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Obese may get less fun per bite, so they eat more February 2, 2001 BY LEE BOWMAN Obese people may eat more because their brains are less sensitive to chemicals that signal satisfaction and pleasure, researchers at the Brookhaven National Laboratory in New York have found. Their study, published this week in a British medical journal, the Lancet, demonstrates that obese people have fewer receptors for dopamine, a neurotransmitter that helps produce feelings of satisfaction and pleasure. The brain chemical also has been tied to addiction to cocaine, alcohol and other drugs. Scientists at Brookhaven and elsewhere have found that addictive drugs increase the level of dopamine in the brain and that addicts have fewer dopamine receptors than people who aren't addicted. "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," said Dr. Nora Volkow, a psychiatrist and co-author of the study. To test this theory, scientists at the Department of Energy lab used brain-imaging techniques to measure the number of dopamine receptors in the brains of 10 severely obese individuals and 10 people of normal weight but similar in age and sex to the overweight group. Radioactive tracer chemicals designed to bind to dopamine receptors in the brain were injected into all the subjects, and then their brains were scanned using positron emission tomography to show how well the tracers were being bound to receptors in the brain. Obese patients, the scientists found, had fewer dopamine receptors than normal-weight subjects. And within the obese group, the number of dopamine receptors decreased as the subjects' body mass index increased. Yet the researchers concede they can't tell whether brain changes they found are a consequence or a cause of obesity. "It's possible that obese people have fewer dopamine receptors because their brains are trying to overcompensate for having chronically high dopamine levels, which are triggered by chronic overeating," said Dr. Gene-Jack Wang, lead scientist for the study. "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." While most drugs that are known to alter dopamine levels are themselves addictive, exercise also is thought to be effective at stimulating dopamine pleasure and satisfaction circuits, Volkow said. "In animal studies done elsewhere, exercise has been found to increase dopamine release and raise the number of dopamine receptors." http://www.suntimes.com/output/news/fat02.html *****************