Gut–brain axis

The gut–brain axis (often brain–gut axis in clinical writing) is the bidirectional communication network between the gastrointestinal tract and the central nervous system. Signals travel through multiple parallel routes, not a single cable:

Route	Examples	What stool reports capture
Neural	Vagus nerve, enteric nervous system, visceral afferents	Nothing directly
Endocrine / HPA	Cortisol, CRF, gut hormones (e.g. 5-HT from enterochromaffin cells)	Nothing directly
Immune	Cytokines, mast cell mediators crossing from mucosa	Indirect via inflammation proxies, see Gut–immune axis
Microbial metabolites	SCFAs, bile acid transformations, tryptophan metabolites	Inferred from taxa/pathways, see SCFAs

A microbiome PDF describes who was detected in stool. It does not score stress at collection, sleep the prior week, or cognitive performance.

Hub: Reading your microbiome report.

Where this site goes deeper (by symptom and mechanism)

Symptoms that feel “in the head” with gut involvement

Concern	Start here	Why not stop at taxa
Brain fog + bloating/diarrhoea	Brain fog with gut symptoms	Rare D-lactic pattern; mostly non-GI causes
Fatigue + GI symptoms	Fatigue with gut symptoms	Anaemia, thyroid, sleep apnoea before sequencing
Abdominal pain, IBS framing	Abdominal pain, IBS subtypes	Visceral hypersensitivity ≠ missing microbe
Bloating without clear diet trigger	Chronic bloating	Motility, SIBO, FODMAP branches

Modifiable drivers with trial evidence in functional gut disorders

Topic	Page	Evidence note
Stress, sleep, exercise	Stress, sleep, and exercise	CBT, hypnotherapy, moderate activity in IBS guidelines (Lacy 2021)
Motility and sensitivity	Gut motility, SIBO & breath testing	Geography limits of stool
Post-infectious onset	Post-infectious IBS	Clinical syndrome, not taxon profile

Microbiome-adjacent mechanisms (often overclaimed for cognition)

Narrative on reports/blogs	Better page	Limit
”Leaky gut → brain fog”	Intestinal barrier	Barrier biology is real; blanket cognition claims are thin
Low butyrate / F. prausnitzii	SCFAs, Faecalibacterium	Local immune/barrier signalling ≠ brain diagnosis
LPS / endotoxemia	Metabolic health & endotoxemia	Stool Gram-negative reads ≠ serum LPS
Dysbiosis label	Dysbiosis	Ecological deviation, not psychiatric diagnosis

What the literature supports vs what reports sell

Supported in functional gut care (symptom endpoints):

Gut-directed psychological therapies and hypnotherapy for IBS symptom burden
Stress and sleep as modifiers of pain and bowel habit, Stress, sleep, and exercise
Visceral hypersensitivity as a core IBS mechanism (Major et al., 2017 context)

Weak or oversold for individual cognition from one stool test:

Generic dysbiosis → brain fog arrows on infographics
Probiotic strain X chosen from genus abundance alone, Probiotics and prebiotics
Pathway scores as proof of neuroactive metabolite flux, Metagenomic pathway scores

Specific gut-linked cognitive pattern (rare): carbohydrate malabsorption / SIBO overlap with D-lactic acidosis after high probiotic load in susceptible patients, detailed on Brain fog and Intestinal barrier, not a default explanation.

Practical routing order

Red flags and non-GI causes, sleep, mood, meds, labs (Red flags)
Symptom pattern, pain vs bloating vs diarrhoea vs constipation (symptom pages above)
Lifestyle and behavioural trials with guideline support
Microbiome sequencing, optional context; Multi-marker synthesis if the report is noisy

What not to conclude

If marketing or a report implies…	Do not conclude…
Low diversity = brain inflammation	Diversity is context-dependent, Alpha diversity
Fix taxa → fix anxiety	Mental health care and GI care both matter
Stool test replaces neurology	Sequencing does not measure CNS function
Vagus / psychobiome supplements required	Evidence tier often low vs behavioural therapies