[twic]

AIUI, insoluble fiber didn't get fermented. Insoluble fiber basically means cellulose; to digest cellulose, you need some pretty serious engineering, like four stomachs [1], or a huge caecum [2], or, er, a two-pass process [3]. Humans don't traditionally have any of those, so we don't digest cellulose to a significant degree. There are other components of food classified as insoluble fiber, but the only significant one is lignin, at on the order of 10-20% of the mass of cellulose [4], and that's so hard to digest you basically have to be a fungus to do it [5]. Apparently even termites don't break it down much.

Soluble fibres are oligosaccharides with weird bonds that we can't break down, but that bacteria can. They're nothing like insoluble fibre, except that they come from plants. They're polymers, but this is biology, every other thing is a polymer. They're not really even fibrous. We absolutely should distinguish them.

This review is pretty accessible, and covers a lot of ground:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3614039/

I would write more, but i'm going to cook some lentils instead!

[1] https://en.wikipedia.org/wiki/Ruminant

[2] https://en.wikipedia.org/wiki/Equine_nutrition#The_cecum_and...

[3] https://en.wikipedia.org/wiki/Cecotrope

[4] http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.1981....

[5] https://en.wikipedia.org/wiki/Lignin#Biodegradation

[epmaybe]

I think the main reason we can't break down cellulose is because we don't have the enzymes to do so. It's not as if cellulose is actually all that different to polysaccharides we are able to digest, it simply has beta 1->4 glycosidic bonds [1] between the glucose molecules instead of alpha 1->6 glycosidic bonds that are digestable by humans.

edit: upon some more skimming of the review article, even with the enzymes we may not have the conditions in which cellulose could be degraded, hence the "four stomachs" and other methods.

[1]: https://en.wikipedia.org/wiki/Cellulose