of dopamine receptors. Body Mass Index, or BMI, is a ratio of weight to height.
The study was published in the journal The Lancet.
Doctors who treat overweight patients welcomed the study as yet more evidence that obesity has physiological causes. Other studies have shown that insulin levels, and levels of a hormone called leptin, also play a role in obesity.
"Obesity is not gluttony," Blackburn explained.
Dr. Richard Atkinson, professor of medicine and nutritional sciences at the University of Wisconsin, agreed.
"This study is important because it tells us more about obesity, gives more evidence that it's not just a failure of willpower. Obesity should be treated like a disease just, like any other disease," he said.
But the doctors were not completely convinced that a lack of dopamine receptors causes obesity -- it could, they said, be the other way around. "It's a little hard to know which is which -- it's like the chicken and the egg," added Atkinson.
They said if future studies show the lack of receptors causes obesity -- and not the other way around -- then scientists could try to develop new drugs that target the receptors.
Those drugs already exist, but doctors often avoid prescribing the compounds, called amphetamines, because they're highly addictive.
The study authors suggest that instead of taking dexamphetamines, obese people could exercise to increase their dopamine levels.
"In animal studies ... exercise has been found to increase dopamine levels and to raise the number of dopamine receptors," said study co-author Dr. Nora Volkow, a psychiatrist at Brookhaven.