Human faeces (stool) – changes in colour and texture

In a healthy person, faeces (stool) is normally brown in colour. If the intestinal contents travel at a normal speed, stool is light to dark brown. If the intestinal contents travel more rapidly, chemical changes to bile (bilirubin) can change the stool green. If there is no bile (bilirubin) in the stool, the faeces is a grey colour, which signifies that the flow of bile into the intestine is blocked. Bleeding that occurs in the lower part of the intestines, particularly in the colon, is likely to cause red or maroon stools since there is less time for chemical changes to take place to the haemoglobin.

The most common ailments of non-brown faeces are:

Gastric or Peptic or Duodenal Ulcers – Bleeding from an ulcer may occur in the stomach or the duodenum, and sometimes is the only symptom. Rapid bleeding causes bowel movements to become black or even bloody.

Gastrointestinal Bleeding – Black, tarry stools usually result from bleeding that occurs high up in the digestive tract – for example, in the stomach or first segment of the small intestine (duodenum). Blood in the stomach turns black when exposed to stomach acid and enzymes. A single severe bleeding episode can produce tarry stools for as long as a week, so continuing tarry stools do not necessarily indicate persistent bleeding.

Pancreatic Disease – When pancreatic enzymes are not present in the intestine, fat is not fully digested. If the content of undigested fat is high enough, stool may become a yellowish colour. The stool will also appear greasy and smell bad.

The most common pancreatic diseases that cause this are pancreatic tumours that block the pancreatic duct through which the enzymes reach the intestines and chronic pancreatitis – usually due to damage from excessive ingestion of alcohol – that destroys the ability of the pancreas to make enzymes. If there is both blood and undigested fat in the stool, the stool may become silver.

Carcinoid Cancer – Significant blood in the stool from a carcinoid tumor can cause bowel movements to become black.

The ingestion of black liquorice, or blueberries, beet and food dyes used for colouring food can all cause black stools or false melena. As can iron and bismuth-containing medicine, e.g. Pepto Bismol.