Read the primary source rather than the write-up and the two do not agree, so here is what is actually in 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.
Where I think it is weakest: the subgroup findings are the part I trust least — with enough subgroups something is always significant, and these were not all pre-registered.
What would genuinely help is knowing what the mechanism is, because if it is transit rather than water then fiber is the wrong lever. If the honest answer is that nobody knows, that is a useful answer and I would rather have it.
Figures above are from the primary publication rather than the press summary. If a number here disagrees with one you have, post yours and we will work out which of us is reading a secondary source.
Dr.GutHealth said:The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work.
No disagreement with Dr.GutHealth. One condition attached. 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.GutHealth 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.
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Shop Reference StandardsShort answer first, then the reasoning. 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.MetabolicMD 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…
Can confirm. Same sequence, different timescale. Nothing to add that would improve it.