Why are faeces brown?

The brown colour of faeces is primarily due to the presence of a pigment called stercobilin. Stercobilin is a compound formed from the breakdown of haemoglobin, which is the oxygen-carrying protein in red blood cells.

After red blood cells have completed their lifespan, they are broken down by the liver and spleen. The haemoglobin is released from the red blood cells and broken down into bilirubin. Bilirubin is then transported to the intestines, where it is converted into stercobilin by bacteria.

Stercobilin is responsible for the characteristic brown colour of faeces. The amount of stercobilin produced can vary depending on factors such as diet and the amount of bile produced by the liver. Certain foods, such as red meat and beets, can also affect the colour of faeces.

In addition to stercobilin, faeces may also contain other pigments that can contribute to its colour, such as urobilin and coproporphyrin. These pigments are also formed from the breakdown of haemoglobin.