Oct. 16, 2008 -- Obese people expect to enjoy food more than lean people do, but when they eat, they enjoy it less, a brain study shows.

And that's a problem. To make up for the missing enjoyment, obese people eat more high-calorie food. Overeating further dulls food enjoyment and locks people in a vicious circle.

The finding comes from real-time brain-imaging studies in obese and lean women by Eric Stice, PhD, of the Oregon Research Institute, and colleagues.

"We originally thought obese people would experience more reward from food. But we see obese people only anticipate more reward; they get less reward. It is an ironic process," Stice tells WebMD.

Stice's team showed women a picture of a chocolate milkshake and a picture of a glass of water. The heavier the woman, the more active the pleasure center in her brain.

Then the women actually tasted a chocolate milkshake or a neutral solution. Heavier women had less activity in their brains' pleasure centers.

"Probably this is related to downregulation of the brain's reward circuit. The more you do things that are rewarding, the less reward you see," Stice says. "The more you eat an unhealthy diet, the more you see this blunted pleasure response to high-energy foods."

Tufts University neuroscientist Emmanuel Pothos, PhD, has seen the same thing in mouse studies. He was not involved in the Stice study.

"Obesity is not only a function of brain systems that regulate body weight, but a function of brain systems that regulate eating for pleasure," Pothos says. "In mice, the central dopamine system -- the system that underlies pleasure from eating -- is defective. The animals have a very low response to stimuli that release dopamine. And food is one of those stimuli."