Like fashion, popular medical topics come in phases. One favored subject now is the gut-brain axis: investigating how our stomachs and guts influence the rest of our bodies, both physically and mentally. 

The first notable focus on “the great abdominal brain” in Western medicine was actually in the nineteenth century, which left us with an abundance of physicians’ writings on the topic. While physicians were particularly interested in the nerves surrounding the guts (these nerves were already known to affect behavior), studying the body holistically lost its popularity in the twentieth century and the focus shifted to studying individual organs and cell types. 

These changes in holistic versus reductionist medical trends reflected the waxing and waning in general curiosity of the gut-brain axis. We are now seeing a renewed trend toward holistic biomedical approaches, with increasing research on the connections between our guts and brains. 

This focus, paired with the recent eruption of neuroscience techniques, has made unique gut-brain axis experiments possible. For example, a study published in the journal Cell Metabolism characterized nerves that relay information from the gut to the brain. They found that in mice, different nutrients recruit distinct nerves, which makes sense given that nutrients are detected by specific receptors. This means that information about nutrients being transmitted from the mice guts to their brains remains segregated.

One type of cell closely linked with food intake in the brain is AgRP neurons (neurons that make a protein called "Agouti-related protein"). An animal eats when these neurons are more active. Conversely, if AgRP neuron activity is suppressed, even starved animals cease caring about food. 

Researchers monitored these AgRP neurons in the brains of mice, while they ingested different foods. Assessing AgRP responses allowed the researchers to infer whether information about how each specific nutrient they were interested in was processed in the brain. Nutrients are primarily detected in the small intestine, and this study reports that certain nutrients are sensed in different parts of the small intestine. By identifying the distinct nerves relaying these signals, the researchers have confirmed that there is a labyrinth of pathways that convey caloric intake from our guts to our brains for maintaining energy balance.