The fibre that worked in IBS was psyllium, not the bran that was studied most

"Take more fibre" is the most common piece of gut advice given, and until this year the evidence on which fibre had not been synthesised recently. Two 2026 reviews now point in a consistent direction: in irritable bowel syndrome the benefit sits with psyllium specifically, and in chronic constipation it sits with viscous soluble fibres [s1] [s2]. Wheat bran — the most-studied fibre in the IBS literature — did not carry the effect [s1].

The IBS meta-analysis

Researchers searched MEDLINE, Embase, Web of Science and Cochrane CENTRAL from inception to 9 April 2026 for randomised trials of fibre supplements against a suitable placebo control in adults with IBS, registering the review prospectively as PROSPERO CRD42024600758 [s1]. The search returned 4,008 records, of which 30 randomised trials in 1,904 people with IBS were eligible [s1].

The distribution of fibre types studied is itself worth knowing. Wheat bran was the most common, appearing in seven trials, followed by psyllium in six, inulin-type fructans in five, and β-galacto-oligosaccharides in two; two trials used fibre co-administration [s1].

Sixteen trials in 1,293 people reported clinical response. Fifty-two percent responded to fibre against 44% to control, a risk ratio of 1.21 (95% CI 1.03 to 1.41, P = .02) with an I² of 38% [s1]. Broken down by type, the benefit was specific: psyllium carried a risk ratio of 1.53 (95% CI 1.06 to 2.19) [s1].

The second outcome complicates the picture rather than confirming it. Nine trials in 497 people reported overall gastrointestinal symptoms using integrative scores, and found no significant effect — a standardised mean difference of −0.25 (95% CI −0.67 to 0.17, P = .25) with an I² of 78% — except for β-galacto-oligosaccharides, which did improve symptoms [s1].

So fibre supplementation produced more responders on a dichotomous endpoint, but did not shift continuous symptom scores overall. Those two results are not contradictory — a treatment can push a subset of people over a response threshold without moving the average — but they are a reason for modest expectations.

The authors also flag a limit that runs through the whole field: subgroup analysis by IBS subtype was not possible for most outcomes, and they call for future trials either to report results by subtype or to recruit a single subtype [s1]. Constipation-predominant and diarrhoea-predominant IBS are different problems, and this literature largely cannot tell you whether fibre helps one and not the other.

The constipation network meta-analysis

A separate network meta-analysis, searching PubMed, Embase, the Cochrane Library and Web of Science to 23 October 2025, compared dietary fibre subtypes in chronic idiopathic constipation across 17 randomised trials in 1,423 adults, assessing bias with the Cochrane RoB 2.0 tool [s2].

Viscous soluble fibres were associated with greater efficacy than placebo in relieving defecation difficulty, at high certainty of evidence, and with improvements in stool consistency and stool frequency relative to placebo at moderate certainty [s2]. On the SUCRA rankings, viscous soluble fibres had a higher probability of favourable performance than other subtypes for straining during defecation (moderate certainty), stool frequency (low certainty) and stool consistency (low certainty), with a relatively favourable tolerability profile at moderate certainty [s2].

The result the authors do not smooth over: insoluble dietary fibres showed the highest ranking probability for improvement in overall constipation symptom scores — but on very low certainty evidence [s2]. Their conclusion is a symptom-oriented and individualised approach to fibre selection rather than a single recommendation [s2].

Certainty ratings doing that much work in one analysis is the useful signal here. High certainty for viscous soluble fibre on defecation difficulty; very low certainty for the finding that points the other way.

Where fibre sits in the guideline

The joint American Gastroenterological Association and American College of Gastroenterology guideline on pharmacological management of chronic idiopathic constipation conducted systematic reviews of fibre alongside osmotic laxatives, stimulant laxatives, secretagogues and prucalopride, using GRADE to assess certainty and an Evidence to Decision framework weighing effects, patient values, costs and health equity [s3].

Fibre received a conditional recommendation, in the same tier as lactulose, senna, magnesium oxide and lubiprostone [s3]. Strong recommendations went to polyethylene glycol, sodium picosulfate, linaclotide, plecanatide and prucalopride [s3].

That placement is the honest calibration. Fibre is recommended, and it is not recommended as strongly as polyethylene glycol.

The practical reading

If the question is which fibre, the two 2026 syntheses converge on soluble, viscous types — psyllium being the archetype — rather than on bran [s1] [s2]. If the question is how much benefit to expect, the IBS figure is 52% responding against 44% on control [s1], which is a real difference and a modest one.

What neither review supports is the general instruction to add fibre in any form. The most-studied fibre in the IBS trials was wheat bran, and the pooled benefit was attributable to psyllium [s1]. Insoluble fibre ranked highest on one constipation endpoint on very low certainty evidence and was not the recommended choice overall [s2].

One further detail is easy to miss: the constipation analysis rated tolerability as a separate outcome, and found viscous soluble fibres relatively favourable on it at moderate certainty [s2]. Tolerability being worth ranking at all is a reminder that these supplements are not uniformly comfortable, and that the fibre which suits one set of symptoms may not suit another — which is precisely the individualised approach the authors conclude with [s2].

This article is informational and is not medical advice. Persistent constipation or a change in bowel habit warrants clinical assessment rather than a supplement chosen from a meta-analysis.

Sources

• Fiber Supplementation in Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis of Randomized Controlled Trials — Gastroenterology, 2026-08-14
• Efficacy of different dietary fibers for chronic idiopathic constipation: a systematic review and network meta-analysis — Food & Function, 2026-06-19
• American Gastroenterological Association-American College of Gastroenterology Clinical Practice Guideline: Pharmacological Management of Chronic Idiopathic Constipation — Gastroenterology, 2023-05-19