Few foods stir brain cells like the chocolate milkshake.

“It’s the cocaine of food,” says Eric Stice, a senior scientist at Oregon Research Institute in Eugene, Ore. “It’s really good at firing up reward circuitry.”

But not so much in overweight people. For them, sipping a chocolate shake just makes them want more because it fails to jolt the brain’s reward centers sufficiently, a new brain imaging study suggests.

The findings by Stice and his colleagues support an emerging idea about the brain’s role in obesity: the notion that many who are overweight gain less emotional satisfaction from eating and so eat more. A clearer understanding of what’s going wrong in the brain could point to better ways to combat obesity and help people stay at a healthy weight.

It’s known from previous research that eating triggers a release of the brain chemical dopamine. And the greater the dopamine release, the greater the feelings of reward. Previous studies also have shown obese people tend to have fewer receptors for receiving dopamine signals.

Here’s where chocolate milkshakes enter the picture. Stice and colleagues at the University of Oregon and two other centers wanted to compare how the brain’s reward center responds to pleasing foods in obese and lean individuals. They needed something delicious that people could eat with a minimum of jaw movement to avoid blurring the brain scans. Volunteers, ranging in age from 14 to 22, were all female because the researchers recruited them from an ongoing study on eating disorders.

The taste of a milkshake stirred activity in the brains of overweight volunteers, but far less than in the brains of lean volunteers. Reward center activity was even more blunted in overweight volunteers who tested positive for a particular gene variation, called TaqIA A1, which seems to reduce brain dopamine receptors.

Researchers tracked weight for one year in some volunteers. Those with low brain activation in response to the milkshake who also had the gene variant were significantly more likely to pack on pounds.

“The more compromised the dopamine signaling, the greater risk of weight gain,” Stice says. The federally funded study appeared Thursday in the journal Science.

While some experts dispute many of the details, they agree that the findings mark a significant shift in obesity research toward recognizing the brain’s higher centers of emotion and motivation.

“Until now, obesity research has been focused on metabolic signals from the gut and the liver,” says Dr. Alain Dagher, an associate professor at the Montreal Neurological Institute. Researchers, he says, tended to regard the regulation of energy and body fat as a function of the lower brain center that controls body temperature and breathing.

“What these researchers have shown is that the brain response in obese individuals is very similar to that in drug addiction,” Dagher says.

But Dagher disagrees with Stice’s conclusion that obese people experience less reward from food.

MRI measurements of lower brain blood flow response don’t necessarily show a blunted reward experience, Dagher says.

Others, such as Dr. Nora Volkow of the National Institutes of Health, have noted that impaired dopamine signaling may act not by undermining feelings of reward, but by promoting compulsive behavior.

Stice and co-authors noted it’s possible that overeating itself causes the reward signaling system to falter, comparable to the way drug abuse de-sensitizes users to the effects of cocaine or heroin. But Stice says the fact that the gene variant can predict weight gain shows “it’s not just a consequence of eating too much.”

He says research shows that the brains of obese people differ in other significant ways. In experiments requiring subjects to complete tasks to earn food rewards, obese people tend to work much harder than lean people, evidence of greater cravings. Brain imaging studies show that obese people experience greater expectation of reward than lean individuals. But Stice is convinced they don’t receive the emotional rewards experienced by people of normal weight.

“The big implication,” he says, “is if you can get people to improve diet quality, you may be able to prevent this reward system from unraveling.”