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Humans don't digest fibre, and nor do any animals, but some creatures have micro-organisms (bacteria, I'm guessing) in their gut which digest fibre for them. My question is, could humans take some sort of pill and establish our own fibre-digesting colonies in our intestines? Has this been tried? Supposing it could be done, would you suppose it would have any negative effects on our health? (i.e.. would those whose gut digested fibre for them be stuck on a low-fibre diet?).

Purpose of question: just wondering. It would be cool to be able to eat bark.

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Probably not without some substantial evolution to change our physiology, depending on what you are wanting. If you just want to digest fibre, that might be done, but if you wanted to see nutritional value out of it, its not clear that's going to be worth it..

Soluable fibre is currently used by the human body, so you are probably be asking about insoluble fibre.

It might be possible to do a little of this experimentation - I'm not recommending this - but there would be several likely issues depending on your goals in such a project, which could vary.

  1. the human gut microenvironment probably will not accommodate the bacteria you need and they might not grow in significant quantity.
  2. the human gut is not physiologically arranged for digesting such fibre - ungulates generally have four stomach chambers that they can use to store some of the hard for the longer time it takes to break the fibre down.
  3. the nutritional value of plant fibre is low XKCD has some numbers on this and a cow can eat 10kg of plant matter a day and in doing so they don't tend to move that much. Humans have to eat a greater volume of vegetables to get the calories they need. Is getting that last 2-5% or so of the energy from digesting fibre worth the trouble? Eat one more apple or a handful of grain.

Lastly, there would likely be some impact on human health - if you succeed then you would be modifying fibre's role in human diet as it stands - which is to help keep the intestines cleaned out.

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Just to add something to the answer from @shigeta - even assuming that you did manage to establish a gut microbiome that was capable of metabolising fibre, this wouldn't be a source of glucose: in ruminants the gut bacteria ferment the sugars released from the cellulose, releasing short chain fatty acids (a.k.a. volatile fatty acids, e.g. propionic acid).

These compounds are used to fuel gluconeogenesis in the liver of the animal: in a lactating cow this can amount to kilograms of glucose synthesised per day. Although you wouldn't be aiming to eat that much fibre, the release of these VFAs into your bloodstream might still cause some metabolic problems.

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