Psyllium, PHGG, methylcellulose or inulin: fiber supplements compared

The fiber shelf at the pharmacy looks simple: powders, capsules and gummies, all promising regularity. Read the ingredient lines, though, and you'll find very different materials, from seed husks to modified cellulose to chicory root extract. They don't do the same job, and for a gut that bloats easily, the differences matter.

Here's how the most common fiber supplements compare, based on what each one has actually been shown to do.

Three questions that separate them

1. Does it form a gel? Highly viscous, gel-forming fibers such as psyllium are the ones linked with lower cholesterol and better blood sugar control. Non-viscous soluble fibers, like inulin and wheat dextrin, don't provide those benefits (McRorie and McKeown, 2017).
2. Does it stay intact in the colon? To soften stool, a fiber has to resist fermentation and either hold on to water or be coarse enough to stimulate the gut lining. Soluble fibers that bacteria ferment readily don't have a laxative effect (McRorie and McKeown, 2017).
3. Has it been tested for your problem? A fiber with good evidence for constipation may never have been studied for bloating, and the reverse.

The supplements, one by one

Psyllium

What it is: a gel-forming soluble fiber from the husk of Plantago ovata seeds.

Best evidence for: constipation. A 2022 meta-analysis of 16 trials with 1,251 people found that fiber supplements improved chronic constipation, with psyllium among the fibers showing significant effects. Benefits were clearest at doses above 10 grams a day and after at least four weeks (van der Schoot et al., 2022). As a gel-forming fiber, it's also one of the supplements linked with cholesterol and blood sugar benefits (McRorie and McKeown, 2017).

Watch out for: it thickens quickly once mixed, so it needs enough liquid and should be drunk right away. Flatulence was more common with fiber than control treatments in that meta-analysis, so expect some gas at first.

PHGG (partially hydrolyzed guar gum)

What it is: guar gum broken into shorter chains, which makes it a water-soluble fiber that doesn't form a gel (Polymeros et al., 2014).

Best evidence for: bloating in IBS, with some data for constipation. In a double-blind trial, 6 grams a day for 12 weeks improved bloating scores compared with placebo in people with IBS, although it didn't change overall IBS severity or quality of life (Niv et al., 2016). In a four-week study of 49 adults with chronic constipation, PHGG shortened colonic transit time and increased bowel movements (Polymeros et al., 2014).

Watch out for: the trials are fairly small, and the constipation study had no placebo group.

Methylcellulose

What it is: a modified form of cellulose, sold as a bulk-forming laxative.

Best evidence for: constipation, though the research is older. In a 1988 study, 4 grams a day increased stool frequency, stool water and fecal solids in 50 healthy people. In 59 people with chronic constipation, daily doses of 1, 2 or 4 grams of methylcellulose, or 3.4 grams of psyllium, all significantly increased stool frequency, water content and fecal solids (Hamilton et al., 1988).

Watch out for: there's much less recent research than for psyllium, so it's harder to say how it compares for bloating or IBS.

Inulin and FOS

What they are: fast-fermenting soluble fibers, often extracted from chicory root and added to bars, gummies, drinks and "prebiotic" blends.

What they don't do: according to a detailed review of fiber supplements, they don't have a laxative effect and don't offer the cholesterol and blood sugar benefits of gel-forming fibers (McRorie and McKeown, 2017).

Watch out for: because they ferment quickly, they can be hard on a gut that bloats easily. Here's a closer look at why inulin can make bloating worse.

Wheat dextrin

What it is: a non-viscous soluble fiber derived from wheat.

What it doesn't do: according to the same review, wheat dextrin doesn't provide a laxative effect or the viscosity-related benefits of gel-forming fibers, and it can even be constipating (McRorie and McKeown, 2017).

Watch out for: it dissolves easily, which is its main appeal, but that doesn't make it the right tool for constipation.

Side by side

• For constipation: psyllium has the strongest evidence. Methylcellulose and PHGG have supporting studies, but they're older or smaller.
• For bloating with a sensitive gut: PHGG has placebo-controlled evidence in IBS. Psyllium is worth considering too, if you build up slowly.
• For cholesterol: gel-forming fibers such as psyllium.
• Best approached with caution if you bloat easily: inulin, FOS and other fast-fermenting "prebiotic" fibers.
• Least useful for regularity: wheat dextrin, inulin and FOS.

How to choose and start

1. Name your main goal: softer, more regular stools, less bloating, or support for cholesterol.
2. Pick one fiber that matches that goal, and check the full ingredient list for added inulin, chicory root or sugar alcohols.
3. Start with about half the label dose and build up over one to two weeks.
4. Take it with enough liquid, as directed on the label, and ask your pharmacist about timing around medications.
5. Give it four weeks before deciding, and track bowel movements and bloating along the way.

If constipation or bloating is new or persistent, or comes with blood in your stool, weight loss, anemia or severe pain, see your doctor before reaching for any supplement.

Sources

1. McRorie JW, McKeown NM. Understanding the physics of functional fibers in the gastrointestinal tract: an evidence-based approach to resolving enduring misconceptions about insoluble and soluble fiber. J Acad Nutr Diet. 2017;117(2):251-264. PMID 27863994
2. van der Schoot A, Drysdale C, Whelan K, Dimidi E. The effect of fiber supplementation on chronic constipation in adults: an updated systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2022;116(4):953-969. PMID 35816465
3. Polymeros D, Beintaris I, Gaglia A, et al. Partially hydrolyzed guar gum accelerates colonic transit time and improves symptoms in adults with chronic constipation. Dig Dis Sci. 2014;59(9):2207-14. PMID 24711073
4. Niv E, Halak A, Tiommny E, et al. Randomized clinical study: partially hydrolyzed guar gum (PHGG) versus placebo in the treatment of patients with irritable bowel syndrome. Nutr Metab (Lond). 2016;13:10. PMID 26855665
5. Hamilton JW, Wagner J, Burdick BB, Bass P. Clinical evaluation of methylcellulose as a bulk laxative. Dig Dis Sci. 1988;33(8):993-8. PMID 2839321

This article is for general information and isn't medical advice. Talk to your healthcare provider or pharmacist before starting a fiber supplement, especially if you take medication or have a digestive condition.