I would like to know whether there are substances that are safe for humans to consume, are not absorbed and do not interact with any digestive process.

So, food is disqualified because although it may be safe, it is processed by the digestive system and is metabolized. I'm looking for things that go entirely unprocessed (other than passage).

One such material, or a discrete class of materials is sufficient.

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    $\begingroup$ Would dietary fiber count? $\endgroup$ – Luigi Jun 30 '15 at 17:37
  • $\begingroup$ Dietary fiber stimulates the intestinal movements. I think the phrase '...do not interact with any digestive process' effectively kills any answer, including laxatives and dietary fibers. $\endgroup$ – AliceD Jul 1 '15 at 4:35
  • $\begingroup$ This is by no way an opinion-based question. $\endgroup$ – WYSIWYG Jul 2 '15 at 4:32
  • $\begingroup$ Dietary fiber might qualify for what I was thinking about; perhaps I just asked the question poorly. Either way, it isn't nearly shocking enough ;-) $\endgroup$ – mHurley Jul 4 '15 at 14:36
  • $\begingroup$ Several sugar substitutes stimulate taste buds but are [largely] indigestible. Whether they are safe in a health sense is debatable. $\endgroup$ – bpedit Oct 10 '16 at 4:51

Simethicone is one example. It is a mixture of silica gel and dimethicone (polydimethyl siloxane); they are individually inert too.

Simethicone/dimethicone is used as antigas (they help the gas bubbles coalesce). It does have a laxative effect, though.

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    $\begingroup$ Do you have any reference for that? $\endgroup$ – rg255 Jul 1 '15 at 5:19
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    $\begingroup$ @rg255 Wikipedia is a basic reference; added that. It is kind of a known fact. I don't know of pharmacology databases. Looking for one. $\endgroup$ – WYSIWYG Jul 1 '15 at 6:07
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    $\begingroup$ Wiki is good enough there I think $\endgroup$ – rg255 Jul 1 '15 at 7:07
  • $\begingroup$ Yeah, Wikipedia is fine for my purposes. $\endgroup$ – mHurley Jul 4 '15 at 14:38

How about gold. You can find it on foods, but it is inert and also not absorbed.

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    $\begingroup$ any reference to that? $\endgroup$ – AliceD Jun 30 '15 at 23:58
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    $\begingroup$ [...] gold nanoparticles [...] absorption across intestinal barriers and accumulation in secondary target organs after oral administration: ncbi.nlm.nih.gov/pmc/articles/PMC3267526 $\endgroup$ – AliceD Jul 1 '15 at 3:51
  • $\begingroup$ So gold nanoparticles can be taken up and subsequently deposited throughout the body $\endgroup$ – AliceD Jul 1 '15 at 6:07
  • $\begingroup$ I doubt there's any research on macroscopic lumps of pure gold and digestion, but i suspect they're entirely unaffected by the process. $\endgroup$ – Resonating Jul 1 '15 at 15:25
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    $\begingroup$ I can look for a ref for this, but we use gold as a tracking source that does not get absorbed/digested. But nanoparticles are a bit unique, it isn't so much the compound as it is the size that facilitates their absorption and subsequent tissue trafficking. So @AliceD, your answer is a bit misleading, because you could really say this for a large swath of materials regardless of how much they get absorbed as Resonanting says, as a 'lump'. $\endgroup$ – The Nightman Jul 1 '15 at 23:09

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