Functional neuroimaging of satiation and satiety

Abstract

The main aim of this research project was to understand the effect of internal state on brain activity associated with different food and odour properties. To this end, the brain activation in response to differential taste and odour stimuli when either being hungry or satiated, and additionally, the process of satiation was studied. Intensity of a taste can influence the satiation process. We investigated the effect of intensity differences on brain activation, by using two basic tastes, sucrose and NaCl as taste stimuli and varied the molarity of the solutions. Both stimuli were rated at the same intensity per concentration. However, the pleasantness of the highest concentration of NaCl was perceived as more aversive. When increasing the concentration of both tastes, we observed that the middle insula activation increased. Amygdala and anterior insula activation were only associated with an increased concentration of NaCl, probably due to the unpleasantness of the stimulus and the physiological function of NaCl. Also the effect of consumption on taste activation was investigated. When consuming a juice the pleasantness of that juice decreased compared to juices not consumed. Using these differences in pleasantness changes as a covariant, we investigated the food specific wanting effect on brain responses. When consuming a sweet or savoury beverage, striatal activation decreased. Taste activation of brain reward areas changed following food consumption. There were no differences in brain activation changes between sweet and savoury juice. When tasting a juice the anterior cingulate response predicted subsequent ad libitum juice intake, independent of the quality of the taste, when the anterior cingulate response was increased, subject drunk less of the associated juice. The effect of consumption on brain responses to food and non-food odours, when perceived retro- or orthonasal, was also studied. Odours sensed orthonasally provide information about objects in the external world, such as the availability of bread, whereas odours sensed retronasally provide information about foods and drinks being consumed. We observed that when an odour is associated with food or non-food, the route of delivery and internal state affect the brain differentially. Insula response is increased for food odours when hungry dependent on the route, this does not occur when full. Last, we investigated the effect of stomach filling on brain activation and the effect on hormone changes. By varying the administration of chocolate milk, by intra-gastrically administrating chocolate milk or ingestion true the mouth. In the process of satiation, amygdala, hypothalamus and hippocampus activation were associated with gastric infusion, independent of the nutrient content of the load. Oral stimulation evoked greater activation in several food intake related brain areas then when infused directly into the stomach, whereas, CCK and insulin concentrations increased more after gastric infusion. In line with the idea that oral stimulation is necessary for optimal absorption and digestion, gastric infusion induced greater hormone responses than oral administration. In conclusion, the internal state affects the physiological need of a stimulus, and thereby influences the brain responses. Sensory stimulation is important for satiation and optimal digestion.