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How is the gut connected to the rest of the body?
The gut is connected to the rest of the body by four routes: nerves that link it to the brain, the immune cells that line its wall, the blood that carries everything it absorbs to the liver, and the chemical messengers made by gut cells and gut bacteria. Through these routes, the state of the gut influences mood, immunity, the liver and metabolism.
In short
- The gut has its own nervous system and is linked to the brain by the vagus nerve.
- A large share of the body’s immune cells sit in the gut wall, where they learn to tell friend from foe.
- Everything absorbed from the gut passes through the liver first.
- Gut bacteria make substances that affect appetite, blood sugar and inflammation.
- These links are real, but a link is not always a cause. The research is strong in some areas and early in others.
The gut and the brain
The gut wall contains a network of nerve cells, the enteric nervous system, that runs digestion largely by itself. It is connected to the brain by the vagus nerve, and most of the traffic on that nerve runs upward, from gut to brain.
This is why stress can bring on cramps or loose stools, and why an upset gut can leave you anxious or low. In conditions such as IBS, treatment that works on this connection is part of standard care. Read more about the vagus nerve.
The gut and the immune system
The gut lining is a single layer of cells that has to let nutrients in and keep bacteria out. Beneath it sits the body’s largest collection of immune cells. Gut bacteria help to train these cells from infancy.
When the lining is damaged or the balance of bacteria is disturbed, the immune system can become more reactive. This is clearest in coeliac disease and inflammatory bowel disease, which can also affect the joints, skin and eyes. Read more about the gut lining and gut bacteria and immunity.
The gut and the liver
Blood leaving the intestine flows directly to the liver, carrying nutrients along with bacterial products. A healthy gut lining limits what gets through. When it does not, more inflammatory material reaches the liver, which is one reason gut health and fatty liver are linked. Read more about the gut-liver axis.
The gut and the skin
People with gut conditions have more skin problems than would be expected by chance, and people with skin conditions have more gut problems. Research into this gut-skin link is still at an early stage. Read more about gut health and the skin.
What does this mean in practice?
Symptoms outside the gut can start in it
Anaemia, tiredness, mouth ulcers or joint pain are sometimes the first sign of coeliac disease or inflammatory bowel disease.
The whole person is treated
For a patient with reflux, I ask about sleep and weight. For fatty liver, about diet and gut symptoms. For IBS, about stress and past infections.
The basics do the most
A varied, fibre-rich diet, regular sleep, physical activity and avoiding unnecessary antibiotics support the gut and everything connected to it.
Where should I be cautious?
The gut does not explain every illness, and treating the gut does not treat every illness. Much of the research linking gut bacteria to mood, weight or skin shows association and not yet cause. Be wary of any claim that one test or one product will fix distant symptoms by “fixing the gut”.
Questions patients ask me
Can a gut problem cause anxiety or low mood?
The two are connected in both directions. Long-standing gut symptoms raise the likelihood of anxiety and low mood, and both make gut symptoms worse. Treating the gut condition and the stress response together works better than treating either alone.
Is the gut really a “second brain”?
The phrase refers to the gut’s own nervous system, which contains hundreds of millions of nerve cells and can run digestion independently. It does not think, but it communicates with the brain constantly.
Does a healthy gut improve immunity?
A healthy gut lining and a diverse population of gut bacteria support a well-regulated immune system. This comes from diet, sleep and activity over time, not from a single product.
Should I have my gut bacteria tested?
Usually not as a first step. See what a stool or microbiome test can tell you.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- Review of the microbiota-gut-brain axis. Cryan JF et al. Physiol Rev. 2019. PubMed 31460832
- Review of the vagus nerve in the microbiota-gut-brain axis. Bonaz B et al. Front Neurosci. 2018. PubMed 29467611
- Review of the gut-liver axis and the microbiome. Tripathi A et al. Nat Rev Gastroenterol Hepatol. 2018. PubMed 29748586
- Review of the microbiota in immunity and inflammation. Belkaid Y, Hand TW. Cell. 2014. PubMed 24679531
- Review of the gut microbiome and the gut-skin axis. Salem I et al. Front Microbiol. 2018. PubMed 30042740
- Review of intestinal permeability: mechanisms, measurement and clinical implications. Camilleri M. Gut. 2019. PubMed 31076401