Dopamine is a neurotransmitter (chemical that signals between neurons) that is a central mediator of reward and motivation in the brain. It has been known for decades that dopamine injections into the brain suppress food intake, and that this is due primarily to its action in the hypothalamus, which is the main region that regulates body fatness (1). Dopamine-producing neurons from reward centers contact neurons in the hypothalamus that regulate body fatness (2). I recently came across a paper by a researcher named Dr. Hanno Pijl, from Leiden University in the Netherlands (3). The paper is a nice overview of the evidence linking dopamine signaling with body fatness via its effects on the hypothalamus, and I recommend it to any scientists out there who want to read more about the concept.
Increased dopamine signaling, particularly through the dopamine D2 receptor, can attenuate and in some cases reverse obesity in diet-induced obese animals, seasonally obese animals (squirrels, Syrian hamsters, etc.), and overweight/obese humans (4). Not only that, it can increase resting metabolic rate and attenuate the metabolic abnormalities associated with obesity even before the fat is lost, which emphasizes that these circuits control metabolism directly, as well as indirectly by influencing fat mass (5)*. Conversely, people who have genetic or drug-induced reductions in D2 receptor signaling gain fat at an accelerated rate (6, 7).
Together, this supports a hypothesis that I've scarcely seen in the scientific literature: that reward centers, and probably food reward itself, can directly influence body fatness and metabolism. Other people have made parts of this argument, however, I've never seen anyone put together the evidence from psychology, pharmacology and neurobiology into a single coherent hypothesis.
Addiction and Obesity
Speaking of dopamine D2 receptors, people who produce fewer of them due to their genetic makeup not only tend to gain more weight, they're also more susceptible to addictive behaviors such as drug abuse and excessive gambling (8, 9, 10). In other words, they respond differently to rewarding stimuli than others and are more susceptible to developing pathological relationships with high-reward things.
When you look at D2 receptors in the human brain, the more fat a person carries, the fewer D2 receptors are available for binding dopmaine** (11). This is the same thing one sees in the brains of people who are addicted to drugs (12). In the case of drugs, reduced dopamine signaling may lead to increased drug-seeking behavior to compensate for a "reward deficit". This is based on the hypothesis that highly rewarding drugs or foods eventually cause the brain to become desensitized to food reward, creating a vicious cycle in which people seek out more and more food reward to reach the same level of reward signaling in the brain. In his nice review article "Food reward, hyperphagia and obesity", Dr. Hans-Rudolph Berthoud and colleagues refer to the chronic intake of highly rewarding food as an "escalating, addictive process leading to obesity" (13). Rodents that are accustomed to eating human junk food will endure foot shocks and extreme temperatures to obtain it, even when regular chow is freely available in unlimited quantity (14).
Commenter "mem" recently pointed me to the research of Dr. Robert Pretlow. He has a nice slideshow online that describes his research (15). He set up a support website for overweight youths, and uses it to conduct his research. He has some interesting observations:
- Many youths describe themselves as addicted to junk food. They know it's unhealthy but they have little control over their cravings.
- Many say they use junk food as a way to cope with stress-- unstable home life, school pressures, lack of sleep, etc.
- Some find that their cravings subside and weight decreases if they can avoid junk food for enough time.
- Ice cream
- Potato chips
- Fast food
Putting it All Together
I believe the evidence as a whole shows that chronic consumption of foods with an excessive reward value causes abnormalities in parts of the brain that regulate body fatness, metabolism and reward/motivation. This can lead to weight gain and metabolic problems, and favor addictive and compulsive relationships to food and other things. The combination of readily accessible, cheap, high-reward food, and stressful lifestyles that drive us to eat it, is probably a major contributor to overweight, obesity, diabetes and perhaps other health problems in affluent nations.
* Thanks to commenter "Gunther Gatherer" for pointing this paper out.
** Specifically in the striatum.
*** Thanks to reader JBG for sending me this.