Overweight women may overeat because the reward centres in their brains seem to respond less than those of lean people, according to researchers who were able to predict weight gain based on the results of a brain imaging study.

In Friday's issue of the journal Science, psychology researchers in Texas, Oregon and Connecticut reported that they were able to predict future weight gain in young women who showed a blunted response to milkshakes in the reward centres of their brains.

"The more blunted your response to the milkshake taste, the more likely you are to gain weight," said study author Dr. Eric Stice, a senior scientist at the Oregon Research Institute who led the work, published in Friday's edition of the journal Science. (Listen to the interview by CBC's Quirks and Quarks.)

Dopamine is the main neurotransmitter involved in the brain's reward pathways. Eating food is associated with releasing dopamine, and the more that is released, the greater the degree of pleasure.

The findings suggested obese people may have fewer dopamine receptors, so they overeat to compensate for the reward deficit, Stice said.

"People with fewer [dopamine] 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."

The timing suggests those with fewer D2 dopamine receptors in the brain are more vulnerable to becoming obese, before the excess weight gain occurs, the researchers said.

Vulnerable to compulsive eating

In the study, researchers used functional magnetic resonance imaging to measure brain responses in 53 female university students and 33 teenage girls as they drank squirts of a chocolate milkshake or a tasteless solution for comparison.

The functional MRI showed how the brain's dorsal striatum was activated after tasting the drinks.

The researchers also tested participants for a genetic variant called Taq1A1, which is linked to a lower number of dopamine D2 receptors.

"This paper takes it one step farther," said Dr. Nora Volkow of the National Institutes of Health, a dopamine specialist who has long studied its link to obesity.

"What is [the genetic variant] doing to the way the brain is functioning that would make a person more vulnerable to compulsively eat food and become obese?"

Prevention strategy?

After tracking changes in participants' body mass index for one year, the researchers found those who showed less striatal activation to the milkshake and the genetic variant had BMIs that increased by 3.6 per cent — a difference of about 2.5 kilograms for a 170-centimetre-tall woman who started out weighing 70 kilograms.

"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," said Stice, who expects the findings would be similar in men.

Given that dopamine levels also affect drug addiction and the ability to control impulses, Volkow said it's possible the results show people with malfunctioning dopamine eat as an impulse, rather than to compensate for lack of pleasure as Stice concluded.

Dr. David Lau, chair of the diabetes and endocrine research group at the University of Calgary and president of Obesity Canada, agreed the study's technique is sophisticated, and offers a new physiological explanation, but the findings need to be confirmed.

It opens new doors in understanding how complex the brain and the "feel good gene" is in response to food, Lau added in an interview with CBC News.

The research was sponsored by the U.S. National Institutes of Health.

With files from the Associated Press