Dopamine is an amazing chemical. It is the feel good chemical of the brain. When we do something that is good or satisfying it typically involves dopamine. All the dopamine in the world, however, will not give you the desired response if you do not have adequate dopamine receptors in your brain. The number of dopamine brain receptors is under genetic control and that is why different individuals require different amounts of a drug or activity to get the same amount of “reward”.
For years, evidence has shown that obese individuals have fewer dopamine (D2) receptors in the brain relative to lean individuals and the suggestion has always been that obese individuals overeat to compensate for this reward deficit. It turns out to be a little more interesting than that.
In a bid to uncover why obese people tend to overeat, researchers have found evidence of the vicious cycle created when an obese individual overeats to compensate for reduced pleasure from food. (1)
The study confirmed that obese individuals have fewer pleasure receptors and overeat to compensate. Furthermore, the evidence shows that the very act of overeating further weakens the responsiveness of the pleasure (dopamine) receptors, further diminishing the rewards gained from overeating. The result is a runaway feedback system. The more you eat, the less you are gratified by the food, so you eat still more, further diminishing the reward, and so on. It is a vicious cycle.
“Although recent findings suggested that obese individuals may experience less pleasure when eating, and therefore eat more to compensate, this is the first prospective evidence to show that the overeating itself further blunts the award circuitry,” says Stice, a senior scientist at Oregon Research Institute, a non-profit, independent behavioral research center. “The weakened responsivity of the reward circuitry increases the risk for future weight gain in a feed-forward manner. This may explain why obesity typically shows a chronic course and is resistant to treatment.”
Using Functional Magnetic Resonance Imaging (fMRI), Stice’s team measured the extent to which a certain area of the brain (the dorsal striatum) was activated in response to the individual’s consumption of a taste of chocolate milkshake (versus a tasteless solution). Researchers tracked participants’ changes in body mass index over six months.
Results indicated those participants who gained weight showed significantly less activation in response to the milkshake intake at six-month follow-up relative to their baseline scan and relative to women who did not gain weight.
“This is a novel contribution to the literature because, to our knowledge, this is the first prospective fMRI study to investigate change in striatal response to food consumption as a function of weight change,” said Stice. “These results will be important when developing programs to prevent and treat obesity.”
To sum it up: There is genetic variability in the population when it comes to brain dopamine receptors. Particularly if you have a low number of these receptors and you overeat, you will need to overeat further to get the same satiability you once got with less food. The solution to obesity, short of gene therapy, is to not start overeating.
There are many parallels between feeding behavior and drug addiction and the reason is that the same dopamine reward system is typically utilized. Drug addicts don’t become addicts overnight. They start out with small doses but as the reward feedback system demands higher and higher doses, the addiction becomes more and more pronounced. Given the many parallels between food and drug cravings, it would make sense to use lessons from drug addiction to aid in the fight against obesity. But then again, we are not too good at fighting drug addiction, either.
- Stice, E. et al. 2010. Weight gain is associated with reduced striatal response to palatable food. Journal of Neuroscience, 30(39):13105-13109.