DataDave said:

Fiber trials in this population are thin, and most of what gets quoted is extrapolated from idiopathic constipation in people eating normally.

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.

Dr.GutHealth 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…

Drug interaction alert relevant to constipation: if you take oral levothyroxine (Synthroid), GLP-1 agonists can reduce absorption due to delayed gastric emptying. My TSH drifted up after starting semaglutide.

Solution: take levothyroxine on an empty stomach, 60 minutes before any food, and have your TSH rechecked 6-8 weeks after starting or changing GLP-1 dose. My endo had to increase my Synthroid by 12.5mcg.

TrialNerd_Beth said:

Worth saying plainly that not all of this is benign.

Heartburn management on constipation: Tums as needed has been my savior. The delayed gastric emptying means stomach acid just sits there.

Don't ignore persistent heartburn — chronic acid exposure can damage your esophagus. If OTC remedies aren't cutting it after 2 weeks, see your doctor for something stronger. You shouldn't suffer through this.

A narrower follow-up, since the general answer is now clear:

What the mechanism is, because if it is transit rather than water then fiber is the wrong lever?

Reporting back.

Update — the osmotic agent was the thing. I had been reaching for a stimulant, which was making the next day worse.