A new brain imaging study has revealed that just the mere sight or smell of food elevates dopamine - a brain chemical linked to reward and motivation - levels in binge eaters.
The findings suggested that the chemical spike could explain compulsive overeating.
The researchers at the US Department of Energy's Brookhaven National Laboratory looked at 18 obese persons - 10 of whom have been diagnosed with binge eating disorder.
They used positron emission tomography (PET) to scan the subjects' brains after injecting a radiotracer designed to bind to dopamine receptors in the brain.
Each subject was scanned four times on two different days, both times after fasting for 16 hours, to test the effects of food stimulation vs. neutral stimulation with and without pre-administration of a drug known to amplify dopamine signals.
The drug, methylphenidate, blocks the reuptake of dopamine from brain synapses, allowing it to linger longer. In scans without methylphenidate, subjects were given a placebo drug.
Then, researchers gave the patients neutral stimulation- clothes and toys brought close enough to smell - while they were scanned.
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.
Subjects with the most severe binge eating disorder, as assessed by psychological evaluations, had the highest dopamine levels in the caudate.
Dopamine levels did not rise significantly in other brain regions or under any other condition (neutral stimulation with or without methylphenidate, or food stimulation without methylphenidate) in either group, and were not correlated with body mass index of the research subjects.
"These results identify dopamine neurotransmission, which primes the brain to seek reward, as being of relevance to the neurobiology of binge eating disorder," said study lead author Gene-Jack Wang.
"The key difference we found between binge eaters and non-binge eating obese subjects was a fairly subtle elevation of dopamine levels in the caudate in the binge eaters in response to food stimulation," he said.
The caudate is believed to be involved in reinforcement of action potentially leading to reward, but not in processing of the reward per se.
"That means this dopamine response effectively primes the brain to seek the reward, which is also observed in drug-addicted subjects," said Wang.
The findings are published online on February 24 in the journal Obesity.