Gut basics
The large intestine: functions, disorders and signs you should not ignore
The large intestine, or colon, is the last metre and a half of the digestive tract. It absorbs water and salts, houses most of the body’s gut bacteria, and forms and stores stool. Its common disorders are constipation, irritable bowel syndrome, diverticular disease, colitis, polyps and bowel cancer.
In short
- The colon turns liquid waste into formed stool by absorbing water.
- Trillions of bacteria live there and ferment fibre into substances that keep its lining healthy.
- Bleeding, a lasting change in bowel habit and unexplained anaemia always need a check.
- Fibre, fluids, movement and screening are what keep the colon healthy.
What are the parts of the large intestine?
It begins in the lower right abdomen at the caecum, where the appendix is attached. It runs up the right side, across the upper abdomen and down the left side, and ends in the S-shaped sigmoid colon, the rectum and the anus.
What does the large intestine do?
Absorbs water and salts
About a litre and a half of liquid enters the colon each day. Most of it is absorbed, leaving a formed stool.
Houses gut bacteria
The colon holds the great majority of the body’s bacteria. They ferment fibre that the small intestine could not digest.
Makes short-chain fatty acids
These products of fermentation are the main fuel of the cells lining the colon and help to keep inflammation in check.
Stores and releases stool
The rectum holds stool until it is convenient to pass it, and the muscles of the anus and pelvic floor control the release.
What are the common disorders?
| Disorder | Main features |
|---|---|
| Constipation | Hard or infrequent stools, straining |
| Irritable bowel syndrome | Pain related to bowel movements, with diarrhoea, constipation or both |
| Diverticulitis | Inflamed pouches in the colon wall, with pain and fever |
| Ulcerative colitis and Crohn’s colitis | Inflammation with bloody diarrhoea |
| Polyps | Growths on the lining, usually silent |
| Bowel cancer | Bleeding, change in bowel habit, anaemia |
| Piles and fissures | Bleeding or pain at the anus |
Which signs should I not ignore?
See a doctor if you have
- Blood in or on the stool
- Looser stools, constipation or narrower stools for more than three weeks
- A feeling that the bowel does not empty
- Tiredness with a low blood count
- Weight loss you cannot explain
- Abdominal pain with fever
Bleeding should not be put down to piles without an examination.
How do I keep my colon healthy?
- Fibre: vegetables, fruit, pulses, whole grains and millets. Aim for some at every meal, and increase gradually. See fibre.
- Fluids: enough to keep the urine pale.
- Movement: daily physical activity speeds the colon.
- Toilet habits: respond to the urge, do not strain, and use a footstool.
- Less processed meat and alcohol, and no tobacco.
- Screening: a colonoscopy from about 45, or earlier with a family history of bowel cancer or polyps.
Questions patients ask me
How often should I pass stool?
Anything from three times a day to three times a week is within the normal range, provided the stool is soft and passes without straining. A change from your own pattern matters more than the number.
Does the colon need cleansing?
No. The colon clears itself. Routine washes and laxative “cleanses” are not needed and can upset the balance of salts and bacteria. A fibre-rich diet does the job.
Why is fibre so important for the colon?
It holds water in the stool, feeds the bacteria that nourish the colon’s lining, and is linked with a lower risk of bowel cancer.
At what age should I have a colonoscopy?
From about 45 if you are at average risk. Earlier if you have bleeding, a change in bowel habit, colitis, or a close relative with bowel cancer or polyps.
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.
- Systematic reviews of carbohydrate quality and human health. Reynolds A et al. Lancet. 2019. PubMed 30638909
- American College of Gastroenterology guideline on colorectal cancer screening. Shaukat A et al. Am J Gastroenterol. 2021. PubMed 33657038
- American Gastroenterological Association and American College of Gastroenterology guideline on chronic constipation. Chang L et al. Gastroenterology. 2023. PubMed 37211380
- Revised estimates of human and bacterial cell numbers in the body. Sender R et al. PLoS Biol. 2016. PubMed 27541692