A study published Wednesday in the Journal of Neuroscience has found that chocolate activates a part of the brain that blunts pain and makes it difficult to stop eating.
But drinking water has the same effect and does not contribute to the growing problem of obesity, according to the study led by University of Chicago neurology professor Peggy Mason and neurobiology research associate Hayley Foo.
Mason and Foo gave rats either a chocolate chip to eat or water to drink as they lit a lightbulb underneath their cages.
The heat from the bulb normally caused the rodents to lift their paws.
But when the rats ate chocolate or drank water, their pain response to the heat was dulled and they did not lift their paws as quickly as when they were not eating. They also kept on eating.
Mason said that eating stimulates a system in the part of the brain that controls subconscious responses, which is known to blunt pain.
The natural form of pain relief may help animals in the wild avoid distraction while eating scarce food, but in modern-day humans, it could be contributing to over-eating and obesity.
"Nature provided for it being difficult to stop eating by making food scarce, particularly energy-dense, high-fat, high-calorie food. But in the modern world, we've completely messed that up," Mason told AFP.
"The cheapest thing you can get is energy-dense food and once it's readily available and you've got it nearby, you're going to eat it and you're not going to stop.
"You're destined to do that because it's a brainstem-mediated effect," she said.
Previous studies have indicated that only sugary substances had a pain-dulling effect, but the response in the rats in the University of Chicago study was the same regardless of whether they were nibbling chocolate or drinking water.
That led Mason to suggest that doctors change the way they calm patients' nerves.
"Stop giving patients lollipops," she said.
"Ingestion is a painkiller but we don't need the sugar. Water blunts pain, too," Mason said.
Two-thirds of American adults are obese or overweight, while about a third of US children are overweight and one in six are obese.