When mammals consume food and digest it or drink fluids that are then filtered by their kidneys, are the waste products generated simply depleted versions of what they consumed?

Are there other byproducts of metabolism and filtration contained in that waste that are generated by the body and are not directly a part of the food or liquid that was consumed that are excreted?

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    $\begingroup$ 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) $\endgroup$
    – biogirl
    Sep 8, 2013 at 14:56

1 Answer 1


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.

  • $\begingroup$ I totally forgot about the bacteria... of course! $\endgroup$
    – tdog2
    Sep 9, 2013 at 2:13

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