Dopamine is an important neurotransmitter that is thought to mediate reward responses to foods, and drives a person to consume more foods. When neuroscientists scanned the brains of obese individuals, viewing images of desirable food, they found greater activity in brain regions rich in dopamine receptors. These same regions are also activated by drugs. However, another group of researchers believe that there is decreased reward activity in the brains of obese individuals. In these individuals overeating acts as a compensatory strategy to normalise the reward deficit. In other words, they eat more to feel normal.
A recent study supports both these theories showing that they are not mutually exclusive, and used brain imaging and genetics data to prove it. Using fMRI the researchers scanned the brains of adolescent girls with differing BMIs while they viewed pictures of appetizing foods and imagined eating junk food. They also genotyped these girls to determine whether they possessed variants of two genes that have been linked with lower dopamine activity. They followed the girls for one year and then checked their BMI. Overall, they found that adolescent girls with greater BMI showed increased reward activation in response to food images. They found that girls with greater reward activation gained more weight. However, the genetic data also showed that girls whose brains had a lower reward response to foods also gained more weight. It would seem that there may be two types of obesity – those who are overly sensitive to food and those who are under sensitive. They propose the Goldilocks priniciple – too much or too little reward system activation may lead to weight gain.
I think I would like to add my opinion on this. Research has also shown that if you are a healthy weight, and are more sensitive to rewards, the heavier you tend to be. I believe that initially, whether your rat or human, the more sensitive you are to food rewards the more likely you are to consume these foods again and again. So at the start you get a huge hit from eating this junk food and want more! This is why oversensitivity to food rewards is found in heavier normal weight people as it is at the initial stages of the addiction process. As you continue to become more addicted, and eat more junk food, your dopamine system begins to shut down reducing the number of dopamine receptors and thus decreasing your sensitivity to a food reward. Thus, you need to eat more and more junk food to compensate (this mechanism has been shown in rats). By this stage you have entered into stage II of the addiction process, and are likely to be obese. This would explain the above pattern. Unfortunately, if this is true, and this pattern is not broken, an addicted individual is destined to become more obese and to enjoy food less and less as time goes by.