What is the gut-brain axis?

The connection between gut and brain via nerves, hormones, the immune system and microbial substances, which helps influence mood, stress and cognition.

How does the gut communicate with other organs?

Via the vagus nerve and the nervous system, gut hormones, the immune system and metabolites such as short-chain fatty acids that circulate in the bloodstream.

Why does this matter?

Because what happens in the gut radiates out to the whole body; a healthy gut environment thus supports a wide range of functions.

CosmoLexicon › Gut–organ axes

Gut–organ axes

EN: gut-organ axes · related terms: microbiome, postbiotics, psychobiotics

Definition Gut–organ axes are the two-way communication lines between your gut (microbiome) and other organs — such as the gut-brain axis, the gut-liver axis and the gut-skin axis. Via nerves, hormones, the immune system and microbial metabolites, gut and organs are in constant conversation and influence each other’s functioning.

In plain language

Your gut does not stand on its own — it is in constant conversation with the rest of your body. We call those lines of conversation gut–organ axes. The best known is the gut-brain axis: your gut and your brain send signals back and forth via nerves (such as the vagus nerve), hormones and substances your microbes make. Hence the feeling of ‘butterflies in your stomach’ or that stress upsets your digestion. But there are more: the gut-liver axis (along which nutrients and bile acids circulate), the gut-skin axis (which plays a part in skin complaints) and connections with your bones, heart and immune system. The through-line: what happens in your gut does not stay there. A healthy gut environment radiates out to your whole body. That makes the gut a kind of central hub — and explains why nutrition and microbiome have such a broad influence.

The science behind

The gut communicates bidirectionally with peripheral organs via several, partly overlapping channels: the autonomic nervous system (with the vagus nerve as the main route), the enteric nervous system, endocrine signals (gut hormones such as GLP-1, PYY), the immune system and circulating microbial metabolites.

• Gut-brain axis. Microbial modulation of tryptophan (serotonin, kynurenine, indoles), GABA and SCFA signalling influences mood, stress response and cognition — the domain of psychobiotics.
• Gut-liver axis. Via the portal vein, microbial metabolites and bile acids reach the liver; the enterohepatic bile-acid cycle steers fat and glucose metabolism (FXR/TGR5).
• Gut-skin axis. Inflammatory tone and microbial diversity are linked to skin condition.
• Further axes. Gut-bone, gut-lung and gut-kidney connections are emerging in the literature.

Short-chain fatty acids (postbiotics) are a recurring common denominator in nearly all these axes: they act locally on the barrier and systemically via receptors and the immune system. The gut–organ axes are thus the anatomical expression of co-metabolism. My InnerSelfie uses these axes as a framework to connect microbiome data with broader health. This reading is supportive and observational — not a medical diagnosis.

Related concepts

Microbiome Postbiotics (SCFA)Psychobiotics Co-metabolism Immunobiomics

Scientific references

• Lee et al. (2024), The human gut microbiome in health and disease.
• Nature, Signal Transduction & Targeted Therapy — microbiota review (s41392-022-00974-4).
• Emily Leeming, Genius Gut.

Frequently asked questions

What are gut–organ axes?: The two-way communication lines between your gut (microbiome) and other organs, such as the gut-brain, gut-liver and gut-skin axes.
What is the gut-brain axis?: The connection between gut and brain via nerves, hormones, the immune system and microbial substances, which helps influence mood, stress and cognition.
How does the gut communicate with other organs?: Via the vagus nerve and the nervous system, gut hormones, the immune system and metabolites such as short-chain fatty acids that circulate in the bloodstream.
Why does this matter?: Because what happens in the gut radiates out to the whole body; a healthy gut environment thus supports a wide range of functions.