The study, published this week in the journal Science, found that obese individuals may have fewer pleasure receptors in their brains, requiring them "to take in more of a rewarding substance such as food or drugs to experience the same level of pleasure as other people," said Eric Stice, a psychology researcher at the University of Texas (UT) at Austin and lead author of the study.
In a throwback to humankind's evolutionary past, the human brain releases the "pleasure chemical" dopamine, a reward to the body for consuming life-sustaining nutrition.
But the researchers theorize that weak "reward centers" in the brain prompt obese people to eat more.
"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 researchers from UT, worked alongside scientists from the Oregon Research Institute, and brain scientists from the Yale University School of Medicine, Connecticut.
Using a technique called functional Magnetic Resonance Imaging (fMRI), researchers examined the extent to which pleasure receptors in individuals were activated in response to a taste of chocolate milkshake versus a tasteless solution.
The participants were next tested for the presence of a genetic variation linked to a lower number of the dopamine receptors.
Researchers then tracked changes in the test participants' body mass index over a one-year period.
The results, said Stice, are key for understanding weight gain, and to helping at-risk individuals.
"Although people with decreased sensitivity of reward circuitry are at increased risk for unhealthy weight gain, identifying changes in behavior or pharmacological options could correct this reward deficit to prevent and treat obesity," he said.