Dr.GutHealth said:

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

That is correct as far as it goes, and here is where it stops going. 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.

Dr.GutHealth said:

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

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. The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work. Less food means less bulk, slower motility means more water reabsorbed, and the result is a smaller, drier, slower stool. Insoluble fiber adds bulk to a system that is already moving slowly, which is why it frequently makes things worse. Soluble fiber holds water and is the better first choice, and an osmotic agent works with the mechanism rather than against it.

mike_nyc said:

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

Second this. Nothing to add that would improve it.