Junk Food Triggers Addiction

Recently, scientists have shown that the same molecular mechanims that drive people into drug addiction are behind the compulsion to overeat, pushing people into obesity. They found that in rats the development of obesity coincides with a progressively deteriorating chemical balance in reward brain circuits. As these pleasure centers in the brain become less and less responsive, rats develop compulsive overeating habits consuming larger quantities of high-calorie food eventually leading to obesity. Scarily, the exact same changes occur in the brains of rats that overconsume heroin or cocaine, and are thought to play an important role in the development of compulsive drug use.

In later studies the researchers showed how eating behaviour changes in ‘addicted’ rats when they have access to high-calorie food. Animals completely lost control over their eating behaviour, the primary hallmark of addiction. They continued to overeat even when they knew they would receive an electric shock. This shows how motivated (addicted) they were to consume palatable food. Control rats that were fed a normal diet decided to not eat rather than receive an electric shock.

Addicted rats were fed a diet that was modeled after the type that was thought to contribute to obesity – easy to obtain high calorie and processed foods like sausage, bacon and cheesecake. Rats always went for the worst type of food consuming double the number of calories as control rats. Rats began to bulk up dramatically soon after the experiments begun. When junk food was removed and they were placed on a nutritious diet (the salad bar option) the rats refused to eat. Not only that this hunger strike lasted for two weeks!!!!

The rats that showed the biggest ‘crash’ in brain reward circuits were the same that showed the biggest food preference for junk food. It was these same rats that kept on eating even when they anticipated being shocked. The authors believe that the reward pathways in the brain were so overstimulated that the system basically turned it itself off, adapting to the new reality of addiction, and it doesn’t matter whether its cocaine or cupcakes. When the animal overstimulates its brain pleasure centers 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 (could be defined as punishment in my book). In other words not eating junk foods makes the rat feel depressed.

After showing that obese rats had clear addiction-like food seeking behaviors, researchers investigated the underlying molecular mechanisms that may explain these changes. They focused on a particular receptor in the brain known to play an important role in vulnerability to drug addiction and obesity – the dopamine D2 receptor. The D2 receptor responds to dopamine, a neurotransmitter that is released in the brain by pleasurable experiences like food or sex or drugs like cocaine. In cocaine abuse, for example, the drug alters the flow of dopamine by blocking its retrieval, flooding the brain and overstimulating the receptors, something that eventually leads to physical changes in the way the brain responds to the drug. The new study shows that the same thing happens in junk food addiction.

In addition, levels of the D2 dopamine receptors were significantly reduced in the brains of the obese animals, similar to previous reports of what happens in human drug addicts. Remarkably, when the scientists knocked down the receptor using a specialized virus, the development of addiction-like eating was dramatically accelerated. The day after the dopamine receptors were knocked out in normal rats they began to show the same compulsive eating behaviours as those rats that had been overeating for several weeks.