Regularity, mechanically
Regularity is the thing people actually notice, and it is also the most misunderstood. Almost everyone assumes fibre works by scratching the gut into action. Mostly it works by holding water.
What stool is made of
Three components, roughly. Water, which is most of it. Bacterial mass, which is a surprisingly large share of the solids. And undigested residue.
Each responds to fibre differently, and that is why different fibres produce different results.
Fermentable fibre adds bacterial mass. Feeding bacteria means more of them, and they end up in the stool. This adds bulk, but a fully fermented fibre adds only what it grew — the fibre itself is gone.
Coarse insoluble fibre works mechanically. Large, rigid particles like wheat bran stimulate the gut wall physically, and particle size matters more than amount. Finely milled bran loses much of the effect, which is why the grind is not a detail.
Gel-forming fibre holds water. This is the one that gets missed, and it is the largest lever.
The gel is the mechanism
Your colon's job includes reclaiming water. Contents arrive liquid and leave formed, and the difference is water taken back across the wall.
A viscous gel-forming fibre resists that. It binds water into a structure the colon cannot easily strip out, so material stays softer and bulkier all the way along. Psyllium is the clearest example: much of it survives to the end still holding its water.
This mechanism explains something that otherwise looks contradictory — that the same fibre is used both for constipation and for loose stool. It is not doing two opposite things. It is doing one thing, holding water inside a gel, and the effect of that depends on where you started. Too little water and it softens. Too much and it binds the excess into a formed mass. The gel normalises rather than pushes in one direction.
It also explains the most common reason fibre disappoints. A gel needs water to be a gel. Take a gel-forming fibre without enough fluid and you have added a water-hungry polymer to a system already reclaiming water, which is the opposite of what you wanted.
Why "fibre makes you regular" is too coarse
The advice collapses three mechanisms into one word, and the three are not interchangeable.
If the problem is hard, dry stool, a gel-forming fibre addresses the mechanism directly and a rapidly fermented one largely does not. If the problem is sluggish movement with adequately hydrated contents, coarse particles may do more. And a fully fermented fibre that leaves nothing behind contributes bacterial mass but no water-holding capacity at all.
This is the practical payoff of the four properties from Chapter 12. "Fibre" is not the unit of analysis. The property is.
Go deeper: transit time cuts both ways
Transit time and fermentation are coupled, and the coupling runs in both directions.
Slower transit means more contact time, so more of a slow fibre gets fermented and the whole profile shifts earlier along the colon. It also means more time for water reabsorption, which is part of why slow transit and hard stool travel together.
Faster transit does the reverse: less complete fermentation, more fermentable material carried further down, less water reclaimed.
So a fibre that changes transit changes its own fermentation profile. There is no fixed answer to where a given fibre ferments in a given person — it depends on how quickly that person's gut is moving, which the fibre itself partly determines.
Most of what a gel-forming fibre does for regularity is hold water where your colon would otherwise reclaim it. Which is why it needs the water, and why it helps in both directions.
What to hold on to
If a fibre is not holding water and is not coarse enough to work mechanically, its effect on regularity comes only from the bacteria it grows — real, but smaller than people expect.
Regularity is the effect nearest the surface. The next few chapters follow fibre into effects you cannot see, starting with the one that has become impossible to discuss without mentioning a class of drugs.
Persistent, severe or changing bowel symptoms are worth taking to a clinician rather than to a supplement.