Fiber Types: Psyllium vs Inulin vs Benefiber vs Citrucel
For constipation, choose psyllium. It forms a gel that softens stool, and it has the best evidence of any fiber supplement. Inulin and wheat dextrin (Benefiber) do not act as laxatives.
By Erin Rose Updated October 12, 2026 How we score
Safety: Take psyllium with a full glass of water. Do not take it if you have trouble swallowing. Take other medicines 2 hours before or after. All the cautions.
The types side by side
Two things decide what a fiber does in your gut: does it form a gel, and do gut bacteria ferment it? A gel holds water in stool and softens it; fermenting makes gas. To work as a laxative, a fiber has to hold water and resist fermentation (McRorie and McKeown 2017).
| Type | Gels? | Fermented (gas)? | Constipation | Other uses |
|---|---|---|---|---|
| Psyllium huskKonsyl, Metamucil, NOW, Yerba Prima | Yes | Resists fermentation; it stays in stool and holds water there (McRorie and McKeown 2017) | Of the fibers with trials in US gut doctors’ 2023 guideline (psyllium, bran and inulin), only psyllium appears to be effective: about 2 more bowel movements a week than placebo (low-certainty evidence). UK dietitians strongly recommend it. | Studied for IBS, cholesterol and blood sugar; see the psyllium page |
| MethylcelluloseCitrucel | Yes | No: a non-fermentable gelling agent (Dellschaft 2026) | Worked as a laxative at doses as low as 1 g a day in an old, small trial (Hamilton 1988). The 2023 US guideline found no trials it could use. | An option if psyllium gives you gas, since gut bacteria do not ferment it |
| Wheat dextrinBenefiber | No; dissolves fully | Yes | Does not act as a laxative, and can be constipating (McRorie and McKeown 2017). | Adding fiber you cannot taste |
| Inulin (chicory root), FOSNOW Inulin, Nutricost Inulin; most fiber gummies | No | Yes | Little to no effect on bowel movements in the US guideline; UK dietitians found it softened stool but did not raise frequency, and made gas worse at 12 to 15 g a day. | Feeds gut bacteria (raises Bifidobacterium), in a review where two authors work for an inulin maker (Nagy 2023) |
| Partially hydrolyzed guar gum (PHGG)Sunfiber, Regular Girl, Thorne | — | — | — | In one trial in IBS, 6 g a day eased bloating more than placebo; overall symptoms did not change (Niv 2016) |
| Acacia (gum arabic)NOW Acacia, Garden of Life Organic Fiber | — | — | — | Adding fiber to food or drinks |
| Wheat branFood, not a supplement here | No | — | Coarse bran can help by irritating the gut wall; finely ground bran can harden stool (McRorie and McKeown 2017; AGA-ACG 2023). | Not advised for IBS (ACG 2021) |
| Glucomannan (konjac)Sold as capsules for weight loss | Swells with water (two of the three labels we read carry choking warnings) | — | — | Weight loss: no meaningful effect, about 0.2 kg (Onakpoya 2014) |
Methylcellulose (Citrucel) also gels, and gut bacteria do not ferment it. Of the fibers US gut doctors could find trials for (psyllium, bran and inulin), only psyllium appears to be effective (AGA-ACG 2023).
| Item | Value |
|---|---|
| Psyllium husk | $0.51 |
| Inulin | $0.75 |
| Wheat dextrin | $1.07 |
| PHGG (guar fiber) | $1.39 |
| Methylcellulose | $4.40 |
- Psyllium husk: NOW Whole Psyllium Husks, $0.51 per 10 g
- Methylcellulose: Citrucel Methylcellulose Fiber Caplets 500 mg, $4.40 per 10 g
- Wheat dextrin: Benefiber Prebiotic Fiber Powder, Unflavored, $1.07 per 10 g
- Inulin: NOW Organic Inulin Prebiotic Powder, $0.75 per 10 g
- PHGG (guar fiber): Tomorrow's Nutrition Sunfiber, $1.39 per 10 g
Psyllium: the one with the evidence
US gut doctors’ 2023 guideline looked at fiber supplements for chronic constipation and found psyllium effective: about 2 more bowel movements a week than a placebo, from low-quality evidence (AGA-ACG 2023). UK dietitians strongly recommend it and found doses over 10 g a day, taken for at least 4 weeks, worked best (BDA 2025). For IBS, guidelines advise soluble fiber such as psyllium over bran (ACG 2021). More on psyllium.
Metamucil is psyllium with a sweetener: Metamucil 4-in-1 Psyllium Powder, Sugar-Free Orange, 3 g of fiber a rounded teaspoon, up to 3 a day on its label.
Methylcellulose (Citrucel)
A gel-forming fiber that gut bacteria do not ferment (Dellschaft 2026), so it is the one to try if psyllium gives you gas. In an older trial in constipated people it worked as a laxative at doses as low as 1 g a day (Hamilton 1988), but the 2023 US guideline found no trials it could use. Citrucel is sold as a drug; we did not find it on a testing list. Citrucel Methylcellulose Fiber Powder.
Wheat dextrin (Benefiber) and inulin
Both dissolve fully and do not gel, and gut bacteria ferment both. Reviews find fermented fibers like these do not act as laxatives; wheat dextrin can even constipate (McRorie and McKeown 2017). Inulin had little to no effect on bowel movements in the US guideline; UK dietitians found it softened stool but did not raise frequency, and made gas worse at 12 to 15 g a day (BDA 2025). Inulin does feed gut bacteria (it raises Bifidobacterium), in a review where two of the authors work for an inulin maker (Nagy 2023).
