Four communication routes between gut and brain
The gut-brain axis is bidirectional and uses several channels at once. The vagus nerve carries direct neural signals; immune messengers (cytokines) report on inflammation; endocrine signals include gut hormones and the stress (HPA) axis; and microbial metabolites such as short-chain fatty acids and neurotransmitter precursors travel through the blood.
These routes overlap and reinforce each other, which is why the system is described as an axis rather than a single nerve or hormone.
How firm the human evidence is
Much of the most striking causal evidence, including effects of microbes on anxiety-like behavior, comes from germ-free and gnotobiotic rodents. Human studies show correlations between microbiome composition and mood or cognition, and some early probiotic trials report modest effects, but the field is young and results are inconsistent.
The gut-brain axis is an active research area, not a basis for self-treating mental health conditions with supplements. This is general educational information, not medical advice; psychiatric or neurological concerns should be discussed with a qualified clinician.