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In other words, does the body actually ADD ANYTHING TO urine or feces?

As the food and drink makes its way from our mouth to our toilets, does the body add anything to it along the way? Or, is the resultant product entirely a subset of the food and drink we have consumed that week?

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If you are saying if it has modified any food, of course it has. Eg. Urea but if you are saying something totally not derived from food in any way, I think that is impossible. (Except from some microbes in urine/faeces) – biogirl Sep 8 '13 at 14:56

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up vote 9 down vote accepted

The answer really depends on what aspect of the urine and feces one is considering.

On the atomic level, no, urine and feces are composed entirely of atoms taken from our environment. As one would expect, as there is no "Humanium" on the periodic table. In fact, all the atoms in urine and feces were originally created by stars.

On the molecular level, yes. The body produces molecules that we do not ingest, or that we ingest in minute quantities, such as urea and bile.

On the cellular level, yes. The body produces, for instance, red blood cells even if the person eats a vegan diet with no blood intake. The proteins that make up the cells are created by breaking down ingested proteins into their constituent amino acids, which are then used to construct new proteins.

Also at the cellular level are the huge amount of bacterial cells that we support. Estimates of the number of bacterial cells in our body range from ten to 100 times the number of our own cells. The bacterial component in feces is massive - up to 1/3 of feces by weight are bacteria. Urine, being produced from the blood by the kidneys, is relatively sterile.

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I totally forgot about the bacteria... of course! – tdog2 Sep 9 '13 at 2:13

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