The gut–brain axis is the bidirectional communication network linking the digestive system, its resident microbes and the central nervous system. It relies on neural, hormonal, immune and metabolic pathways. The vagus nerve provides a direct neural route from gut to brain, rapidly transmitting information about gut activity and inflammation. Hormones such as cortisol and neurotransmitters such as serotonin and GABA help translate gut signals into changes in mood, stress and cognition.

Intestinal microbes are central players. They ferment dietary fibers into short chain fatty acids like butyrate, acetate and propionate, which can cross physiological barriers and influence brain cells, neuroinflammation and blood brain barrier integrity. Many microbes either produce or modulate neurotransmitters, shaping emotional behavior and stress responsiveness. Germ free animal studies show that the absence of gut microbes alters brain development, stress circuits, anxiety like behavior and learning, effects that can be partly reversed by microbial colonization.

Immune signaling also links gut and brain. Microbes regulate intestinal barrier permeability, modulate immune cell activity and influence systemic inflammation. Chronic low grade inflammation is associated with depression and neurodegenerative diseases, and altered gut communities are observed in conditions such as autism spectrum disorder, Parkinson disease, multiple sclerosis and major depression. Although causality in humans remains under investigation, evidence suggests that gut microbiota can contribute to disease risk and progression.

Lifestyle factors that influence the gut microbiome, including diet, antibiotics, probiotics, prebiotics and stress, may therefore indirectly affect brain health. This has driven interest in microbiome targeted therapies to complement existing treatments for psychiatric and neurological disorders.