There’s something poignant about the possibility that one of the reasons obese people eat too much is because they are unable to take pleasure in the taste of their food. But according to a new study published in Science, that’s exactly what happens:
The dorsal striatum plays a role in consummatory food reward, and striatal dopamine receptors are reduced in obese individuals, relative to lean individuals, which suggests that the striatum and dopaminergic signaling in the striatum may contribute to the development of obesity. Thus, we tested whether striatal activation in response to food intake is related to current and future increases in body mass and whether these relations are moderated by the presence of the A1 allele of the TaqIA restriction fragment length polymorphism, which is associated with dopamine D2 receptor (DRD2) gene binding in the striatum and compromised striatal dopamine signaling. Cross-sectional and prospective data from two functional magnetic resonance imaging studies support these hypotheses, which implies that individuals may overeat to compensate for a hypofunctioning dorsal striatum, particularly those with genetic polymorphisms thought to attenuate dopamine signaling in this region.
The experiment was elegantly designed. Subjects were given sips of a chocolate milkshake inside an fMRI machine. The activation of the striatum – an area rich in dopamine neurons and involved in the processing of hedonic rewards – was then monitored. Sure enough, obese people tended to have reduced activation in the striatum: they kept on sipping the milkshake in a manic search for satisfaction. A second study found that, over time, people with a polymorphism that leads to reduced dopamine receptors in the striatum also tended to put on weight, suggesting that obesity is, at least in part, triggered by a shortage of neural pleasure. Of course, this contradicts the popular (and deeply unfair) cultural stereotype of obesity, which assumes that people who are overweight are gluttons, unable to resist temptation. In fact, they are the opposite of gluttons: the reason they eat too much is because they don’t enjoy their food enough. They keep on sipping the milkshake precisely because it isn’t pleasurable. The finding reminds of that old Jewish joke in the beginning of Annie Hall:
There’s an old joke – um… two elderly women are at a Catskill mountain resort, and one of ’em says, “Boy, the food at this place is really terrible.” The other one says, “Yeah, I know; and such small portions.”
For another really cool finding on obesity and dopamine, check out this blog post.