RetaRick_CA said:

The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work.

RetaRick_CA has the substance of this right. The condition it depends on is worth stating. 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.

RetaRick_CA 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.

Answering 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.

LarryQC_SD 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…

This is my experience too, for whatever a second data point is worth. Nothing to add that would improve it.