# Metabolism and the Mind: Investigating the Link Between Glucose Control and Reinforcement Learning in Humans

## TL;DR

Poor blood sugar control is linked to changes in how people learn from rewards, which may explain why diabetes and depression often occur together.

## Contribution

This study identifies a novel link between glucose control and reward learning, potentially explaining the comorbidity of diabetes and depression.

## Key findings

- Poorer glucose control was associated with greater reliance on recent rewards during learning.
- Greater reliance on recent rewards was linked to higher depression symptoms.
- There was modest evidence connecting glucose control directly to depression symptoms.

## Abstract

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 poorly understood. Here, we focus on the link between glucose control and cognition, motivated by the observation that there is substantial, unexplained comorbidity between type 2 diabetes and depression. In rodents, insulin modulates dopamine signaling in the ventral striatum. We therefore hypothesized that, in humans, differences in glucose control would be associated with altered reward learning.

To test this hypothesis, we recruited 48 participants from the general population, who each completed a glucose tolerance test, a monetary reward learning task known to relate to dopamine function, and mental health questionnaires. We fitted an established reinforcement learning model to the task data to obtain computational parameters characterizing participants’ learning, and then examined the associations between these parameters and their glucose control.

We discovered that poorer glucose control was associated with greater reliance on recent rewards during learning, which was in turn associated with higher levels of depression symptoms. There was also more modest evidence for the association between glucose control and 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 such as type 2 diabetes and depression.

Signals from the body influence the way we think and feel. We studied whether differences in blood sugar regulation affect learning and mood. A total of 48 adults completed tests measuring blood sugar control, reward-based learning, and mental health. Those with poorer blood sugar control relied more on recent rewards when learning—a pattern linked to higher depression symptoms. Although preliminary, these results suggest that metabolic signals may impact reward learning beyond eating behavior, connecting metabolism and mental health. This could help explain why type 2 diabetes and depression often occur together.

Signals from the body influence the way we think and feel. We studied whether differences in blood sugar regulation affect learning and mood. A total of 48 adults completed tests measuring blood sugar control, reward-based learning, and mental health. Those with poorer blood sugar control relied more on recent rewards when learning—a pattern linked to higher depression symptoms. Although preliminary, these results suggest that metabolic signals may impact reward learning beyond eating behavior, connecting metabolism and mental health. This could help explain why type 2 diabetes and depression often occur together.