This gut-released hormone has already been called "the hunger hormone" but what we must know about it is that ghrelin not only increases appetite, it also makes you perceive food as more appealing, as signaled by a new research published in the Cell Metabolism journal. Functional magnetic resonance imaging (fMRI) of the brain revealed that ghrelin shots made reward centers react in a more pronounced manner to food images. Hunger seems to be driven by metabolic and pleasure pathways.

"When you go to the supermarket hungry, every food looks better. 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. In our plentiful environment, however, it is likely a significant cause of obesity and its associated diseases," said co-author Alain Dagher of the Montreal Neurological Institute at McGill University.

Ghrelin amounts increase before eating and plummet afterwards, pointing to a clear link between the hunger sensation and this hormone. "Indeed, both lean and obese people administered ghrelin eat significantly more calories from a free-choice buffet relative to people administered a placebo. Overall, acute and chronic nutritional states seem to influence naturally circulating levels of the hormone," Dagher explained.

Ghrelin was found to turn on hypothalamus neurons, which have a lot of ghrelin receptors. But this new research shows that the hormone also impacts the reward and motivation nuclei. The team assessed the reaction of various brain nuclei to food and nonfood images before and after ghrelin shots in the case of 12 subjects. Other 8 subjects watched the same images, but the shot they were given was just a saline solution.

"The hunger hormone" was observed to boost brain activity when food images were presented, in several brain nuclei connected to pleasure and reward in case of visual cues, like the amygdala, orbitofrontal cortex, insula, visual areas, and the striatum.

"It's not one or two brain regions, but the whole network. [After ghrelin infusion], 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," said Dagher.

The decrease of the ghrelin activity could be used in the fight against obesity. However, since the hormone is also involved in the brain's pleasure nuclei, this could also impact the mood of the subject, leading him/her to depression. The same reward nuclei impacted by the ghrelin are also connected to drug addiction. "That shows it's reasonable to think of high-calorie food as having addictive potential," Dagher concluded by saying.