Figure 1.

Avenues of communication between the gut and the brain. The four primary avenues of microbiota–gut–brain communication. Damaged epithelium leads to increased translocation of gut bacteria into circulation (A). Microbial metabolites like short-chain fatty acids (SCFAs), neurotransmitters, and cytokines enter circulation to affect central structures (B). The vagus nerve provides a bidirectional communication pathway for the gut and brain (C). Activation of the hypothalamus–pituitary–adrenal (HPA) axis leads to the production of cortisol, which increases intestinal permeability and alters microbial composition (D).