Fibre is defined by what human digestion cannot do to it, which makes it an odd nutrient. Its effects arise from where it ends up rather than from anything the body absorbs directly.
Human enzymes cannot break the bonds
Digestive enzymes handle starch, protein and fat but cannot cleave the linkages holding most plant structural carbohydrates together.
Those components therefore travel through the small intestine largely intact and arrive in the large intestine still recognisably food.
What happens there is carried out by the bacterial population living in that section, which possesses the enzymes humans lack.
Fermentation produces active compounds
Bacteria break fibre down for their own energy, and the process releases short-chain fatty acids as by-products.
These compounds are absorbed by the gut wall, where some serve as the preferred fuel for the cells lining the colon and others enter general circulation.
That is the route by which something the body cannot digest still influences processes elsewhere, including appetite signalling and inflammatory regulation.
Soluble and insoluble do different jobs
Soluble fibre dissolves into a gel that slows gastric emptying and the absorption of glucose, and it binds bile acids, which affects cholesterol handling.
Insoluble fibre does not dissolve. It adds bulk and holds water, which shortens transit time and makes stool easier to pass.
Most plant foods supply both in varying proportions, which is why advice tends to point at food groups rather than at fibre types.
Variety matters more than quantity alone
Different bacterial groups specialise in different substrates, so a diet drawing fibre from a narrow range supports a correspondingly narrow population.
A wider range of plant sources gives more groups something to work on, which is associated with a more diverse microbial community.
Diversity in that community is one of the more consistent markers distinguishing populations with differing gut health, though the causal direction is not fully settled.
Why increasing intake quickly causes trouble
A sudden increase gives the existing population more substrate than it is adapted to process, and fermentation produces gas as well as useful compounds.
Discomfort and bloating in the first weeks reflect that mismatch rather than intolerance, and it typically settles as the community shifts.
Increasing gradually and with adequate fluid gives the adjustment time to happen, and persistent digestive symptoms that do not settle deserve clinical attention rather than further dietary experimentation.