Binge eaters' brains surge with dopamine at the sight and smell of food, a new study finds, priming the brain to see reward similar to the way drug addicts' brains do. Researchers at the Energy Department's Brookhaven National Laboratory say the chemical spike could explain compulsive overeating.

The scientists looked at 18 obese persons--ten of whom have been diagnosed with binge eating disorder. Their brains were scanned four times on two different days, both times after fasting for 16 hours. On one day, patients took the drug methylphenidate, which allows dopamine to swirl around in the brain longer because it prevents the brain from "reuptaking" it. Next, while the subjects were being scanned, their favorite foods were waved in front of their faces and tiny tastes were swabbed on their tongues. Then, researchers gave the patients  neutral stimulation--clothes and toys brought close enough to smell--while they were scanned. The process was repeated without the drug on a different day.

When they smelled food after taking methylphenidate, the binge eaters showed big increases in dopamine levels in the caudate and putamen parts of the brain. The caudate is a key part of the brain's learning system--scientists think it plays a role in reinforcing behavior that could lead to a reward. "That means this response effectively primes the brain to seek the reward, which is also observed in drug-addicted subjects," the study's lead author, Gene-Jack Wang, said.

The worse the subjects' binge eating disorder, the bigger their dopamine spike. There were no dopamine spikes among the non-binge eating group, nor when the binge eaters saw neutral stimuli or didn't take methylphenidate.