The reason why it is difficult to digest is that the lack of corresponding decomposing enzymes in the body is related to microorganisms.

The reason why Flammulina velutipes is not fully digested is that it contains a lot of "fungal polysaccharide" fiber. Fungal polysaccharide is a very stable substance, and weak acid and weak alkali can't hurt it. So it is not suitable for digestion.

Bean sprouts are because the enzymes needed to decompose beans can only be found in the bacteria in the abdomen, but if you don't keep the habit of eating beans, there may not be enough enzymes in the abdomen to digest beans.