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, according to media reports on Friday.
"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 said.
Using a technique called functional Magnetic Resonance Imaging (fMRI), researchers examined 43 female college students aged 18 to 22 and 33 teenagers aged 14 to 18, while they drank either a chocolate milk shake or a tasteless solution.
Cells in the brain's "reward" centers release dopamine when people eat, causing that feeling of pleasure, researchers explain.
The brain scans showed that activity in the brain's dorsal striatum area was much weaker in weighty women. After a year, however, participants who displayed the blunt response were more likely to have put on weight.
"The research reveals obese people may have fewer dopamine receptors, so they overeat to compensate for this reward deficit," said Stice, who has studied eating disorders and obesity for almost two decades.
Although past research has shown that biological factors play a major part in obesity, the study is one of the first to positively identify factors that increase people's weight gain risk in the future.
The results, said Stice, are key for understanding weight gain, and to helping at-risk individuals.
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