Fibre is classified as a carbohydrate but behaves almost nothing like the rest of that group. The difference comes down to a chemical bond human digestive enzymes cannot break.
Human enzymes cannot open the bonds
Starch and sugars are held together by links that human digestive enzymes recognise and split, releasing glucose for absorption in the small intestine.
Fibre is built from similar sugar units joined by different bonds, and no human enzyme cleaves them, so the material arrives in the large intestine essentially intact.
That single structural difference is why fibre contributes little directly absorbable energy while still occupying volume and altering the passage of everything around it.
Gut bacteria do what human enzymes cannot
Bacteria in the large intestine possess the enzymes to ferment many fibres, breaking them down for their own use.
The products of that fermentation include short-chain fatty acids, some of which are used directly as fuel by the cells lining the colon.
This is why fibre intake shapes the composition of the gut community: it supplies the substrate that determines which populations have something to grow on.
Soluble and insoluble forms act differently
Soluble fibres dissolve and form a viscous gel, which slows gastric emptying and the rate at which glucose from the same meal reaches the bloodstream.
Insoluble fibres do not dissolve; they add bulk and hold water, which speeds transit and softens stool consistency.
Most fibre-containing foods provide both in varying proportions, so the categories describe behaviour rather than dividing foods into two separate groups.
Effects on fullness are partly mechanical
Fibre-rich foods occupy more volume for the same energy, and stretch receptors in the stomach wall respond to volume rather than to calories.
Slower emptying keeps that stretch signal active for longer and moderates the rise and subsequent fall in blood glucose that follows a meal.
Both effects contribute to the reported difference in fullness between whole and refined versions of the same food, alongside the difference in how quickly they are eaten.
Why increases should be gradual
A sudden large increase supplies more fermentable material than the existing bacterial population is adapted to process, producing gas and discomfort.
The community shifts over a few weeks as populations able to use the new substrate expand, which is why the same intake becomes comfortable with time.
Anyone with an established bowel condition should change fibre intake with clinical guidance, since the type and amount that helps in one condition can worsen another.