In short: soluble fiber, and psyllium in particular, is the fiber supplement with the best evidence in irritable bowel syndrome (IBS). In a 2026 analysis of 16 placebo-controlled trials, 52% of people responded to fiber compared with 44% on placebo, and psyllium was the one type with a clear benefit of its own¹. Wheat bran, an insoluble fiber, did not help in an earlier analysis², and the British IBS guideline advises avoiding it³. Psyllium is only slowly fermented and produced little gas in imaging studies⁴, which is why it is usually better tolerated than bran or inulin. There are no trials of fiber supplements in people with SIBO, apart from one trial of partially hydrolyzed guar gum given with an antibiotic.

Fiber is often the first thing people with IBS are told to try, and also the first thing many people with SIBO are told to avoid. Both pieces of advice can be right, because “fiber” covers very different substances. This guide explains which types help bowel habit and which tend to cause gas, what the trials show for psyllium, bran, methylcellulose and guar gum, how to start a supplement, and how fiber fits with SIBO and the low-FODMAP diet.

Soluble vs insoluble fiber

Dietary fiber is the part of plant food that the small intestine cannot digest. It reaches the colon (large intestine), where some of it is fermented: gut bacteria break it down and release gases such as hydrogen and carbon dioxide, along with short-chain fatty acids that feed the cells lining the colon. Fibers differ in three ways that matter for IBS:

Solubility. Soluble fibers dissolve or swell in water. Insoluble fibers, such as wheat bran, stay mostly intact and add bulk.
Viscosity. Some soluble fibers, psyllium among them, form a thick gel that holds water in the stool. Others, such as inulin and partially hydrolyzed guar gum, dissolve without thickening much.
Fermentability. Short-chain fibers such as inulin and galacto-oligosaccharides are fermented quickly and produce gas early. Psyllium is fermented slowly because its structure limits bacterial access⁴.

The fibers that help IBS most in trials are soluble, gel-forming and slowly fermented. The ones most likely to cause trouble are either insoluble (bran) or rapidly fermented (inulin, FOS and GOS, which are also FODMAPs).

What the trials show

Several meta-analyses, which pool the results of many trials, have reached the same conclusion:

2014: an analysis of 14 randomized trials with 906 people found fiber helped IBS overall, but the benefit came from soluble fiber. About 1 in 7 people treated with soluble fiber improved who would not have on placebo. Bran had no significant effect, although the analysis found no evidence that it caused harm².
2026: the most recent analysis included 30 placebo-controlled trials with 1,904 people. The most studied fibers were wheat bran (7 trials), psyllium (6), inulin-type fructans (5) and galacto-oligosaccharides (2). In the 16 trials that counted responders, 52% responded to fiber and 44% to placebo. Psyllium improved the response rate by about half (risk ratio 1.53). Scores of overall gut symptoms did not improve significantly across fibers, except with galacto-oligosaccharides. Most trials did not report results by IBS subtype¹.

The largest single trial compared psyllium and bran directly. In 2009, 275 people with IBS in general practice took 10 g a day of psyllium, bran or a rice-flour placebo for 12 weeks. More people had adequate relief with psyllium than placebo in the first month (57% vs 35%) and second month (59% vs 41%), and symptom severity fell by 90 points with psyllium compared with 49 with placebo. Bran was better than placebo only in the third month, and that result did not hold up in a stricter analysis. People on bran were the most likely to drop out early, mainly because their symptoms got worse⁵.

Guideline recommendations

The 2021 American College of Gastroenterology guideline recommends soluble, but not insoluble, fiber to treat overall IBS symptoms⁶.
The 2021 British Society of Gastroenterology guideline describes soluble fiber such as ispaghula (another name for psyllium) as an effective first-line treatment for overall symptoms and abdominal pain, advises avoiding insoluble fiber such as wheat bran because it may make symptoms worse, and recommends starting at a low dose of 3 to 4 g a day and building up gradually to avoid bloating³.

