Obese people may get less satisfaction from food due to a blunted response in brain circuitry relating to pleasure, and may overeat to make up for the decreased pleasure, particularly if they carry a specific gene variant, U.S. researchers said on Thursday.
When a person is eating, the body releases dopamine, a chemical in the brain's reward centers. The amount of pleasure from food depends on the amount of dopamine released.
Brain scans show that obese people had less activity in the dorsal striatum, the part of the brain that releases dopamine in response to eating, when they drank a chocolate milkshake, compared to leaner people, the researchers said.
This diminished response to food was even more pronounced in those who had a variant of a gene called TaqlA1, which is related to dopamine signaling in the dorsal striatum.
Those who showed the most diminished brain response to the milkshakes and also had this genetic characteristic were at high risk for unhealthy weight gain in the ensuing year, the researchers wrote in the journal Science.
"The evidence of blunted response leading to future weight gain clearly seems to suggest that people are over-eating in response to this diminished reward that they experience when they eat," Eric Stice of the Oregon Research Institute, who led the study, said in a telephone interview.
"It's much the same way that people who smoke regulate cigarettes. If you give them the low-tar cigarettes, they make up for the lost tar by smoking more efficiently, and get more of it," Stice said.
The findings help explain the biology of obesity at a time when obesity rates are rising in many parts of the world.
The study involved 43 female college students ages 18-22 and 33 adolescent girls ages 14-18. The researchers tracked their weight for a year after performing the brain scans.
The researchers performed brain scans to view blood flow to the dorsal striatum, showing brain activity, as the girls and women drank a chocolate milkshake or a flavorless liquid.
Typically the brains of those drinking the milkshake lit up when drinking the milkshakes, but not so much among those with a higher body mass index, a measure of obesity.