Interoception and gut-brain communication.

Interoception - the internal sensing of bodily states and their communication to the brain - has recently become a 'buzzword'. However, the scientific study of how the brain and the body communicate has a much longer and well-established history. In the 16^th century, the French philosopher and scientist René Descartes (1596-1650) proposed that the mind and body are two fundamentally distinct entities, yet they interact with each other to regulate animal and human behaviors. Descartes argued that the body, which operates like a machine and follows the laws of physics, is controlled by the mind (or the brain), the source of our cognitive functions. Today, it is widely recognized that the interactions between the body and brain are critical for our survival and overall well-being (Figure 1). At the same time, we are just beginning to learn about the mechanistic bases of these interactions. In this primer, we will discuss the origins of interoception research and how body-brain interactions govern some of our most critical homeostatic functions, such as energy balance. Given their key roles in health and disease, we will focus on the neural and hormonal pathways involved in gut-brain communication and their impact on feeding behavior and metabolism.