Psyllium for IBS-C, IBS-D and bloating

Psyllium (ispaghula husk) is the husk of Plantago ovata seeds. It is sold as a powder, capsules and wafers, often under brand names such as Metamucil. It absorbs water and forms a gel, which softens hard stool and can firm up loose stool. Its effects differ by IBS subtype.

IBS with constipation (IBS-C). The evidence is strongest here. In a 1987 double-blind trial of 80 people with IBS, 82% rated ispaghula satisfactory compared with 53% on placebo; constipation and gut transit time improved only with ispaghula⁷. Constipation guidelines agree: the 2025 British Dietetic Association guideline, based on randomized trials in chronic constipation, found psyllium increased the number of people who benefited, made stools more frequent and softer, and reduced straining⁸.

IBS with diarrhea (IBS-D). The evidence is thinner. Psyllium’s gel holds water, and in a 1998 study of 8 healthy volunteers given the laxative sugar lactulose, psyllium slowed gastric emptying and transit through the colon⁹. In a 1987 dose study in people with IBS, ispaghula improved constipation and pain more than diarrhea¹⁰. In a 2022 pilot trial of 37 people with loose stools and fecal incontinence, 6 g a day of psyllium cut accidents by at least half in 50% of people, a similar rate to a low-FODMAP diet (39%)¹¹. No large trial has tested psyllium in IBS-D alone, and the 2026 meta-analysis could not separate results by subtype¹.

Pain. In a 2017 double-blind trial in 84 children and teenagers with IBS, 6 weeks of psyllium reduced the number of pain episodes about twice as much as placebo. Breath hydrogen and methane did not change¹².

Bloating. Psyllium does not reliably relieve bloating, but it does not usually cause much either. In the 1987 trial, bloating improved about equally with ispaghula and placebo⁷. The 2025 constipation guideline found fiber supplements as a group increased flatulence but not bloating, and suggests kiwifruit as an alternative for people who get bloating, pain or wind with psyllium⁸. Psyllium labels warn of “minor bloating as your body adjusts to increased fiber intake”¹³, which is why a slow start matters.

Why psyllium causes less gas

Imaging studies show what happens in the gut. In a 2022 crossover trial, 19 people with IBS drank 20 g of inulin, 20 g of psyllium, both together, or a placebo, and had MRI scans every hour for 6 hours. Psyllium alone increased the volume of the colon without raising colonic gas or breath hydrogen. Inulin caused the largest rise in colonic gas, and adding psyllium to inulin cut that rise to a level not significantly different from placebo. In laboratory tests with stool from people with IBS, psyllium produced less gas than inulin⁴.

Psyllium does not stop fermentation; it slows it down. A 2025 trial in 17 healthy adults found psyllium delayed the rise in breath hydrogen from inulin but did not reduce the total over 24 hours, apparently by slowing inulin’s arrival in the colon¹⁴. These were short studies with single doses. Whether taking psyllium with meals makes FODMAP-rich foods easier to tolerate over weeks has not been tested.

Other fiber supplements

Methylcellulose is a semi-synthetic fiber made from plant cellulose that bacteria cannot ferment. It is sold as a bulk laxative (for example, Citrucel). Trials in IBS are few. In a 2013 exploratory primary-care trial of 135 people with IBS, methylcellulose capsules for 6 weeks were no better than placebo on symptom scores¹⁵. In a 2026 crossover trial in 22 healthy volunteers, a methylcellulose gel reduced the colonic gas produced by inulin about as well as psyllium and drew more water into the bowel¹⁶, although in a second trial of 30 healthy volunteers it did not slow the early rise in breath hydrogen as well as psyllium did¹⁷. Being non-fermentable, it is a reasonable option for people who get gas even from psyllium, but the evidence for IBS symptoms is weak.

Partially hydrolyzed guar gum (PHGG) dissolves without forming a thick gel. In a 2002 open trial of 188 people with IBS, half of those given wheat bran switched to PHGG after 4 weeks, compared with 11% who switched the other way, and PHGG was better tolerated¹⁸. In a 2016 placebo-controlled trial of 121 people with IBS, 6 g a day for 12 weeks improved bloating but not other symptoms or quality of life¹⁹. PHGG is also the fiber tested alongside an antibiotic in SIBO; see our prebiotics guide.

