Fiber Compared: Psyllium vs Inulin vs Methylcellulose vs Wheat Dextrin
Informational summary of published research — not medical advice. If you have a GI condition, check with your clinician before adding a fiber supplement.
Quick answer
The type of fiber matters more than the gram count. Psyllium is the best all-rounder — it is the one fiber with an FDA heart-health claim and it works for constipation and IBS. Methylcellulose (Citrucel) if psyllium gives you gas. Inulin only to feed gut bacteria — not for constipation, and avoid it if you have IBS. Wheat dextrin dissolves clear but is a weak laxative. Match the fiber to the job.
Which fiber for your goal?
The fiber types, side by side
| Fiber | Soluble? | Gels? | Fermentable (gas) | Best for | Avoid if |
|---|---|---|---|---|---|
| Psyllium husk | Mostly soluble | Yes — gels | Low (survives the colon) | Constipation, cholesterol (FDA claim), IBS | Esophageal narrowing/dysphagia; low fluid intake |
| Inulin / FOS (chicory) | Soluble | No | High + fast (most gas) | Feeding gut bacteria (prebiotic) | IBS, SIBO, FODMAP sensitivity |
| Methylcellulose (Citrucel) | Soluble | Yes — gels | None (not fermented) | Constipation with the least gas | Anyone wanting a prebiotic effect (there is none) |
| Wheat dextrin (Benefiber) | Soluble | No | Moderate (less gas than inulin) | Adding fiber invisibly (dissolves clear) | Relying on it to relieve constipation |
| Beta-glucan (oat/barley) | Soluble | Yes — gels | Fermentable | Cholesterol, post-meal blood sugar | Celiac (use certified gluten-free oats) |
| Acacia / gum arabic | Soluble | No (stays thin) | Slow + distal (best tolerated) | A gentle prebiotic at higher doses | Expecting proven weight/lipid outcomes |
| Glucomannan (konjac) | Soluble | Very — most gel-forming | Slow | Modest short-term weight/LDL support | Swallowing trouble; tablets dry — choking/obstruction risk |
| PHGG (guar gum, partially hydrolyzed) | Soluble | No | Gentle (low gas) | Bloating in IBS | Expecting a global IBS cure |
Machine-readable version: /fiber/forms-compared.json (CC BY 4.0).
Three properties explain every fiber
Forget "soluble vs insoluble" as the headline — it is a weak predictor of what a fiber actually does. What matters is a combination of three properties (McRorie & McKeown, 2017):
- Does it gel (viscosity)? Gel-forming fibers hold water in the stool and slow digestion — that is what softens stool and lowers cholesterol and post-meal glucose. Psyllium, methylcellulose, beta-glucan and glucomannan gel; inulin and wheat dextrin do not.
- Is it fermented? Bacteria fermenting fiber is what produces gas. Highly fermentable fibers (inulin, FOS) feed the microbiome but cause the most bloating; barely-fermented fibers (psyllium, methylcellulose) cause the least.
- Does it survive the colon? A fiber that is fully fermented is gone before it can bulk the stool — which is why fermentable, non-viscous "fibers" like wheat dextrin are weak laxatives despite the label.
This is why psyllium is the standout: it gels (so it works), it is barely fermented (so little gas), and it survives the colon (so it bulks) — the one combination that does all three jobs. It carries the only FDA-authorized fiber heart-health claim, at 7 g/day of soluble fiber (21 CFR 101.81).
The IBS trap: not all fiber helps, and one type hurts
"Eat more fiber" is the wrong advice for IBS. A meta-analysis of 22 trials found soluble fiber (psyllium) improved global IBS symptoms and pain, while insoluble fiber showed no benefit (Nagarajan 2015). Worse, inulin and FOS are fructans — a core high-FODMAP trigger that reliably provokes bloating, gas and pain in sensitive people (Staudacher 2017). So the same "fiber" aisle contains both the fiber recommended for IBS (psyllium) and one of the fibers most likely to flare it (inulin). If you have IBS, the answer is a specific fiber, not more of any fiber. Interestingly, in a small trial of 19 IBS patients, adding psyllium to inulin reduced the gas inulin produced (Gunn 2022) — an early finding, not yet general guidance.
