"The more blunted your response to the milkshake taste, the more likely you are to gain weight," said Dr. Eric Stice, a senior scientist at the Oregon Research Institute who led the work, published in today's edition of the journal Science.
A healthful diet and plenty of exercise are the main factors in whether someone is overweight. But scientists have long known that genetics also play a major role in obesity - and one big culprit is thought to be dopamine, the brain chemical that's key to sensing pleasure.
Eating can temporarily boost dopamine levels. Previous brain scans have suggested that the obese have fewer dopamine receptors in their brains than lean people. And a particular gene version, called Taq1A1, is linked to fewer dopamine 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 the obesity link. "It takes the gene associated with greater vulnerability for obesity and asks the question why. What is it doing to the way the brain is functioning that would make a person more vulnerable to compulsively eat food and become obese?"
First, Stice's team had to figure out how to study the brain's immediate reactions to food. Moving inside an MRI machine skews measurements, which ruled out letting the women slurp milkshakes. Yale University neuroscientist Dana Small solved that problem, with a special syringe that would squirt a small amount of milkshake or, for comparison, a tasteless solution, into the mouth without study participants moving.
They were told when to swallow, so researchers could coordinate the scans with that small motion.
Then they recruited volunteers: 43 female college students ages 18 to 22, and 33 female teenagers ages 14 to 18. Body mass index calculations showed the young women spanned the range from very skinny to obese.
