Dr.EndoIndy said:

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

Agreeing with Dr.EndoIndy, 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.

Dr.EndoIndy 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.

Taking the question as asked, rather than the general version of it. 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.

Correct me if the detail matters more than I have assumed.

Dr.BariatricHTX 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 matches mine closely enough to be worth saying so out loud. Posting only so the count is not one.