When you see a person weighing more than usual, the first thought that crosses your mind is that he/she must enjoy his/her food, when actually it’s not that way. More exactly, they seem to have a diminished ability to experience the pleasure of eating, prompting them to overindulge to boost their satisfaction, a study published in the Oct. 17 issue of the journal Science showed.

The study is the latest in a series focusing on the brain’s response to food using the neurotransmitter dopamine. The human brain releases the "pleasure chemical" dopamine, a reward to the body for consuming life-sustaining nutrition. But the study showed that obese people have fewer “reward centers” in the brain which prompts them to eat more.

“The research reveals obese people may have fewer dopamine receptors, so they overeat to compensate for this reward deficit,” said Eric Stice, a psychology researcher at the University of Texas (UT) at Austin and lead author of the study. The more a person overeats, the less potent the rewards from eating become and that creates a pattern of overeating.

The study was done by researchers at the Oregon Research Institute in Portland in collaboration with colleagues at Yale University and at the University of Texas at Austin. It involved 43 female college students aged 18 to 22 and 33 teenagers ages 14 to 18 who were brain scanned while drinking either a chocolate milk shake or a tasteless solution.

Using an fMRI machine, the researchers measured the activity in an area of the brain that tends to be a hub for dopamine, called the dorsal striatum, when women had either a pleasurable food (the chocolate milkshake) or a control food (a tasteless solution). Obese women showed less activity in that region of the brain when they drank the milkshake compared to their leaner counterparts. The participants were also tested for the Taq1A1 gene, which is linked to fewer dopamine D2 receptors. The researchers then followed the participants for 12 months and monitored changes in their body mass index (BMI). The results showed that those having the weaker response to the milkshake and who also had the Taq1A1 gene were the ones most likely to put on weight over the follow up period.

The study demonstrates "an association between an abnormal response to food and future weight gain -- and it shows that this relationship depends upon your genetic makeup," said one of the researchers, Dana Small, an associate professor at Yale.

The results are key for understanding weight gain, and to helping people at risk of obesity.

"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," Stice said.