Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. It is about constipation and the lower-GI picture, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work. Less food means less bulk, slower motility means more water reabsorbed, and the result is a smaller, drier, slower stool. Insoluble fiber adds bulk to a system that is already moving slowly, which is why it frequently makes things worse. Soluble fiber holds water and is the better first choice, and an osmotic agent works with the mechanism rather than against it.
The condition it depends on
One important qualification: fiber without adequate fluid is worse than no fiber. If intake is down, fluid is usually down too, and adding bulk to a dry system is how people end up in real trouble.
The practical version
Soluble versus insoluble is the distinction that matters: psyllium and oat beta-glucan hold water; wheat bran adds bulk. On slowed transit with low intake, the first helps and the second often does not.
What I am not sure about
The narrow version of the question is what the mechanism is, because if it is transit rather than water then fiber is the wrong lever. I would rather have one careful answer than five confident ones.
carlos_SATX said:The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work.
Agreeing with carlos_SATX, and the qualification matters more than the agreement. The escalation order that reflects the mechanism: water and salt first, then soluble fiber, then magnesium or an osmotic agent, then a stimulant only briefly and only if the osmotic has failed. Stimulants first is the commonest error and the one most likely to end in a cycle of dependence.
carlos_SATX said:The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work.
Worth saying plainly that not all of this is benign. Severe constipation on delayed gastric emptying has produced obstruction and ileus cases, rarely but genuinely. Pain with vomiting and no bowel movement is not a fiber conversation.
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View ResultsAnswering the narrow version, because the broad one does not have a single answer. Fiber trials in this population are thin, and most of what gets quoted is extrapolated from idiopathic constipation in people eating normally. The extrapolation is not unreasonable but it is an extrapolation, and it fails specifically for insoluble fiber at low intake — the case that comes up here most often.
pete_manc_UK said:The escalation order that reflects the mechanism: water and salt first, then soluble fiber, then magnesium or an osmotic agent, then a stimulant only…
Second this. The detail I would add is minor and it is already implied above.