The daily food supply for gut microbes
The microbiome is often discussed as if it is a separate organ. It is better understood as a living community that responds to what reaches the colon. Human enzymes cannot fully digest fibre in the small intestine, so part of that material travels further down the digestive tract.
There, microbes can ferment selected fibres and turn them into acids, gases, and other metabolites. That does not make every fibre identical. Some fibres ferment quickly, some slowly, and some behave more as water-holding structure. The useful question is not only whether a food contains fibre. It is what kind of fibre reaches the gut and what work it gives microbes to do.