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This might sound funny but my friend asked me why most - if not every - faeces are of a brown or brownish color? Why not grey or slightly bluish or green for example? Is there any simple scientific explanation for this?

Thanks

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    $\begingroup$ The brown color of faeces is mainly due to the presence of bile and bilirubin. Bilirubin is a breakdown product of haemoglobin, which is destroyed by the body as part of red-blood-cell turnover. The half-life of a human red blood cell is about 120 days. Faeces may be green if bilirubin is not being produced (although it has never happened to me) $\endgroup$ – tomd Apr 26 '15 at 13:38
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    $\begingroup$ This question more likely belongs to Biology SE $\endgroup$ – Freddy Apr 26 '15 at 14:17
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    $\begingroup$ @TomD you should consider adding this as an answer! $\endgroup$ – user15489 Apr 27 '15 at 7:43
  • $\begingroup$ @TomD: I don't think green - green would be the primary decomposition products of haemoglobin decomposition which are excreted from the bile. Sometimes these can go back to the stomach and are vomited with greenish color, but when they pass the colon especially the large intestine they get oxidized to the brown coloured product. If the bile gets stuck from some liver infection or stones or the like and no billirubin enters the colon the faeces appear yellowish to white - not green. $\endgroup$ – Rudi_Birnbaum Mar 29 '16 at 16:33
  • $\begingroup$ @Franky_GTH. Biliverdin, a key breakdown product of heme, is certainly green, and NADPH-linked biliverdin reductase plays a key role in converting this green catabolic intermediate to bilirubin (which is excreted). $\endgroup$ – tomd Mar 29 '16 at 17:49

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