A marked reduction in chemical receptors in the brains of obese people may trigger overeating in an effort to stimulate the brain's "reward and motivation" circuits, a new study shows.
A similar finding has been identified in drug addicts.
Dr. Gene-Jack Wang and his colleagues at Brookhaven National Laboratory on Long Island discovered that obese people have about 20 percent fewer receptors for the chemical dopamine in the area of the brain that regulates reward.
The idea to study eating behavior grew from findings that medicines that increase dopamine stimulants such as Ritalin suppress appetite. By contrast, medicines that block dopamine lead to a weight gain. Animal studies also have shown that many of the reinforcing aspects of food even the expectation that food will be coming triggers an increase in dopamine. This suggests that the brain chemical is intimately tied to circuits in the brain that govern pleasure and reward.
The Brookhaven study is scheduled to appear today in the medical journal Lancet.
Last year, Brookhaven's Dr. Nora Volkow published a paper suggesting that cocaine addicts have reduced levels of a specific type of dopamine receptor called D2. Addicts use drugs to activate these reward circuits. Many behaviors associated with a reinforcing response from the brain drugs, sex, gambling and now food can increase dopamine, Volkow said.
"It's one of the most powerful strategies nature has created to make people do something," she added. "Activation of reward circuits creates a sense of well-being."
She thinks that a reduced number of dopamine receptors can put people at risk for drug use and overeating, as this latest study suggests.
The scientists recruited 10 extremely obese but otherwise healthy people. The average age was 35; the average weight, 300 pounds. Each subject received a PET scan, which can measure specific brain chemicals. The scientists were interested in the dopamine D2 receptors. The receptors were significantly decreased in the obese people compared with a group of 10 volunteers of normal weight. The higher the body-mass index, or BMI, the lower the number of dopamine receptors.
If the link between obesity and dopamine receptors can be replicated in other studies, the finding might help guide treatments for obesity.