Brain Imaging May Clarify Why Some Overweight People Continue to Eat, Despite Full Stomachs

Researchers have found new insights into why some people overeat and gain weight whereas others do not. By examining how the human brain responds to "satiety” signals delivered when the stomach is in various stages of fullness, the scientists have identified brain circuits that motivate the desire to overeat.

Treatments that target these circuits may prove beneficial in controlling chronic overeating, according to the investigators from the U.S. Department of Energy's Brookhaven National Laboratory (Upton, NY, USA).

"By simulating feelings of fullness with an expandable balloon we saw the activation of different areas of the brain in normal weight and overweight people,” said lead author Dr. Gene-Jack Wang of Brookhaven Lab's Center for Translational Neuroimaging. The overweight individuals had less activation in the areas of the brain that signal satiety in normal weight participants. The overweight subjects were also less likely than normal weight individuals to report satiety when their stomachs were moderately full. "These findings provide new evidence for why some people will continue to eat despite having eaten a moderate-size meal,” said Dr. Wang.

Dr. Wang and colleagues studied the brain metabolism of 18 individuals with body mass indices (BMI) ranging from 20 (low/normal weight) to 29 (extremely overweight/borderline obese). Each study participant swallowed a balloon, which was then filled with water, emptied, and refilled again at volumes that varied between 50 and 70%. During this process, the researchers used functional magnetic resonance imaging (fMRI) to scan the participants' brains. Subjects were also asked throughout the study to describe their feelings of fullness. The higher their BMI, the lower their likelihood of saying they felt "full” when the balloon was inflated 70%.

One notable region of the brain--the left posterior amygdala--was activated less in the high-BMI subjects, whereas it was activated more in their thinner counterparts. This activation was turned "on” when study subjects reported feeling full. Subjects who had the highest scores on self-reports of hunger had the least activation in the left posterior amygdala.

The scientists also looked at a range of hormones that regulate the digestive system, to see whether they played a role in responding to feelings of fullness. Ghrelin, a hormone known to stimulate the appetite and cause short-term satiety, showed the most relevance. Researchers found that individuals who had greater increases in ghrelin levels after their stomachs were moderately full also had greater activation of the left amygdala. "This indicates that ghrelin may control the reaction of the amygdala to satiety signals sent by the stomach,” noted Dr. Wang.


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Brookhaven National Laboratory