Faecal colour: normal and pathological

By Cristiano Antonino Last updated Jul 17, 2022

Under normal conditions, the colour of stools varies from light to dark brown. This characteristic pigmentation is imparted by bile residues that have escaped intestinal reabsorption and are metabolised by the local flora (bile is produced by the liver and poured into the intestine, where it aids digestion and absorption of dietary lipids)

Although the colour of stools is heavily influenced by eating habits, any abnormal colour may also be due to morbid conditions

Therefore, if the change is not attributable to particular dietary changes or is accompanied by other symptoms – such as diarrhoea, constipation, weakness, abdominal pain or dizziness – it is important to promptly report it to your doctor.

Green-coloured stools

The composition of bile includes a substance called bilirubin, which is derived from the breakdown of red blood cells.

This is metabolised by the intestinal flora into stercobilin, which gives stools their typical brown colour.

Bilirubin is in turn derived from a precursor, called biliverdin, which is also present in bile and sometimes in faeces, to which it gives a green colour.

This occurs when intestinal transit is so fast that it causes incomplete transformation of biliverdin into bilirubin and its derivatives.

Green stools are thus a typical expression of diarrhoea and the pathological and non-pathological conditions (e.g. laxative abuse) that cause it.

Certain antibiotic treatments or iron supplements can also give the same inconvenience.

The green colour of stools may be related to a high intake of foods rich in chlorophyll, which is found especially in spinach, rocket, parsley, green beans and green leafy vegetables in general.

Orange colour of stools

Orange-coloured stools can be the consequence of abundant consumption of foods rich in beta-carotene, recognisable by their yellow-orange hues (carrots, pumpkins, apricots, mangoes, sweet potatoes, etc.).

Abuse of supplements containing this antioxidant pigment, taking rifampin (an antimycobacterial agent) or foods with similar colouring agents can also cause orange stools.

Red stools

When the colour of stools takes on reddish hues, the fear is that the event is somehow related to serious illnesses associated with the presence of blood in the excrement.

Fortunately, even in this case there is a possible correlation with the abundant consumption of red-coloured foods (tomato juice, red fruit and beets).

On the other hand, we must not forget that bleeding in the various tracts of the digestive tract can result in the evacuation of red or bloody faeces.

In the worst case scenario it could also be colon cancer or an intestinal polyp with a tendency to develop into a cancerous form.

The red colour of stools may be uniform or altered by bright red filaments or stains, which can also be seen on toilet paper or toilet walls; this condition occurs when bleeding affects the last part of the intestine (proctitis, diverticulitis, haemorrhoids, anal fissures, polyps or tumours of the rectum).

If the faeces are dark red in colour, the bleeding is more likely to come from the upper tract of the digestive tract (oesophagus, stomach and duodenum).

White, grey or pale-coloured stools (acholic)

Pale-coloured faeces may be due to conspicuous intake of white or beige-coloured foods, such as rice, potatoes or tapioca.

The ingestion of antacids (based on aluminium hydroxide) or barium used as a contrast agent for X-rays of the digestive tract can give the faecal mass a chalky white colour.

In the opening section we mentioned that the colour of faeces is mainly due to the presence of bilirubin and its metabolites.

It follows that a faecal hypochromia is often due to the failure of bile to reach the intestine, for instance due to gallstones or more rarely to a tumour of the bile duct or pancreas.

White-coloured faeces may also signal a number of serious liver disorders involving blockage of the bile ducts, such as cirrhosis, hepatitis and liver cancer.

Shiny, greasy, pale-coloured faeces are typical of steatorrhea (excessive fat in the excrement, usually caused by intestinal malabsorption as in coeliac disease).

Black, piceous, dark, tarry faeces

Dark grey-coloured stools may indicate the presence of metals, such as iron (e.g. from excessive ingestion of chocolate and/or meat) or bismuth; they may also be the result of charcoal intake or a high liquorice intake.

More generally, however, blackish and tarry stools may indicate the presence of partially digested blood (melena); such a condition may testify to haemorrhages in the upper digestive tract, e.g. at oesophageal, gastric or duodenal level (peptic ulcer).

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