Some of us tend to eat in spite of being full. A new research carried out by brain imaging has given clues as to why some people overeat despite a full stomach.
While examining how the human brain responds to 'satiety' messages delivered when the stomach is in various stages of fullness, the researchers at the U.S. Department of Energy's Brookhaven National Laboratory identified brain circuits that stimulate the desire to overeat.
According to the researchers, treatments that target these circuits may prove useful in controlling chronic overeating.
"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 Gene-Jack Wang of Brookhaven Lab's Center for Translational Neuroimaging and lead author of the study.
The findings of the study showed that the overweight subjects had less activation in parts of the brain that signal satiety in normal weight subjects. The overweight subjects were also less likely than normal weight subjects 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 Wang.
For the study, Wang and colleagues looked at the brain metabolism of 18 individuals with body mass index (BMI) ranging from 20 to 29.
Each participant in the study swallowed a balloon, which was then filled with water, emptied, and refilled again at volumes that varied between 50 and 70 percent. During this process, the researchers used functional magnetic resonance imaging (fMRI) to scan the subjects' brains.
Throughout the study, participants were also asked 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 percent.
The findings revealed that one notable region of the brain, the left posterior amygdala, was activated less in the high-BMI subjects, while it was activated more in their thinner counterparts. This activation was turned on when study subjects reported feeling full.
The researchers found a correlation between brain activation signal in the right amygdala and BMI. Overweight subjects showed less response in the right amygdala to moderate stomach fullness than normal weight subjects.
"This study provides the first evidence of the connection of the left amygdala and feelings of hunger during stomach fullness, demonstrating that activation of this brain region suppresses hunger. Our findings indicate a potential direction for treatment strategies - be they behavioral, medical or surgical -- targeting this brain region," Wang said.
The researchers 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. It was 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," Wang said.
The study is published online and will appear in the February 15, 2008 issue of NeuroImage.