Fibre is defined by what human digestion cannot do to it. That definition explains why its effects are indirect and why different fibres behave so differently from each other.
Human enzymes cannot break the bonds
Digestive enzymes are specific to particular chemical bonds, and the bonds linking the sugar units in most plant structural material are not among them.
Fibre therefore arrives in the large intestine essentially as it was eaten, which is where the resident bacterial population encounters it.
Those bacteria possess enzymes humans lack, so material that was indigestible upstream becomes a substrate downstream, and the fermentation that follows is the origin of most of fibre's effects.
Fermentation produces active compounds
Bacterial fermentation yields short-chain fatty acids, and one of them is the preferred fuel of the cells lining the colon.
These compounds are absorbed and act locally on the gut lining, while some enter the bloodstream and interact with signalling involved in appetite and glucose handling.
Which is why fibre's influence extends beyond the digestive tract despite the fibre itself never being absorbed at all.
Soluble and insoluble do different jobs
Soluble fibres dissolve and form viscous gels, which slow gastric emptying and the absorption of sugars, and they ferment readily.
Insoluble fibres do not dissolve, add bulk, and speed transit through the intestine, and they ferment much less.
The categories overlap in real foods, since a wholegrain or a legume contains both, which is one reason food-based recommendations outperform supplement-based ones here.
The bacterial population responds to what it is fed
Different bacterial groups specialise in different substrates, so a diet dominated by one fibre type supports a narrower community than a varied one.
Community composition shifts within days of a dietary change, though the extent of the shift depends heavily on what was already present.
Variety of plant sources therefore appears in dietary advice for a reason distinct from total fibre quantity, since the two do not measure the same thing.
Increasing intake has predictable side effects
A sudden increase supplies substrate faster than the bacterial population adjusts, and the resulting gas and bloating are a fermentation artefact rather than a sign of intolerance.
Gradual increases with adequate fluid are the standard suggestion, since the community composition shifts over days to weeks rather than immediately.
People with inflammatory bowel conditions, strictures or diagnosed irritable bowel syndrome are the exception, as fibre type and quantity require individual guidance in those situations rather than general advice.