While having food, the body releases dopamine, a neurotransmitter in the reward centers of the brain, but Eric Stice, psychology researcher at the university, found that obese people show less activation in the striatum relative to lean people.
He also found that individuals with a blunted response were more likely to show unhealthy weight gain, particularly if they had a gene associated with compromised dopamine signaling in the brain's reward circuitry.
Previous studies have revealed that biological factors play a major role in causing obesity, but few studies have identified factors that increase people's risk to gain weight in the future.
Stice and his colleagues set out to explore how blunted responses in the brain relate to weight gain in young females.
"The research reveals obese people may have fewer dopamine receptors, so they overeat to compensate for this reward deficit. People with fewer D2 receptors need to take in more of a rewarding substance-such as food or drugs-to experience the same level of pleasure as other people," Stice said.
With the help of Functional Magnetic Resonance Imaging (fMRI), the researchers measured how the dorsal striatum was activated in response to the taste of a chocolate milkshake (versus a tasteless solution).
The researchers also tested participants for the presence of a genetic variation linked to a lower number of dopamine D2 receptors, the Taq1A1 allele.
For one year, the researchers tracked participants' changes in body mass index and found that participants with decreased striatal activation in response to the milkshake who also had the A1 allele were more likely to gain weight over time.
"Understanding the abnormalities in activation of reward circuitry in response to eating is critical to helping people regulate their weight because dopamine serves as the primary neurotransmitter in the reward pathways of the brain," Stice said.
"Although people with decreased sensitivity of reward circuitry are at increased risk for unhealthy weight gain, identifying changes in behaviour or pharmacological options could correct this reward deficit to prevent and treat obesity," he added.
The study appears in the Oct. 17 issue of the journal Science.