Obese individuals may over-eat because they experience less satisfaction from eating food due to a reduced response in their brains' reward circuitry, according to a new study by Eric Stice, psychology researcher at The University of Texas at Austin.
While eating, the body releases dopamine, a neurotransmitter in the reward centers of the brain, but Stice found obese people show less activation in the striatum relative to lean people. He also found 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.
Stice and a team of researchers have published their findings in the Science article, "Relation Between Obesity and Blunted Striatal Response to Food is Moderated by TaqIA A1 Allele."
Although research has revealed biological factors play a major role in causing obesity, few studies have identified factors that increase people's risk to gain weight in the future.
With support from the National Institutes of Health, Stice led a research team--comprising clinical psychologists from the university and Oregon Research Institute and sensory scientists from the John B. Pierce Laboratory and Yale University--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 over-eat to compensate for this reward deficit," Stice, who has studied eating disorders and obesity for almost two decades, said. "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."
Using Functional Magnetic Resonance Imaging (fMRI), Stice's team 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. The results revealed 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 behavior or pharmacological options could correct this reward deficit to prevent and treat obesity."