The communication channels between gut and brain
The gut and brain talk through several routes at once. The vagus nerve carries signals directly from gut to brainstem; immune messengers (cytokines) relay inflammatory status; short-chain fatty acids from fiber fermentation can influence the nervous system; and microbes affect the supply of neurotransmitter precursors such as tryptophan. Together these form what is called the gut-brain axis, a two-way system rather than a one-way street.
What the evidence supports and what it does not
Animal studies show clear effects of gut microbes on behavior and brain chemistry, and human research links diet and microbiome features with mood and stress measures. However, most human data are associative, effects are modest, and the field is young, so claims that a particular fiber or 'psychobiotic' reliably treats anxiety or depression go beyond current evidence.
This is general educational information, not medical advice. Mental-health symptoms should be addressed with a qualified professional, not self-managed through supplements.