Overweight people actually don’t enjoy eating fatty food and sweets, a brain study found. The thought of future gain weight triggers for obese women a brain’s response to food which makes them get less pleasure from food.

Two studies, one in 43 female college students aged 18 to 22 and the other in 33 teenagers aged 14 to 18, measured the brain’s activity using a technique called functional Magnetic Resonance Imaging (fMRI) while they drank a chocolate milk shake or a tasteless drink. The researchers also tested for a particular genetic variant - TaqA1 - which is linked to fewer dopamine receptors in the brain.

The brain scans showed that women with one form of the D2 dopamine receptor gene had the lowest pleasure response when drinking a milkshake. Obese women had less activity in their brain’s pleasure centers. Activity in the brain’s dorsal striatum was much weaker in weighty women. Moreover, they had to drink more than one milk shake to get the same pleasure response. After a year, these participants were also more likely to gain weight over the following year.

The study, published in the Oct.17 issue of Science, was done by researchers at the Oregon Research Institute in Portland in collaboration with researchers at Yale University and at the University of Texas at Austin.

“This is the first imaging study which found less activation of dopamine receptors in [some] humans,” said study lead author Eric Stice, a scientist at the Oregon Research Institute in Portland. “The research reveals obese people may have fewer dopamine receptors, so they overeat to compensate for this reward deficit,” he added.

Previous studies had linked this form of the gene, called Taq1A1, to obesity and this new study reveals that overweight people have fewer dopamine receptors in the brain, so they overeat to compensate this deficit. These people who carry a variant gene that dulls dopamine responses are more likely to get less pleasure from eating even if they are not obese. This process determines a vicious circle: people tend to eat more and overcompensate the fact that they don’t get enough reward.

Dr Eric Stice, from the Oregon Research Institute said this is the first study to link the fact that obese people get les pleasure when eating with their future weight gain. The results of the study are crucial for understanding weight gain and for helping individuals who are at risk to become obese. Studies of obese animals have shown that dieting increases the D2 dopamine receptor’s response to food. Stice said that diet pills won’t work, but physical activity may be the right solution for overweight people, as exercise also activates the dopamine pathway.

An active lifestyle may help people combat obesity. Studies found that being genetically predisposed to obesity had no effect on those with above average level of exercise. The increased risk of obesity due to the fat mass and obesity associated gene can be blunted through physical activity.