It’s a valid question. I just don’t think we have much of an understanding on what our gut culture does with the food we don’t absorb. Eventually we might be able to consider the biome as a secondary digestion much as we think of the multiple stages of ruminant digestion.
One of the many things that Dr Phinney said last year that I found interesting was in relation to the care and feeding of the cells that line the gut. It’s commonly known that enterocytes prefer to be fueled by short chained fats in the gut rather than glucose from circulation. So much so that diets low in fibre have a weak correlation with bowel cancer. This is apparently one of the justifications from dietitians for a diet high in fibre. Namely to feed bacteria that convert indigestible fibre into the short chained fatty acid, butyric acid.
The “interesting thing” was in a low carb dietary context that enterocytes prefer betahydroxybutyrate in circulation over glucose and being a 4 carbon molecule it apparently replaces butryic acid as the preferred fuel for the gut. Which means for those of us on a low carb diet, we may also be able to get away with a low fibre diet.