A study that looked at the mechanism behind dietary fibers effect on the microbiome, they hypothesized that «dietary fiber
resists digestion in the small intestine, and enters the colon where it is fermented to produce [short chain fatty acids] that may enhance the healthy composition of gut microbiota.»
And what about the other issues from the article: Phytates in nuts and seeds also interfere with the enzymes we need to digest our food, including amylase (required for the breakdown of starch), pepsin (needed to breakdown proteins in the stomach) and trypsin (needed for effective
protein digestion in the small intestine).
A resistant starch is one that resists
digestion in the small intestine and is digested through a fermentation process by bacteria in the large intestine.
They can use resistant starch as food because it resists
digestion in our small intestine, and moves on to the large bowel.
Resistant starches (RS) resist
digestion in the small intestine, so they pass unchanged to the large intestine, where beneficial colonic bacteria partially break them down (ferment) to short - chain fatty acids (SCFA) and gases.