Acacia gum and inulin, FOS and GOS are mainly used as prebiotics, to feed beneficial bacteria, rather than to regulate bowel habit. Inulin-type fiber supplements did not improve stool frequency in constipation trials and increased flatulence⁸. Our prebiotics guide covers their evidence in SIBO.

Wheat bran is insoluble and fermentable. It did not help IBS in the meta-analyses² and worsened symptoms in some trial participants⁵.

Comparison of fiber types

Fiber	Type	Fermentation and gas	Evidence in IBS
Psyllium (ispaghula)	Soluble, gel-forming	Slow; little gas on MRI⁴	Best: benefit in meta-analyses and guidelines¹
Methylcellulose	Soluble, semi-synthetic	Not fermented	Weak: no benefit in one small trial¹⁵
PHGG	Soluble, low viscosity	Moderate; usually well tolerated	Moderate: helped bloating in one trial¹⁹
Acacia gum	Soluble, low viscosity	Gradual	Not well studied in IBS; see prebiotics
Inulin, FOS, GOS	Soluble, short-chain (FODMAPs)	Fast; most gas⁴	Mixed; GOS improved symptom scores¹
Wheat bran	Insoluble	Fermentable	No benefit; may worsen symptoms²³

How to start and dose a fiber supplement

Start low. The British guideline recommends 3 to 4 g of soluble fiber a day at first, increased gradually to avoid bloating³. One rounded teaspoon of a typical psyllium powder contains about 3.4 g of psyllium; labels advise new users to start with one dose a day and increase to three if needed¹³.
Build up over weeks. Trials in IBS mostly used 10 to 20 g a day⁵¹⁰. In constipation, doses above 10 g a day and at least 4 weeks of use worked best⁸. In a small 1987 dose study, 20 g a day of ispaghula worked better than 10 g, with no extra benefit from 30 g¹⁰.
Take it with plenty of water. Mix each dose with at least 8 ounces (a full glass) of water or other fluid¹³.
Space it from medication. Take psyllium at least 2 hours before or after other medicines taken by mouth, because laxatives can affect how other drugs work¹³.
Give it time. In the largest trial, the benefit of psyllium was clear from the first month⁵. If symptoms are worse after a few weeks at a modest dose, try a different fiber or stop.
Check the label. Flavored products can contain sugar or sweeteners. Sweeteners ending in “-ol”, such as sorbitol, are polyols, one of the FODMAP groups.

Safety

Psyllium is generally safe, but it swells quickly. Labels warn that taking it “without adequate fluid may cause it to swell and block your throat or esophagus and may cause choking”, and that people with difficulty swallowing should not take it¹³. Case reports describe blockages, for example a 76-year-old man with Parkinson’s disease, which can slow swallowing, who needed an endoscopy to remove a psyllium mass near the stomach²⁰. Always take it stirred into a drink, never as dry powder.

Ask a doctor before starting a fiber supplement if you have abdominal pain, nausea or vomiting, or a change in bowel habit that has lasted more than 2 weeks, and stop and seek advice if you have rectal bleeding or constipation lasting more than 7 days¹³. These can be signs of something other than IBS. People with a narrowed bowel, previous bowel obstruction or severe slow motility should also check first.

Fiber and SIBO

There are no trials of psyllium, methylcellulose or bran in people with diagnosed SIBO. A 2026 review of SIBO diet recommendations from North America, South America, Europe and the Asia-Pacific region found that the role of fiber “is not clearly established and depends on the type of fiber and the clinical context”; guidelines either gave no clear recommendation or advised an individual approach, noting that excess fermentable fiber may worsen symptoms in some people²¹. The one controlled finding is for PHGG: added to the antibiotic rifaximin, it raised breath-test normalization rates in a single small trial, covered in our prebiotics guide.

