PurityPaulOR 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.

BenResearch_OR said:

Constipation replaced nausea at around week five and has been the more persistent of the two by a wide margin.

Gastric emptying studies and constipation: semaglutide delays gastric emptying by ~40% at steady state, as measured by acetaminophen absorption testing and scintigraphy[1].

Clinically relevant implications:

• Explains early satiety and nausea
• Affects absorption of co-administered oral medications
• Creates aspiration risk for procedures under anesthesia
• Contributes to constipation via slowed GI transit

Importantly, the gastric emptying delay attenuates with continued use (tachyphylaxis), which correlates with the transient nature of GI side effects.

COA_Karl said:

Worth saying plainly that not all of this is benign.

Acid reflux and constipation: I developed GERD symptoms around week 6 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.

One thing that is still open after VendorMark’s answer:

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

Closing the loop on my own question.

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.