Science Questions

Why is food multicoloured, but poo brown?

Sun, 5th Jul 2009

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Question

Raphaela in Norwich asked:

Why is it that food that was once red or green or yellow, et cetera, comes out one color when it comes out of the human body?

Answer

Chris - And we know what color she is referring to and the answer actually is that the food itself doesn’t necessarily contribute very much to the color of what comes out the back end.  The reason being that the dominant determinant of the color of what comes out the back end is bile and bile salts, what your liver squirts into your small intestine to help you to absorb fat.  And the reason for that is that there is a chemical which is called bilirubin which is a breakdown product of hemoglobin, the stuff that makes your red blood cells red, that goes into your intestines and it gets modified by a bacteria in the small bowel and as a result of that modification, it gets oxidized into a chemical first called urobilinogen which is what gets reabsorbed into your blood stream and makes your wee go yellow but then it also ends up back in your gut and gets turned into an even browner chemical called stercobilin and stercobilin is the brown stuff that it makes pooh a brown color.  And if you have a blockage in the supply of bile into your intestines from the liver, what that does is actually prevent you from getting any of this stercobilin being made and you actually do very pale colored poohs and so, you can use that as a way to diagnose people who have gall stones or liver problems.

Kat -   Or if you’re Gillian McKeith, just so you know, look at it for fun. 

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