Gut-brain reinforcement: Insights from investigations in humans and in rodents

Ingestive behavior is orchestrated by a complex interplay of neural circuits that sense and integrate nutritional information from the external environment and the internal milieu. A dynamic interaction between these circuits unfolds over time and allows organisms to learn to associate the nutritional benefits of foods and drinks realized in the body with antecedent actions (e.g., reaching, chewing) and sensory experiences in the environment (e.g., sight, smell) and oral cavity (e.g., flavor). This process of sequential gut-to-brain association learning optimizes behavior and metabolism by enabling the value of foods to be learned and updated based on their physiological consequences. As a result, when cues are encountered in the future, they can support predictive coding, such as the simulation of potential future states to guide adaptive food decisions, and they can acquire the capacity to elicit conditioned responses in the body to optimize metabolism. Here, we review recent advances in gut-brain reinforcement learning and highlight outstanding questions and controversies.