NOW inulin vs Benefiber: buy Benefiber
For most people, buy Benefiber over NOW inulin. Benefiber is on NSF/ANSI 173, so an outside program checks that the tub holds what the label says. It dissolves fully, with no taste. NOW Organic Inulin is on no testing list, and inulin made gas worse at 12 to 15 g a day (BDA 2025). NOW inulin does cost less: $0.75 per 10 g of fiber against $1.07.
Choose NOW inulin if you want a prebiotic, a fiber that feeds gut bacteria. Neither one is for constipation; for that, buy psyllium.
- Benefiber Prebiotic Fiber Powder, Unflavored: 3 g of fiber in 2 teaspoons (4 g), $0.32 a serving at list price.
- NOW Organic Inulin Prebiotic Powder: 4 g of fiber in 1 level teaspoon (about 4 g), $0.30 a serving at list price.
Guar fiber (PHGG) and acacia
PHGG (sold as Sunfiber) dissolves clear. In one trial in people with IBS, 6 g a day eased bloating more than a placebo, though overall symptoms did not change (Niv 2016). For acacia, we found no trial we could cite for constipation.
Wheat bran and glucomannan
Coarse wheat bran can help constipation by irritating the gut wall; finely ground bran can harden stool, and bran is not advised for IBS (McRorie and McKeown 2017; AGA-ACG 2023; ACG 2021). Glucomannan is sold for weight loss, but a review of trials found no real effect, about 0.2 kg (Onakpoya 2014).
Questions
Which type of fiber is best for constipation?
Psyllium. Of the fibers with trials, it is the only one US gut doctors’ 2023 guideline found effective, and UK dietitians strongly recommend it (AGA-ACG 2023; BDA 2025). Methylcellulose (Citrucel) also worked in an older trial.
Which fiber causes the least gas?
Fibers that gut bacteria do not ferment, because fermentation is what makes fiber gas (ACG 2021). Methylcellulose is not fermented (Dellschaft 2026), and psyllium resists fermentation (McRorie and McKeown 2017). Inulin is fermented and made gas worse at 12 to 15 g a day (BDA 2025).
Is Benefiber as good as Metamucil?
Not for constipation. Benefiber is wheat dextrin, which dissolves fully and does not act as a laxative (McRorie and McKeown 2017). Metamucil is psyllium, the fiber with constipation evidence. Benefiber is fine if you just want fiber you cannot taste.
Is inulin a good fiber supplement?
It feeds gut bacteria, but it is not a constipation fix: it did not raise bowel movements in the US guideline or the UK review, and it causes gas (AGA-ACG 2023; BDA 2025). Start with a few grams. In one study of 26 healthy adults, a single 10 g dose at breakfast caused only mild gas and bloating (Bonnema 2010).
NOW inulin or Benefiber: which should I buy?
Benefiber, for most people. It is on NSF/ANSI 173, so an outside program checks what is in the tub, and it dissolves with no taste. NOW Organic Inulin is on no testing list, though it costs less per gram of fiber. Neither works as a laxative; for constipation, buy psyllium.
What is the difference between soluble and insoluble fiber?
Soluble fiber dissolves or swells in water; insoluble fiber does not. What matters more for a supplement is whether it forms a gel and resists fermentation: a fiber needs both to soften stool as a laxative (McRorie and McKeown 2017).
More: the best fiber supplement, how much fiber a day and psyllium husk vs powder vs capsules.
Sources
- Chang L, et al. AGA-ACG Clinical Practice Guideline: Pharmacological Management of Chronic Idiopathic Constipation. Gastroenterology. 2023. PubMed 37211380
- Dimidi E, et al. British Dietetic Association guidelines for the dietary management of chronic constipation in adults. J Hum Nutr Diet. 2025. PubMed 41081513
- Lacy BE, et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol. 2021. PubMed 33315591
- McRorie JW Jr, McKeown NM. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract. J Acad Nutr Diet. 2017. PubMed 27863994 (McRorie is listed at Procter & Gamble, which makes Metamucil, on a 2015 paper (Gibb 2015, PubMed 26561625).)
- Dellschaft N, et al. Randomised Placebo-Controlled Trial of Methylcellulose and Psyllium Gels on Inulin-Induced Colonic Gas. Aliment Pharmacol Ther. 2026. PubMed 42728685
- Hamilton JW, et al. Clinical evaluation of methylcellulose as a bulk laxative. Dig Dis Sci. 1988. PubMed 2839321
- FDA. 21 CFR 201.319, warning for bulk fibers such as psyllium (eCFR, read October 6, 2026). Metamucil drug facts label (DailyMed, read October 6, 2026).
- Nagy DU, et al. Effect of chicory-derived inulin-type fructans on abundance of Bifidobacterium and on bowel function. Crit Rev Food Sci Nutr. 2023. PubMed 35833477 (co-authors from BENEO, an inulin maker). Bonnema AL, et al. J Am Diet Assoc. 2010. PubMed 20497775
- Niv E, et al. Nutr Metab (Lond). 2016. PubMed 26855665
- Onakpoya I, et al. The efficacy of glucomannan supplementation in overweight and obesity. J Am Coll Nutr. 2014. PubMed 24533610
- Prices: each brand’s own list price, or Walgreens, iHerb, Vitacost or Sam’s Club where named, checked October 6, 2026.