Are the obese addicted to food, much as some are addicted to drugs? The results of a new study suggest the answer may be yes. Gene-Jack Wang and Nora Volkow of the Brookhaven National Laboratory and their colleagues have discovered that obese people seem to share a neurochemical deficiency with many cocaine and alcohol abusers. In short, they have fewer brain receptors for dopamine, a neurotransmitter associated with producing feelings of satisfaction. The results will appear in the February 3rd issue of the journal theLancet.

"Since eating, like the use of addictive drugs, is a highly reinforcing behavior, inducing feelings of gratification and pleasure, we suspected that obese people might have abnormalities in brain dopamine activity as well," Volkow says. To find out, the researchers injected 20 volunteers10 obese subjects and 10 control subjectswith a radioactive chemical tag designed to bind to dopamine receptors in the brain. Then they scanned these individuals using positron emission tomography (PET) and counted the numbers of receptors they saw (see image). The obese subjects not only had fewer dopamine receptors than did the normal-weight subjects, but the number of receptors was lower for patients who were heavier.

The scientists now face a chicken-and-egg problem: "It's possible that obese people have fewer dopamine receptors because their brains are trying to compensate for having chronically high dopamine levels, which are triggered by chronic overeating," Wang notes. "However, it's also possible that these people have low numbers of dopamine receptors to begin with, making them more vulnerable to addictive behaviors, including compulsive food intake." Regardless of the answer, treatments aimed at improving dopamine function in obese people are likely to help as much as current strategies. And, the scientists point out, exercise is one of the safest ways to increase dopamine levels in the brain.