When it comes to potato chips, cookies or chocolates, it's hard to stop at just one. Now, new research may explain why: junk food can actually become addictive -- even as addictive as heroin, the new research suggests.
Researchers in the U.S. have found that rats that had access to an unhealthy diet that included everything from bacon to chocolate to fat-laden desserts got so "hooked" on the foods that they refused to eat anything else, and needed more and more just to feel satisfied.
While it's too soon to say whether the findings in the rats might also apply to humans , the researchers say the study certainly helps shed more light on the emerging area of food addiction and compulsive bingeing.
The study, conducted by Scripps Research Institute neuropsychopharmacologist Paul J. Kenny and graduate student Paul M. Johnson, is published in the journal Nature Neuroscience. It looked at rats given access to two kinds of diets:
- One group of rats ate a healthy, balanced diet
- The other group received regular healthy meals, but had also access to high-calorie foods and snacks
The rats in the second group quickly developed a preference for the high-calorie food. When they were given unlimited access to the foods, they began eating it all day long. Within 40 days, the body weight of the rats in that group had risen 25 per cent.
"They always went for the worst types of food, and as a result, they took in twice the calories as the control rats," Kenny said in a news release announcing the findings.
The rats not only became obese, they also showed addiction-like changes in their brains -- the same changes that have been reported in humans addicted to drugs.
Specifically, the obese rats showed altered levels of a receptor in the brain called the D2 dopamine receptor. The D2 receptor responds to dopamine, the brain chemical that is released in the brain by pleasurable experiences, such as sex or drugs like cocaine.
The more junk food the rats ate, the more they overloaded the brain's reward centre. Kenny says the same thing happens in those who become addicted to drugs. The reward pathways in the brain become so over-stimulated that the system essentially turns on itself.
"The body adapts remarkably well to change -- and that's the problem," said Kenny. "When the animal over-stimulates its brain pleasure centres with highly palatable food, the systems adapt by decreasing their activity. However, now the animal requires constant stimulation from palatable food to avoid entering a persistent state of negative reward."
The researchers found that the levels of the D2 dopamine receptors were significantly reduced in the brains of the obese animals, similar to what happens in human drug addicts, Kenny said. So as the pleasure centres in the brains became less responsive, the rats ate even more compulsively, in an effort to get "their fix."
The addictions became so intense that the rats still sought out the unhealthy foods even if it meant risking pain.
Before the experiment began, the rats had been trained to expect a minor shock to their feet shortly after a cue light began to flash. But when the rats that had unlimited access to high-calorie food were shown the cue light, they braved the shock so that they could keep on eating.
Kenny's team then decided to see what would happen when they cut off all access to the junk-food-addicted rats' "drug." They removed the junk food and replaced it with an exclusively nutritious diet.
The result? The rats simply refused to eat.
"The change in their diet preference was so great that they basically starved themselves for two weeks after they were cut off from junk food," Kenny said.
The rats that had begun to eat most compulsively and that showed the greatest changes in their brains were the most likely to refuse any food.
"These same rats were also those that kept on eating even when they anticipated being shocked," Kenny said.
To take the experiment one step further, the researchers artificially knocked out the D2 dopamine receptor in some of the rats, using a special virus. They found that the treated with the virus developed compulsive eating much faster – but only among those eating the tasty, unhealthy food. The rats eating the healthy diet continued to eat as normal. But when they were switched over to junk food, they also very rapidly showed addiction-like changes.
"The very next day after we provided access to the palatable food, their brains changed into a state that was consistent with an animal that had been overeating for several weeks," Kenny said.
It's unclear is the results among rats apply to people, who, of course, are able to use reason in their food choices. But the researchers say this is the first study to support the idea that overeating of tasty but unhealthy food can become habitual, and that the same brain changes seen in drug addicts can also occur to junk food addiction.
