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People may experience the passage of green stool for numerous reasons with the most common ones being the intake of high-chlorophyll containing plants and vegetables such as kale and spinach and food and drinks that contain artificial colouring.

Causes of Green Bowel Movements

In people who do not eat the mentioned foods and colourants, passing green-coloured stool could be caused by any of the following factors:

  • Antibiotics - the bacteria that are present in the gastrointestinal system influence the normal colour of stool. Using antibiotics reduces the number of bacteria present in the gut and this influences the colour of stool.
  • Bile pigment - the normal colour of stool is brown and this is due to the presence of broken down bile which comes from the liver. Bile is produced to help break down fats in the diet and is naturally a green colour. If food moves too quickly through the gastrointestinal tract, there is less time available to break down bile and it ends up colouring stool green. A possible cause for this includes diarrhoea.
  • Infection with a parasite or bacterium - in the case of an infection of the gut with a microorganism, stool can turn green, not due to the pathogen itself but rather because it causes diarrhoea which, as mentioned, doesn't allow for bile to break down enough.
  • Medical procedures - green stools can be caused by graft versus host disease in cases where a bone marrow transplant is rejected, for example.

Other Colour Changes in Bowel Movements

Although passing green stool is usually not a cause for concern, some changes of colour in one's bowel movements should be investigated further to rule out any serious medical condition.

Consistently relating a specific colour of stool to an illness can be difficult to perform but the following characteristics can help serve as a general guideline:

  • Bright red - probably the most common colour change after green, this can occur when eating products such as beetroot, drinks and food with red colouring, cranberries, red gelatin, and tomato-based products. This can also be caused by bleeding from the lower digestive tract such as the rectum and includes conditions such as haemorrhoids.
  • Black - the intake of iron supplements can cause the stool to appear black in colour. A more serious reason could include bleeding in the stomach or first few centimetres of the small intestine as blood mixes with the strong acid of the stomach. In this case, the stools texture is described as tar-like and produces a foul odour.
  • White, light, or clay-coloured - a lack of bile in stool can cause it to appear lighter in colour and this may occur in conditions where the fluid is not produced or obstructed from passing into the small intestine. These conditions include liver disease, gallbladder issues, and pancreatic cancer.
  • Greasy and yellow - this description of stool can be caused by a bacterial infection of the small intestine as well as a malabsorption disorder of the gut.

These guidelines can help one determine when the need is urgent to consult with a primary care doctor.

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