What can be said from the IBS research:

Fermentable fibers are the ones likely to cause trouble. Inulin, FOS and GOS are fermented quickly and produced the most gas on MRI⁴. In SIBO, where bacteria are already present higher in the gut, these are the fibers most people find hard to tolerate.
Psyllium is fermented slowly. It did not raise breath hydrogen or methane in children with IBS¹² or colonic gas in adults⁴. That makes it a reasonable first choice if a fiber supplement is needed, but nobody has tested whether it changes SIBO itself.
Constipation needs addressing. People with methane-dominant overgrowth, intestinal methanogen overgrowth, are often constipated. Cutting fiber too far can make constipation worse, and soluble fiber is one of the treatments with the best evidence for constipation⁸. Our guide to gut motility covers other options.

Practitioner approaches. These are based on clinical experience, not trials. SIBO practitioners commonly keep fermentable fiber supplements low during antimicrobial treatment, then add fiber back gradually afterwards, starting with less fermentable types such as psyllium or PHGG and moving to prebiotic fibers later. Many also ask patients to pause fiber supplements before a breath test, since test preparation involves avoiding fermentable foods for about 24 hours²²; see our guide to breath tests and follow the instructions from the lab or clinic.

Fiber and the low-FODMAP diet

FODMAPs are fermentable oligosaccharides, disaccharides, monosaccharides and polyols, a group of short-chain carbohydrates that are poorly absorbed and fermented quickly. Psyllium is a long-chain polymer (an arabinoxylan) that resists digestion⁴, so plain psyllium is not a FODMAP. Inulin, FOS and GOS are. The low-FODMAP diet cuts out many high-fiber foods such as wheat, legumes and some fruit, and can lower total fiber intake and beneficial bifidobacteria.

Adding psyllium is one way to keep fiber up during the diet. In a small 2026 placebo-controlled trial, 26 people with IBS on a low-FODMAP diet took a blend of psyllium and resistant starch or a placebo for 3 weeks. Symptoms stayed stable, fiber intake rose, and gut bacteria changed, without the blend making symptoms worse²³. The MRI studies suggest psyllium may also soften the gas response to FODMAPs eaten with it⁴, but this has not been tested as a long-term strategy.

Common questions

What is the best fiber supplement for IBS?

Psyllium has the strongest evidence. It is the only fiber with a clear benefit in the 2026 meta-analysis¹, and both the American and British guidelines recommend soluble fiber over bran⁶³. It works best for IBS with constipation.

Is Metamucil good for IBS?

Metamucil is a brand of psyllium, so the psyllium trials apply to it. Some versions contain sugar, sweeteners or flavorings; plain or unflavored psyllium avoids those. The same applies to other psyllium brands and to generic ispaghula husk.

Does psyllium cause bloating?

It can at first, especially at a high starting dose; labels warn of minor bloating while the body adjusts¹³. In trials, bloating was no worse than with placebo⁷, and psyllium produced little gas on MRI⁴. Starting at 3 to 4 g a day and increasing slowly helps³.

Can I take psyllium with IBS-D?

Yes, although the evidence is weaker than for constipation. Psyllium’s gel can firm loose stools, and it reduced accidents in people with loose stools in a small trial¹¹. Start with a low dose.

Should I avoid fiber if I have SIBO?

Not all fiber. The fibers most likely to worsen gas are rapidly fermented ones such as inulin, FOS and GOS. Slowly fermented fibers such as psyllium are usually better tolerated, and stopping fiber altogether can worsen constipation. There are no SIBO trials of psyllium²¹, so let symptoms guide you and see our SIBO diet guide.

Is psyllium low FODMAP?

Plain psyllium is not a FODMAP, because it is a long-chain fiber rather than a short-chain carbohydrate⁴. Check flavored products for added FODMAPs such as inulin, chicory root fiber or polyol sweeteners.

When is the best time to take psyllium?

There is no best time of day from trials. What matters is taking each dose with a full glass of water and keeping it at least 2 hours apart from other medicines¹³.

Works Cited & Scientific References 23

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Fiber supplements for IBS and SIBO: psyllium, bran and more