Not surprisingly, the rats that gorged themselves on the human food quickly became obese. But their brains also changed. By monitoring implanted brain electrodes, the researchers found that the rats in the third group gradually developed a tolerance to the pleasure the food gave them and had to eat more to experience a high.

The rats in the study kept eating the junk food even in the face of pain:

They began to eat compulsively, to the point where they continued to do so in the face of pain. When the researchers applied an electric shock to the rats' feet in the presence of the food, the rats in the first two groups were frightened away from eating. But the obese rats were not. "Their attention was solely focused on consuming food," says Kenny.

In previous studies, rats have exhibited similar brain changes when given unlimited access to cocaine or heroin. And rats have similarly ignored punishment to continue consuming cocaine, the researchers note.

I wonder what would happen if, after the rats ate some fattening food, they put some cocaine out next to the food. Would the rats take a little coke, which should make them get un-hungry, and stop eating?

Or, if they gave the rats some coke first, would they'd get un-hungry enough to only eat a little of the junk food?

The study says it's all related to dopamine levels in the brain.

The neurotransmitter dopamine appears to be responsible for the behavior of the overeating rats, according to the study. Dopamine is involved in the brain's pleasure (or reward) centers, and it also plays a role in reinforcing behavior. "It tells the brain something has happened and you should learn from what just happened," says Kenny.

Overeating caused the levels of a certain dopamine receptor in the brains of the obese rats to drop, the study found. In humans, low levels of the same receptors have been associated with drug addiction and obesity, and may be genetic, Kenny says.

Maybe they need to find a time-released "happy pill" that keeps everyone's levels of dopamine at an optimal level.

If you're looking for some healthy foods that raise dopamine production, try bananas, dairy products, pumpkin seeds, almonds and avocados. They raise levels of an amino acid named tyrosine which is a prescursor to dopamine,