What the real thing costs
Because psyllium works at the studied doses (7 g/day soluble for cholesterol; >10 g/day for constipation), the honest value question is cost per gram of fiber, not price per tub. Plain psyllium powder is dramatically cheaper per gram than branded blends or methylcellulose:
| Product | Fiber / serving | Price | Cost / gram fiber |
|---|---|---|---|
| NOW Organic Inulin Prebiotic Powder (8 oz) | 3 g | $10.00 | $0.043 |
| NOW Psyllium Husk Powder, Organic (12 oz) | 7 g | $14.10 | $0.053 |
| Konsyl Daily Psyllium Fiber Powder (19 oz) | 5 g | $25.99 | $0.058 |
| Benefiber Prebiotic Fiber Powder (wheat dextrin) | 3 g | $13.48 | $0.073 |
| Metamucil 4-in-1 Psyllium, Sugar-Free (72 servings) | 3.4 g | $17.97 | $0.074 |
| Citrucel Methylcellulose Fiber Powder | 2 g | $18.59 | $0.310 |
NOW Organic Inulin Prebiotic Powder (8 oz) works out to about $0.04/gram of fiber — roughly 7× cheaper per gram than methylcellulose (Citrucel). You are paying for flavoring, convenience and brand, not for more or better fiber.
If you want the best all-rounder (psyllium)
Pure organic psyllium husk. 7 g fiber (6 g soluble) per 9 g serving — just under the FDA 7 g/day soluble-fiber target (two servings clears it). Cheapest per gram of fiber in the category.
100% psyllium husk, 5 g fiber (3 g soluble) per 6 g dose, unflavored, no additives. 90 servings.
As an Amazon Associate we earn from qualifying purchases. Picks are ranked by cost per gram of fiber and evidence, never commissions. See the full best fiber supplement ranking.
Frequently asked questions
What is the best type of fiber supplement?
For most goals, psyllium husk — the only fiber with an FDA heart-health claim (7 g/day soluble), with RCT support for both constipation and IBS, and little gas because it is barely fermented. Pick a different fiber only for a reason: methylcellulose if psyllium is gassy, inulin only as a prebiotic (not for constipation, not with IBS), oat beta-glucan mainly for cholesterol.
Which fiber is best for constipation?
A gel-forming fiber that holds water: psyllium first, or methylcellulose (Citrucel) if psyllium causes gas. Avoid inulin and wheat dextrin for constipation — they are non-viscous and largely fermented, so they do not reliably bulk stool.
Is soluble or insoluble fiber better for IBS?
Soluble. A 22-trial meta-analysis found soluble fiber (psyllium) improved IBS symptoms and pain while insoluble fiber did not. And fermentable fibers like inulin/FOS are high-FODMAP IBS triggers — so the answer is soluble and low-fermentable (psyllium), not just more fiber.
Why does fiber make me gassy?
Gas is bacteria fermenting the fiber. Inulin and FOS ferment fast and produce the most; wheat dextrin is milder; psyllium and methylcellulose are barely or not fermented, so they cause the least. Ramping up slowly over 1-2 weeks with more water also cuts gas.
Related
- Fiber Dosage Guide — how much per day, and how to ramp without the bloat
- Best Fiber Supplement — ranked by cost per gram of fiber
- Psyllium: The Deep Dive — full cholesterol/glycemic/constipation evidence, dosing, and husk vs powder vs capsules
- All Fiber Guides
Sources
- McRorie JW Jr, McKeown NM. "Understanding the Physics of Functional Fibers in the Gastrointestinal Tract." J Acad Nutr Diet. 2017. PMID: 27863994
- Nagarajan N, et al. "The role of fiber supplementation in the treatment of irritable bowel syndrome." Eur J Gastroenterol Hepatol. 2015. PMID: 26148247
- Staudacher HM, Whelan K. "The low FODMAP diet: mechanisms and efficacy in IBS." Gut. 2017. PMID: 28592442
- Gunn D, et al. "Psyllium reduces inulin-induced colonic gas production in IBS." Gut. 2022. PMID: 34353864
- Timm DA, et al. "Wheat dextrin, psyllium, and inulin produce distinct fermentation patterns and gas volumes." J Med Food. 2010. PMID: 20482283
- van der Schoot A, et al. "The Effect of Fiber Supplementation on Chronic Constipation in Adults." Am J Clin Nutr. 2022. PMID: 35816465
- U.S. FDA. "Health claims: Soluble fiber from certain foods and risk of coronary heart disease." 21 CFR 101.81.