[cjdell]

I wonder if you might have a microbiome problem, i.e. you lack the microorganisms necessary to break down certain foods such as fibres. There are now treatments which have been demonstrated to have success in restoring the missing microorganisms that lead to food intolerances, the most successful being FMT.

[delichon]

As I understand it the capacity of the human gut to break down insoluble fiber is quite limited, especially compared to ruminants that can turn cellulose into fatty acids in their rumen. I'm sure that a healthier gut than mine can handle insoluble fiber much better, but I'm skeptical that even healthy humans can do that by digesting a significant portion of their dietary insoluble fiber.

[nradov]

Humans can't really digest fiber (like ruminants), but fiber is metabolized to some degree by bacteria in the colon. Specifically the bacteria ferment fiber into butyrate which is important for immune system function and other things. If you don't get enough fiber then that is likely to cause gut microbiome problems.

https://peterattiamd.com/colleencutcliffe/