Neuroeconomics of sugars: Glucose vs. fructose effects on reward signaling

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Metabolic state affects behavior, in large part to defend homeostasis. However, the pathways linking metabolism to decision-making are not well understood. The core idea of this revised proposal is that by combining insights and methods from endocrinology and neuroeconomics, we can better identify mechanisms linking food consumption and behavior. Here we focus specifically on comparing the effects of two sugars (glucose and fructose) on reward signaling and “delay discounting” (the tendency to value rewards less as a function of their delay). Glucose and fructose have dramatically different effects on endocrine function (including greater circulating insulin and GLP-1 after glucose) and central nervous system activity (including greater attenuation of dopamine release within the VTA and substantia nigra after glucose). There is reason to believe that these identified differences may lead to differences in delay discounting. Specifically, we hypothesize that the tendency to prefer immediate reward over larger but more delayed reward will be greater after fructose consumption than after glucose consumption. Given links between immediate-oriented decision making and negative health consequences like obesity and problem drug use, this would have significant health implications. Participants (N = 36) will complete three functional Magnetic Resonance Imaging (fMRI) sessions, each after overnight fast followed by consumption of either: water, a 75g fructose drink, or a 75g glucose drink. Outside the scanner participants will complete a task that uses convex budgets to estimate utility and discount functions. During fMRI, participants will complete: 1) a modified “Monetary Incentive Task” with immediate and delayed rewards, and 2) a “Delay-Discounting Task” in which the participant makes choices between immediate and delayed money. In the latter, options will be individualized to be difficult (the immediate and delayed options similarly attractive given the participant’s discounting and utility). Neuroimaging analyses will focus on established reward pathways and on fronto-parietal networks recruited during the decision making task. By attaching to an ongoing separate study that includes measurement of endocrine responses to the same sugars, we will also be able to investigate whether variance in the primary study measures is associated with (and possibly mediated by) individual differences in endocrine response to glucose and fructose.