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This is number 4 in the new Questions you never asked, but now that I mention it, yeah, that's a good point series.

So, although you probably never thought about it, now that I mention it, yeah, that's a good point. why are faeces brown?

The colour comes chiefly from bilirubin, a product of the break down of haemoglobin. You see, red blood cells transport oxygen around your body. They do so by a protein called haemoglobin, which bonds to the oxygen. red blood cells live about 120 days, after which they are broken down in the spleen, liver and bone marrow. The main by-products are iron, which is mostly re-used (to make new red blood cells), and bilirubin. the bilirubin is excreted, through bile, into the small intestine. Bilirubin at this stage is green. In the small intestine, bacteria transform the bilirubin. The resulting molecule is brown, and this is excreted in the faeces, to give them their brown color.

an intersting side note: bird faeces are white. This is because, unlike mammals, birds do not urinate. Their kidneys produce urea, like us, but it is excreted in the form of uric acid, which is insoluble, and thus comes out as the white paste we all know and love.

Since we're talking about the colour of faeces, It's a well known fact that if you drink a lot Guinness on a night out, your stool will reflect this fact the next day by it's almost indentical colour to the pints of the black stuff you were drinking the night before.

I don't know why this happens, probably due to the amount of iron that a pint of Guinness contains.

Also if you eat a lot of beets on any given day... in the next cycle your faeces will have a nice shade of red to them. That and your pee will turn pink.

Don't Panic, that's not blood in your stool.

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