The human body produces a hormone called ghrelin, which stimulates appetite by making food seem more desirable.
In a breakthrough with possible ramifications for treating obesity and controlling weight, a new study suggests that metabolic and pleasure signals in our brains work together to drive the desire to eat.
"Ghrelin has widespread effects, not just on one or two brain regions, but the whole network," said researcher Alain Dagher, a neurologist at McGill University in Montreal, "[After Ghrelin levels are increased], food pictures become even more salient — people actually see them better. It influences not only visual processing, but also memory. People remembered the food pictures better when ghrelin was high."
In an example of ghrelin in action, people who go to a supermarket when they are hungry find the food they see more appealing and buy more.
Eating behavior is both hunger-driven and pleasure-driven. Homeostatic (hunger-driven) feeding, which is related to the nutritional needs of our bodies, is thought to be regulated by a different set of hormones.
Hedonic (pleasure-driven) feeding, on the other hand, involves parts of the brain that are associated with reward and motivation. This kind of eating is stimulated by the smell and appearance of food.
Researchers knew that ghrelin, a hormone secreted by the gut, stimulates both hunger and food consumption. However, the underlying mechanisms, particularly the brain regions involved, had been unclear.
To learn more about ghrelin's effects on eating, Dr. Dagher and colleagues studied 20 healthy volunteers.
Three hours after eating a meal, the subjects viewed a series of pictures of food and scenery and subjectively rated their appetite and mood. They were also tested for their recall of the pictures.
Twelve participants were then given ghrelin by intravenous infusion and eight received a placebo infusion. All participants underwent a brain MRI, after which they viewed the same pictures again.
Those who received ghrelin showed a significant increase in hunger and recall of food pictures. Brain imaging revealed heightened neural response in the amygdala, orbitofrontal cortex, anterior insula and striatum.
"These regions encode the salience and the hedonic and incentive value of visual cues," the authors say. "This effect likely accounts for the ability of ghrelin to trigger and promote feeding."
Dagher and his colleagues report their findings in the May issue of the journal Cell Metabolism.