Metabolic interoceptive rewards shape affect, but not action

Central to survival is our ability to learn that the sensory properties of food are associated with metabolic interoceptive signals (e.g., changing blood glucose). These signals influence cognition and brain activity, shaping the hedonic evaluation of flavours, and activating reward-related brain regions such as the ventral striatum. However, there remains a substantial gap in understanding how metabolic rewards shape behaviour, particularly in humans. We hypothesised that metabolic interoceptive rewards may function as Pavlovian stimuli, eliciting a Pavlovian approach bias which modulates everyday instrumental decision-making via Pavlovian-instrumental transfer. To test this, in a double-blind design, over one week, participants (N = 53) consumed two novel, flavoured drinks: one containing the tasteless carbohydrate maltodextrin, and one a calorie-free control. In a subsequent lab session, participants completed a Pavlovian-instrumental transfer task, performing a points-based instrumental decision-making task while tasting calorie-free versions of each flavour. Participants also rated liking and wanting of the flavours. As predicted, flavours previously experienced with calories were rated as significantly more liked after conditioning. However, counter to our hypothesis, the calorie-associated flavours did not enhance instrumental responding; computational modelling instead indicated a suppression of action. These findings reveal a dissociation between hedonic preferences and action: while metabolic rewards shaped liking, they did not invigorate behaviour. This highlights the complexity of interoceptive reinforcement learning and points to the need for further work to understand how and when internal metabolic expectations shape action.