UC San Diego

The metabolic syndrome (MetS) is a constellation of vascular and metabolic abnormalities that commonly occur together and increase risk for cardiovascular disease (CVD) and type II diabetes (T2DM). Having MetS, especially during middle-age, increases the risk for dementia in later life. Because of these issues, prevention of MetS has become an important public health initiative. Abdominal obesity is causally linked to MetS; therefore, increased efforts to prevent obesity and to better understand what predicts weight gain is of extreme importance. Related research suggests that altered processing of food reward in the brain of obese persons is a possible mechanism related to overeating. Therefore, the study aimed to investigate potential differences in brain response according to metabolic status during pleasantness evaluation of sweet and bitter tastes when hungry and sated. Fifteen healthy middle-aged (44-54 years old) adults and sixteen middle-aged adults with MetS were recruited from the San Diego community. The subjects participated in two functional magnetic resonance imaging (fMRI) scans: (1) after a 12-hour fast; and (2) after a nutritional preload. While in the scanner, participants rated the pleasantness of caffeine (bitter) and sucrose (sweet) solutions. Data were analyzed using voxelwise linear mixed-effects modeling and region of interest (ROI) analyses. Exploratory analyses were also conducted to examine associations between ROI activation and adiposity, and differential brain responses in participants diagnosed with T2DM. The results suggest that middle-aged individuals with MetS have decreased brain responses during pleasantness evaluation of sweet and bitter tastes in regions involved in sensory and higher-level taste processing. Additionally, hypothalamic activation was positively associated with adiposity when hungry, and negatively associated with adiposity when sated. This suggests the presence of altered hypothalamic functioning in obese middle-aged adults. It is speculated that this is due to altered hormonal nutrient signaling and/or chronic overeating. Finally, the results suggest that insulin resistance plays a large role in central taste processing during pleasantness evaluation. It is hypothesized that altered fMRI responses in participants with T2DM may be related to greater disinhibition in combination with increased necessity for dietary monitoring and carbohydrate restrictions in these individuals