A gut hormone that causes people to eat more does so by making food appear more desirable, suggests a new report in the May issue of Cell Metabolism, a publication of Cell Press. In a brain imaging study of individuals, the researchers found that reward centers respond more strongly to pictures of food in subjects who had received an infusion of the hormone known as ghrelin.
The findings suggest that the two drives for feeding-metabolic signals and pleasure signals-are actually intertwined.
"When you go to the supermarket hungry, every food looks better," said Alain Dagher of the Montreal Neurological Institute at McGill University. "Your brain assigns a cost versus benefit to every food item. Now, we've found that it is ghrelin that acts on the brain to make food more appealing."
Such a hedonic feeding behavior, which can occur in the absence of nutritional or caloric deficiency, may have once provided an adaptive advantage to humans, Dagher added. In our plentiful environment, however, it is likely a significant cause of obesity and its associated diseases.
Ghrelin levels are known to rise before a meal and fall afterwards, suggesting that it causes hunger and encourages eating. Indeed, Dagher noted, both lean and obese people administered ghrelin eat significantly more calories from a free-choice buffet relative to people administered a placebo. Overall, he said, acute and chronic nutritional states seem to influence naturally circulating levels of the hormone.
It has also been well established that ghrelin activates feeding through its effects on the hypothalamus, where ghrelin receptors are densely concentrated. However, ghrelin also has specific effects on many brain regions implicated in reward and motivation.
In the new study, the researchers investigated further using functional magnetic resonance imaging (fMRI) to measure the brain's response to food and nonfood images following single-blinded ghrelin infusions. Twelve people viewed pictures before and after ghrelin administration, and eight others viewed the same pictures in two identical blocks without receiving ghrelin. (All participants were told they might receive ghrelin.)