To read the full abstract: Cell Reports. 2018 Dec 4;25(10):26432652.e4.
More on the gut and the reward response: Ghrelin has been reported to encourage eating through dopamine that is important for the reward response. Here, the authors injected 38 subjects with ghrelin, while exposing them to various odors, both food and non-food based. They were also shown random images of objects, so that over time subjects associated the images with the odors. Using functional magnetic resonance imaging (fMRI), they monitored activity in brain regions known to be involved in reward response from dopamine.
They found that activity in these regions was higher in subjects injected with ghrelin, but only when responding to the images associated with food smells. This means that ghrelin controls the extent to which the brain associates reward with food odors. Following ghrelin injection, participants responded faster to food odor-associated cues and perceived them to be more pleasant, but ghrelin had no effect on their reaction to images associated with non-food odors. Ghrelin also increased functional connectivity between the hippocampus and the ventral striatum.
Obesity is associated with enhanced hypothalamic responses to food, but also to an abnormal reward response to food-related cues that are abandoned in our environment, for example fast food advertising. In 1930, Newburgh and Johnston wrote: All obese persons are alike in one fundamental respect; they literally overeat. This study shows that ghrelin may be a major factor in our intensified response to food cues. The brain regions identified have been linked to a vulnerability to obesity, suggesting a genetically-based hypersensitivity to food-associated images and smells.
Reference: 1. Newburgh LH, Johnston MW. The Nature of obesity. J Clin Invest. 1930 Feb;8(2):197213.