Overweight People May Not Know When They’ve Had Enough
Researchers at the U.S. Department of Energy’s Brookhaven National Laboratory have found new clues to why some people overeat and gain weight while others don’t. Examining how the human brain responds to “satiety” messages 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 useful in controlling chronic overeating, according to the authors.“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 Gene-Jack Wang of Brookhaven Lab’s Center for Translational Neuroimaging. 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.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 percent. During this process, the researchers used functional magnetic resonance imaging (fMRI) to scan the subjects’ 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 percent.
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. Subjects who had the highest scores on self-reports of hunger had the least activation in the left posterior amygdala.
“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,” said Wang. “Our findings indicate a potential direction for treatment strategies – be they behavioral, medical or surgical — targeting this brain region.”
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,” said Wang.Neuroimage. 2007 Nov 22 [Epub ahead of print]
Brookhaven National Laboratory, Upton, NY, USA; Mt. Sinai School of Medicine, NY, USA.
Gastric distention during meal ingestion activates vagal afferents, which send signals from the stomach to the brain and result in the perception of fullness and satiety. Distention is one of the mechanisms that modulates food intake. We measured regional brain activation during dynamic gastric balloon distention in 18 health subjects using functional magnetic resonance imaging and the blood oxygenation level-dependent (BOLD) responses. The BOLD signal was significantly changed by both inflow and outflow changes in the balloon’s volume. For lower balloon volumes, water inflow was associated with activation of sensorimotor cortices and right insula. The larger volume condition additionally activated left posterior amygdala, left posterior insula and the left precuneus. The response in the left amygdala and insula was negatively associated with changes in self-reports of fullness and positively with changes in plasma ghrelin concentration, whereas those in the right amygdala and insula were negatively associated with the subject’s body mass index. The widespread activation induced by gastric distention corroborates the influence of vagal afferents on cortical and subcortical brain activity. These findings provide evidence that the left amygdala and insula process interoceptive signals of fullness produced by gastric distention involved in the controls of food intake.