Vicious cycle of overeating and obesity unravelled
In a bid to uncover why obese people tend to overeat, researchers have found evidence of the vicious cycle created when an obese individual overeats to compensate for reduced pleasure from food.
Obese individuals have fewer pleasure receptors and overeat to compensate, according to a study conducted by senior scientist Eric Stice and his colleagues at the University of Texas.
Stice showed that overeating could further weaken the responsiveness of the pleasure receptors ('hypo-functioning reward circuitry'), further diminishing the rewards gained from overeating.
Food intake is associated with dopamine release. The degree of pleasure derived from eating correlates with the amount of dopamine released.
Evidence has shown that obese individuals have fewer dopamine (D2) receptors in the brain relative to lean individuals and suggested that obese individuals overeat to compensate for this reward deficit.
People with fewer dopamine receptors need to take in more of a rewarding substance — such as food or drugs — to get an effect which other people get with less.
"Although recent findings suggested that obese individuals may experience less pleasure when eating, and therefore eat more to compensate, this is the first prospective evidence to show that the overeating itself further blunts the award circuitry," said Stice.
"The weakened responsivity of the reward circuitry increases the risk for future weight gain in a feed-forward manner. This may explain why obesity typically shows a chronic course and is resistant to treatment."
Using functional magnetic resonance imaging technique (fMRI), Stice's team measured the extent to which a certain area of the brain (the dorsal striatum) was activated in response to the individual's consumption of a taste of chocolate milkshake (versus a tasteless solution).
Researchers tracked participants' changes in body mass index (BMI) over six months. Results indicated that those participants who gained weight showed significantly less activation in response to the milkshake intake, at a six-month follow-up relative to their baseline scan, and to participants who did not gain weight.
"This is a novel contribution to the literature because, to our knowledge, this is the first prospective fMRI study to investigate change in striatal response to food consumption as a function of weight change. These results will be important in developing programmes to prevent and treat obesity," said Stice.
The study has been published in The Journal of Neuroscience.