PeptideChemSF 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.

NeuroNate said:

I added fiber, made it noticeably worse, and only later worked out that I had added the bulk without adding the water.

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.

VendorMark said:

Worth saying plainly that not all of this is benign.

Acid reflux and constipation: I developed GERD symptoms around week 7 that I'd never had before. The delayed gastric emptying means food and acid sit in your stomach longer.

Management that works for me: elevate head of bed 6 inches, don't eat within 3 hours of lying down, avoid trigger foods (tomato, citrus, spicy, chocolate), and famotidine 20mg at bedtime. Much better now.

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.

Resolved: water and magnesium first, soluble fibre second, bran gone entirely. Two months of doing it in the wrong order and a week to undo.