Gut basics
What is the gut? A beginner’s guide to your second brain
The gut is the digestive tract: a continuous tube that runs from the mouth to the anus, helped by the liver, gallbladder and pancreas. It digests food and absorbs nutrients, forms a barrier against harmful organisms, houses trillions of bacteria, and is in constant communication with the brain through its own nervous system.
In short
- The gut is one long tube with specialised sections, each with its own job.
- It does three things: digests and absorbs, defends, and communicates.
- It contains its own network of nerves, which is why it is called the second brain.
- Trillions of bacteria live in it, mostly in the large intestine, and they help with digestion and immunity.
What are the parts of the gut?
| Part | What it does |
|---|---|
| Mouth | Chews food and mixes it with saliva, which begins to digest starch |
| Food pipe (oesophagus) | Carries food to the stomach by muscular waves |
| Stomach | Mixes food with acid and enzymes, and releases it slowly |
| Small intestine | Completes digestion and absorbs almost all nutrients |
| Large intestine (colon) | Absorbs water and salts, houses most gut bacteria, forms stool |
| Rectum and anus | Store and release stool under your control |
Three organs assist it. The liver makes bile and processes everything that is absorbed. The gallbladder stores bile and releases it after a meal. The pancreas makes the enzymes that digest fat, protein and carbohydrate.
What does the gut do?
It digests and absorbs
Food is broken down into small molecules that pass through the wall of the intestine into the blood. See how digestion works.
It defends
The lining of the gut is a barrier one cell thick. Beneath it lie many of the body’s immune cells. See the gut lining.
It communicates
Nerves, hormones and immune signals carry information between the gut and the rest of the body. See how the gut affects the rest of the body.
Why is the gut called the second brain?
The wall of the gut contains a web of several hundred million nerve cells, called the enteric nervous system. It controls the movement of the gut, the release of digestive juices and local blood flow, and it can do so without instructions from the brain.
It is also linked to the brain by the vagus nerve. This two-way link is why emotions are felt in the stomach and why gut trouble can affect mood. See the enteric nervous system.
Who lives in the gut?
The gut holds trillions of bacteria, roughly as many as there are human cells in the body. Most live in the large intestine. They ferment the fibre we cannot digest, make useful substances for the gut lining, crowd out harmful organisms and help to train the immune system. See what gut health means.
How do I look after my gut?
- Eat a variety of vegetables, fruit, pulses and whole grains
- Include fermented foods such as curd and buttermilk
- Eat at regular times, and chew well
- Sleep seven to eight hours
- Be physically active every day
- Use antibiotics and painkillers only when needed
When should I see a doctor?
See a doctor if you have
- Difficulty swallowing, or food sticking
- Blood in the stool, or black stools
- Weight loss you cannot explain
- Persistent vomiting
- A change in bowel habit lasting more than three weeks
- Abdominal pain that wakes you at night
Questions patients ask me
How long is the gut?
The whole digestive tract is several metres long. Most of that length is the small intestine, which is coiled inside the abdomen.
How long does food take to pass through?
Usually between one and three days. Food leaves the stomach in two to four hours, crosses the small intestine in a few hours more, and spends the longest time in the large intestine.
Are all gut bacteria good?
Most are harmless or helpful when they are in the right place and in balance. Problems arise when harmful organisms take hold or when bacteria grow in the wrong part of the gut.
Is gut health the same as having no symptoms?
Not quite. A healthy gut digests well, has regular and comfortable bowel movements, and does so without calling attention to itself. Some conditions, such as polyps and fatty liver, cause no symptoms and are found only by screening.
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 enteric nervous system. Furness JB. Nat Rev Gastroenterol Hepatol. 2012. PubMed 22392290
- Revised estimates of human and bacterial cell numbers in the body. Sender R et al. PLoS Biol. 2016. PubMed 27541692
- Review of the microbiota-gut-brain axis. Cryan JF et al. Physiol Rev. 2019. PubMed 31460832
- Review of the microbiota in immunity and inflammation. Belkaid Y, Hand TW. Cell. 2014. PubMed 24679531