Dopamine is a neurotransmitter that motivates us to engage in rewarding activities such as eating and sex. Animals without dopamine stop eating and starve to death.
Certain drugs are able to activate dopamine receptors, and, as revealed by brain scans, meth heads, cocaine addicts, and alcoholics have lower levels of dopamine in the reward areas of their brains. Such people are thus chronically starved for satisfaction and need drugs.
Now here's the interesting part.
The same is true of fat people. Like drug addicts, they have less dopamine in the reward areas of their brains. How come?
Scientists fed rats diets of calorie intense food (cookies, cheese, chocolate chips, marshmallows, etc.). After a few months, the rats became obese. They then examined their brains—sure enough, the obese rats had fewer dopamine receptors than rats stuffed with the same number of calories of normal chow.
So, it's the calorie intense food that does it, not how much one eats. Not only that, but the changes in the calorie-intense-food-fed rats were long lasting.
Junk food actually changes the brain to make one chronically crave junk food.
- Paul J. Kenny, "Reward Mechanisms in Obesity: New Insights and Future Directions", NIH Public Access