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Scientists say they've discovered one reason why overweight people eat more than they need to and find it so hard to stop. The report comes out tomorrow in the Journal Science. And their research involves high tech brain scans, fancy genetic tests, and some chocolate milkshakes. And we've got some milkshakes, too, in this story from NPR's Richard Knox.

RICHARD KNOX: You could say we're all addicted to food. But we all know some foods are more addictive than others, like the milkshakes that a worker's whipping up at Gifford's Ice Cream in downtown Washington.

Ms. WALRITA JORDAN: Make sure you get the ice cream all blended up.

KNOX: Today, one of the customers, Walrita Jordan (ph) is rewarding herself with a sundae. Chocolate milkshakes also make her happy.

Ms. JORDAN: Mm, warm and fuzzy and, you know, I just feel good, and I'm like, whoo, that was great.

Dr. ERIC STICE (Senior Scientist, Oregon Research Institute, University of Oregon): There's very few things that excite the brain as much as chocolate.

KNOX: Eric Stice of the Oregon Research Institute says chocolate is just the thing to investigate the brain's reward system, the circuits that light up when people eat or drink something tasty. And chocolate milkshakes are ideal. The yummy flavor can be squirted through a straw while volunteer's heads are clamped into an MRI scanner.

Stice and his colleagues looked at how the brains of 77 young women responded to the milkshakes. Some women were lean, some obese, and some had a gene that makes them less sensitive to dopamine, a chemical central to the pleasure response, ao they don't get as much pleasure out of food as others.

Dr. STICE: When obese individuals eat food, they seem to have a blunted response to this food, and if you're a genetic risk, this effect is amplified.

KNOX: We're used to thinking that overweight people love to eat and get more pleasure out of rich treats like ice cream. The Oregon researchers say, this notion is only half true.

Dr. STICE: If you look at the brain response to when people are about to get the milkshake, obese individuals show greater activation of reward circuitry, not less. So, ironically, they expect more reward but seem to experience less.

KNOX: Kent Berridge of the University of Michigan says the new research helps explain the biological difference between wanting and liking. By wanting, he means anticipating something delicious. Liking means the pleasure that comes from actually tasting it.

Dr. KENT BERRIDGE (Department of Psychology, University of Michigan): It's an exciting result. The wanting and the liking start to come apart. The wanting is up and the liking might be blunted. And it's very intriguing that the blunting might happen in some individuals with particular gene.

KNOX: As with other addictions, Stice thinks people overeat to compensate for the mismatch between the strong cravings they have and the reduced enjoyment they get. The best way to avoid this vicious cycle is to develop healthy eating habits before pleasure responses get blunted by overeating.

Dr. STICE: Dont get your body used to eating high fat, high sugar foods because it will be really hard to stop.

KNOX: It's best to do this when you are young, Stice says. If you are already obese, then the research may have other implications.

Dr. STICE: So if you try not to eat chocolate, eating just a little bit of chocolate every day is not going to be the way to go. It's better to stop eating chocolate and say that's the health improvement I am going to make to my diet. And after a month, maybe six weeks, your craving for chocolate will finally go down and stay down.

KNOX: Walrita Jordan ponders the new research over her sundae.

Ms. JORDAN: I think that, a lot of times, what you see and what you think may fill you up. You know, it's not necessarily what your body really needs. I think, medically, some people just can't get past this. I'm a mind over matter type of girl.

KNOX: If the Oregon researchers are right, that's truer than she realizes. Overeating has a lot to do with what's going on in the mind. Richard Knox, NPR News.

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