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.
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."
New Genetic Risk for Obesity
Some people carry a variant gene that dulls dopamine responses. These people, Stice found, are more likely to be obese. And even if they are not obese, they get less pleasure from eating -- putting them at risk of overcompensating by overeating.
"People with the most blunted reward circuits are at the most risk of overeating, and the more they engage in eating, the more you see downregulation of their reward circuitry," Stice says. "They eat more to get the same reward."
"Of course it is a vicious circle," says Pothos. "A person says, 'I do not get pleasure from high-energy food, so I am eating even more but getting less pleasure, I don't know what to do. So obesity and weight gain may result from what we may call addiction to high-energy food."
The term "addiction" isn't a metaphor. Stice and Pothos note that the same vicious circle, involving the same brain circuits and the same underlying genetic susceptibility, occurs in people addicted to drugs.
However, both researchers are quick to point out that a dysfunctional pleasure system is only half the answer to the puzzle of obesity. Metabolic functions that control body weight also play a major role.
"We don't want to say obesity is an addictive disorder and not a metabolic disorder. We just want to say, 'Pay attention to both,'" Pothos says.
Stice is now looking at whether obese people who switch to a healthy diet can reset their pleasure circuitry. He finds that when obese people stop eating energy-dense foods, their craving for such foods goes down, not up.
"If we can get obese people to improve the quality of their diets and stay the course for long time, eventually they do much better in craving and their pleasure circuits should go back to their old balance," he says.
Pothos and colleagues are looking at whether parents' unhealthy eating behavior has an effect on children -- even before they are born.
"How did the obesity epidemic happen? Something is passed from parents to their offspring," he suggests.
Stice and colleagues report their findings in the Oct. 17 issue of the journal Science. Stice's colleagues included researchers from Yale University and the University of Texas at Austin.