Fibre is counted as a carbohydrate on a nutrition label, but the human gut cannot break most of it down. What digests it is the bacterial population of the large intestine.
The enzymes humans do not have
Digestion runs on enzymes that cut specific chemical bonds. Human enzymes handle starch and simple sugars efficiently, because those are the bonds our system is equipped to open.
Fibre is built from bonds those enzymes do not recognise. It passes through the stomach and small intestine largely intact, arriving in the colon close to the form it was eaten in.
That survival is the point rather than a failure of digestion. Material reaching the colon undigested is the only material available to the bacteria living there.
What the bacteria do with it
Colonic bacteria carry enzymes humans lack, and they ferment fibre for their own energy. The by-products of that fermentation are short-chain fatty acids.
Those acids are absorbed through the gut wall, and the cells lining the colon use one of them as a preferred fuel. The host gains energy indirectly, through a microbial intermediary.
Which means the usable calorie content of fibre is neither zero nor fixed. What the body recovers depends partly on which species are present and how well they ferment.
Soluble and insoluble describe behaviour in water
The familiar split between soluble and insoluble fibre describes what happens when the material meets water. Soluble fibre forms a gel; insoluble fibre holds its structure.
Gel formation slows how quickly stomach contents empty into the intestine, which flattens the rise in blood glucose after a meal. Insoluble fibre adds bulk and shortens transit time.
Most plant foods contain both in some ratio, so the categories explain effects better than they guide food choices.
Why intake fell across industrial diets
Refining grain removes the bran and germ, where most of the fibre sits. Refined flour stores well and behaves predictably in baking, so it displaced whole grain across industrial production.
Peeling, juicing and extruding do something similar at other points in the supply chain. Each step trades fibre for shelf life, texture or convenience.
A diet built mostly on processed products therefore delivers far less fibre than one built on whole plants, without anyone deciding to eat less of it.
The bacterial population adjusts slowly
A change in fibre intake shifts which species have food available, and the composition of the population follows. That shift takes days rather than hours.
A sudden large increase can produce gas and discomfort while fermentation capacity catches up. Gradual change gives the population time to adapt.
Anyone with a diagnosed gut condition should work out changes with a clinician, because the same fermentation that suits most people can aggravate specific disorders.