What is diarrhoea?

Diarrhoea is an abnormal increase in the frequency and liquidity of the stools. The amount of water in the stool each day is generally not more than 200 ml for an adult. When it exceeds this amount, it is called diarrhoea.

Some authorities consider stool frequency of > 3 times per day as abnormal. However, any change in the frequency and consistency of stool to what is normal for that particular individual is relevant.

As aforementioned, diarrhoea is not itself a disease, but can be a symptom of several underlying diseases. If a stool takes the shape of the container it is in – this is considered a diarrhoeal stool.

Diarrhoea can be acute or chronic

Acute diarrhoea by definition lasts for less than two weeks and is usually self-limiting.

Chronic or persistent diarrhoea lasts for more than four weeks. 

This has a significant impact on quality of life and overall health and can be disabling and even life-threatening in vulnerable groups such as infants, children and the elderly.

Diarrhoea and intestinal functioning 

The colon, or large intestine, absorbs fluid from digested food particles that pass through it, forming semi-solid stools. If the intestinal lining becomes irritated, the intestinal tract either fails to reabsorb fluid effectively or promotes the secretion of intestinal fluid in the lumen, resulting in a watery stool.

The intestinal tract produces 10 litres of fluid (acid, enzymes, bile, and lubricants) to aid digestion daily. Eight litres are reabsorbed by the small intestine and the remainder reaches the colon where 1.9 litres are reabsorbed. Only 0.1 litres of fluid is excreted in the stools. Slow rhythmic contractions propel the digested food along, allowing the absorption of nutrients.

Diarrhoea is caused by rapid transit through the intestine which does not allow for re-absorption of liquid, inflammation of the intestines which does not allow for absorption of nutrients or liquid and finally by excessive secretion of fluid from the intestinal lining and the presence of osmotically active substances, including certain medications,  in the lumen of the intestine.

Often more than one of these mechanisms is active when a person has diarrhoea. The inflamed intestinal lining itself may also leak excess fluid, adding to the water lost through diarrhoea. Diarrhoea has a positive effect in that it helps to rapidly expel dangerous microbes from the digestive tract.


Almost everyone gets diarrhoea occasionally and adults average four bouts a year. Diarrhoea is more common in developing countries and areas, and is a serious risk for babies and young children.

Read more:

How can diarrhoea be treated?

Risk factors of diarrhoea

Causes of diarrhoea 

Revised and reviewed by Dr Saville Furman, MBChB MFGP (SA). Family Physician, Part time lecturer in family medicine and primary care at UCT, Red Cross Children’s Hospital and Groote Schuur hospital, Cape Town. February 2015.

Previously reviewed by Dr Naayil Rajabally MBBCh (Wits), FCP (SA), Cert Gastroenterology (Phys) Division of Gastroenterology, Department of Medicine, Groote Schuur Hospital. Updated, October 2011

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