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Rats made obese on a diet of chocolate, cheesecake, pound cake and frosting couldn't stop eating the rich food, even when they knew they'd get a shock. ((CBC))

Compulsive eating of rich food triggers some of the same addiction-like responses in the brain as heroin, a study on rats suggests.

When U.S. researchers offered rats high-calorie foods such as bacon, sausage, cake and chocolate on top of their healthier but less appetizing chow, the animals overconsumed calories, quickly gained weight and continued to overeat — even when they knew they'd receive an unpleasant electric shock.

On the other hand, rats fed a healthy diet with little access to junk food would stop eating the snacks when they knew a shock was coming.

The study, published in the May issue of the journal Nature Neuroscience, found similar changes in the brain's reward circuits occur in both obese and drug-addicted rats.

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"These data are, as far as we know, the strongest support for the idea that overeating of palatable food can become habitual in the same manner and through the same mechanisms as consumption of drugs of abuse," said study co-author Paul Kenny, a neurobiologist at the Scripps Research Institute in Jupiter, Fla.

Eating food is associated with the release of dopamine, and the more that is released, the greater the degree of pleasure. It is thought that people with fewer dopamine receptors may need to take in more of a rewarding substance — such as food or drugs — to experience the same level of pleasure as other people.

The Scripps researchers found levels of Type 2 dopamine receptors in the brain's reward circuit decreased as the rats became obese. As a result, the reward pathways no longer worked as well, the researchers said. 

Beyond control

The over-eating animals showed many of the hallmarks of excessive drug use, such as compulsive behaviour, Kenny said.

"In other words, it almost becomes beyond your control, and you'll keep on taking the drug even when really you shouldn't," Kenny said in an interview with the Australian Broadcasting Corporation.

In the diet experiment, levels of type 2 dopamine receptors plummeted for at least two weeks, even after the high-calorie treats were no longer available.

The findings have parallels for human research into overeating, said one of the funding organizations of the study.

"This research opens the door for us to apply some of the knowledge we have gathered about drug addiction to the study of overeating and obesity," Dr. Nora Volkow, director of the National Institute on Drug Abuse, said in a release.

Addicts, whether rat or human, will compulsively consume a substance even when it is detrimental to them.

The Scripps researchers trained rats to expect painful foot shocks when they saw a light signal. Lean rats stopped eating the junk food when the light came on. Obese rats accustomed to a high-calorie diet kept overeating, despite the shocks, which shows how motivated they were to consume the tasty food, Kenny said.

When the junk food was removed and the researchers tried to offer just the healthier "salad bar option," the rats refused to eat, and the animals nearly starved themselves for two weeks.

Repeated over-consumption

The researchers also used a specialized virus to knock down or artificially lower levels of the dopamine receptors. The day after junk food was offered, the animals' brains changed to a state resembling those that had been overeating for several weeks.

The findings also suggest that unlimited access to unhealthy foods can trigger repeated over-consumption and obesity, said study co-author Paul Johnson, a graduate student in the department of molecular therapeutics at the Scripps Research Institute.

Prof. Boyd Swinburn, an Australian obesity expert from Deakin University in Melbourne agreed, pointing to changes in the food industry. 

"The products have become much more processed and manufactured and therefore energy-dense, and they worked out what things to add like sugar, salt and fat and a whole bunch of other chemicals to make it tasty," Swinburn said.

Over a decade, prices of these foods have fallen in relative terms, the food is "absolutely everywhere that you turn," and the promotion and marketing has become more sophisticated than it was 30 years ago, Swinburn added.

Swinburn cited Japan, South Korea and some Scandinavian countries as countries where obesity has not increased with individual wealth, which he attributed to maintaining traditional cuisine and resisting the lure of junk food. European culture also embraces more active transport such as riding to work and school, he said.

The study was funded by the National Institutes of Health's National Institute on Drug Abuse, a Bank of America Fellowship, and the Margaret Q. Landenberger Research Foundation.

With files from The Australian Broadcasting Corporation