https://nutritionsource.hsph.harvard.edu/carbohydrates/fiber...
> Resistant starch – Soluble fermentable fiber found in legumes, unripe bananas, cooked and cooled pasta, and potatoes that acts as a prebiotic. Adds bulk to stools but has minimal laxative effect. May help to normalize blood sugar and cholesterol levels.
For stronger studies, these guys have you covered: https://www.ncbi.nlm.nih.gov/books/NBK559033/
This includes a part of the "resistant starch", which is poorly digested by humans, so a big part of it reaches the large intestine.
So yes, the "resistant" starch counts as "fiber", but not all of it, because some part of it is hydrolyzed into glucose, which is absorbed.
looking into RS's in general, it seems like there is a special carve-out after a lot of industry lobbying for very specific forms of RS's to be classified as fibers and only if they have physiological effects similar to the classic definition of fiber: https://www.fda.gov/media/113663/download
for practical reasons, I suppose I'm just not understanding why you don't eat more 'classical' fibers. psyllium husk, for eg, which is cheap and effective and has been utilized as a digestive aid in Chinese traditional medicine for centuries (one of the few to survive rigorous empirical evaluation, naturally)
fiber is great and under-consumed (especially soluble) and is very well-studied on modern populations [0]. we don't really need to make mechanistic reasonings about evolutionary adaptations that occurred hundreds of thousands of years ago or rely on studies of a small, homogenized population whose lived realities are far from yours and mine
plus, following the the hierarchy of evidence, mechanistic guesses are considered extremely low quality, only ranking above things like mouse studies [1]
[0] https://www.uptodate.com/contents/healthy-diet-in-adults#H61... [1] https://en.wikipedia.org/wiki/Hierarchy_of_evidence