Junk food as addictive as cocaine, study finds

New research has found that Junk food is as addictive as smoking and hard drugs.

Rats hooked on sausage and cheesecake confirmed the link between compulsive eating and cocaine addiction.

Binge-eaters and drug addicts are both at the mercy of the same molecular effects in the brain, the research suggests.

The findings may explain why some people find it so hard to stop gorging on junk food.

Scientists found that obesity coincides with a progressive chemical imbalance in the brain’s “reward” circuits.

These produce feelings of pleasure from activities such as eating and sex, and are known to play a role in addiction.

As the reward centres become less responsive, rats given easy access to fatty food quickly develop compulsive over-eating habits, the US research showed.

The animals consumed ever larger amounts of calories until they become obese.

Exactly the same changes in the brain were seen to occur when the rats were given cocaine or heroin.

Dr Paul Kenny, from the Scripps Research Institute in Jupiter, Florida, said: “In the study, the animals completely lost control over their eating behaviour, the primary hallmark of addiction. They continued to overeat even when they anticipated receiving electric shocks, highlighting just how motivated they were to consume the palatable food.”

The rats were fed a diet based on those contributing to obesity in humans which included high-fat foods such as sausage, bacon and cheesecake. Their progress was compared with that of a group of “control” rats given a normal diet.

Soon after the experiment began, the rats on the junk food diet started to bulk-up dramatically.

“They always went for the worst types of food, and as a result, they took in twice the calories as the control rats,” said Dr Kenny.

“When we removed the junk food and tried to put them on a nutritious diet - what we called the ’salad bar option’ – they 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.

“It was the animals that showed the ’crash’ in brain reward circuitries that had the most profound shift in food preference to the palatable, unhealthy diet. These same rats were also those that kept on eating even when they anticipated being shocked.”

The research, reported today in the journal Nature Neuroscience, showed how the brain over-reacted to excess stimulation by tasty fatty food.

“The body adapts remarkably well to change, and that’s the problem,” said Dr 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 scientists focused on brain chemistry involving dopamine, a neurotransmitter that helps messages pass between nerve cells.

Dopamine is released during pleasurable experiences, but taking cocaine causes it to flood the brain. The brain adapts by becoming less responsive to the drug, leading to addiction.

A similar process causes compulsive eaters to become fixated on junk food, said Dr Kenny.

He added: “These findings confirm what we and many others have suspected, that over-consumption of highly pleasurable food triggers addiction-like neuro-adaptive responses in brain reward circuitries, driving the development of compulsive eating.

“Common mechanisms may therefore underlie obesity and drug addiction.”

Levels of D2 receptors – cell molecules that trigger dopamine’s biological effects – were significantly reduced in the brains of obese rats, the scientists found. Loss of D2 receptors has also been documented in human drug addicts.

When the researchers erased the receptor in rats using a specialised virus, the development of compulsive eating accelerated dramatically.

“This addiction-like behaviour happened almost from the moment we knocked down the dopamine receptors,” said Dr Kenny.

“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 over-eating for several weeks.”