Dopamine is a neurotransmitter that helps produce a reward response in the brain. This response kicks into action when we do something pleasurable- like eating highly palatable food. It is known that there is a reduction in this reward response in obese people. However, it is unclear whether the reduction in reward precedes obesity and causes people to overeat to compensate or if it is a consequence of overeating.
Recent research published in the journal Nature Neuroscience sheds some light on this question. Drs. Paul Johnson and Paul Kenny from The Scripps Research Institute in Florida used rats to investigate the effect of overeating on the dopamine reward system. They allowed one group of rats almost unlimited access to a cafeteria diet made up of cheesecake, pound cake, frosting, chocolate, bacon and sausage.
The control group was given access only to rat chow food. All rats had similar pre-experiment baseline dopamine reward responses to food.
During the experiment the caloric intake for the cafeteria diet group was almost double that of the control group. After 40 days the cafeteria diet rats had increased their body weight by about 50% and were obese. Importantly, the dopamine reward response for this group progressively decreased during the experiment. This research demonstrates that easy access to highly palatable food reduces the reward response leading to compulsive eating behavior and obesity. This is similar to the changes in dopamine reward responses associated with cocaine and heroin addiction.