Differences in glucose control are associated with altered reward learning

Signals from the body profoundly influence cognition. This process is known as interoception, and has been extensively studied in the cardiac, respiratory, and gastric domains; in contrast, metabolic influences remain much more poorly understood. Here, we focus on the link between glucose regulation and cognition, motivated by the observation that there is substantial, unexplained comorbidity between type-2 diabetes and depression. In rodents, insulin modulates dopamine signalling in the brain, but how this translates to humans is unknown. Based on this prior work, we hypothesised that differences in insulin sensitivity might be associated with altered reward learning. To test this hypothesis, we recruited 48 participants drawn from the general population, who each completed a glucose tolerance test (a measure of insulin sensitivity), a monetary reward learning task, and several mental health questionnaires. We discovered that poorer glucose control is associated with greater reliance on recent rewards during learning. We also found that poorer glucose control and greater reliance on recent rewards both predict higher depression symptoms. Together, our results identify a specific neurocognitive process, reward learning, by which metabolic information may influence cognition, and which may explain the link between metabolic diseases like type-2 diabetes and depression.