Fiber
Dietary fiber is carbohydrate that escapes digestion in the small intestine. What a fiber does in the body depends on two physical properties, viscosity and fermentability, more than on the "soluble" or "insoluble" label on the package. Average US intake is about half the Adequate Intake.
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What It Does
Viscous, gel-forming fibers such as psyllium, oat beta-glucan, and guar gum thicken intestinal contents, which slows glucose absorption and traps bile acids. The liver replaces those bile acids from cholesterol, pulling LDL out of circulation. Fibers that form a gel and resist fermentation, psyllium most of all, also hold water all the way through the colon, which softens hard stool and firms loose stool (McRorie 2017). Fermentable fibers such as inulin and partially hydrolyzed guar gum feed colonic bacteria, which produce short-chain fatty acids, including butyrate, the main fuel of colon cells. Fermentation also produces gas.
The observational evidence for dietary fiber is consistent. A Lancet series of meta-analyses found that people eating 25-29 g a day had 15-30% lower all-cause mortality, coronary disease, type 2 diabetes, and colorectal cancer than those eating the least (Reynolds 2019).
For supplements, psyllium has the most trial data. About 10 g a day lowered LDL by roughly 13 mg/dL in a meta-analysis of 28 trials (Jovanovski 2018). In type 2 diabetes it lowered fasting glucose and HbA1c, with no effect in people with normal glucose; the benefit was proportional to how poor control was (Gibb 2015). In irritable bowel syndrome, soluble fiber such as psyllium helped and wheat bran did not (Moayyedi 2014).
The diversity argument holds that eating many different plant fibers produces a more diverse microbiome. American Gut participants who ate more than 30 plant types a week had more diverse gut bacteria than those eating fewer than 10 (McDonald 2018), but that cohort was self-selected and observational. In a 10-week randomized diet study, a high-fiber diet did not increase microbiome diversity, while a fermented-food diet did (Wastyk 2021). Varied whole-food fiber is reasonable advice; the microbiome rationale for it is thinner than often claimed.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Absorption | Typical dose | Cost |
|---|---|---|---|
| Psyllium Huskispaghula, Plantago ovata | Not absorbedSoluble, gel-forming, minimally fermented. The gel holds water to soften hard stool and firms loose stool, and traps bile acids, which lowers LDL cholesterol. | 5-10 g powder 1-3 times daily with ample water | Low |
| MethylcelluloseCitrucel | Not absorbedSemisynthetic, soluble, non-fermented fiber that adds water-holding bulk to stool. | 2 g 1-3 times daily | Low |
| Inulinchicory root fiber, fructooligosaccharides (short-chain) | Not absorbedSoluble, non-viscous fructan rapidly fermented by colonic bacteria, selectively increasing bifidobacteria and producing short-chain fatty acids. | 5-10 g daily, increased gradually | Low |
| Wheat DextrinBenefiber, resistant dextrin | Not absorbedSoluble, non-viscous, readily fermented. Dissolves clear in liquids. | 3-6 g 1-3 times daily | Low |
| Partially Hydrolyzed Guar GumPHGG, Sunfiber | Not absorbedGuar gum enzymatically shortened to reduce viscosity. Fermented slowly, which limits gas. | 5-6 g daily | Moderate |
| Glucomannankonjac fiber, konjac root | Not absorbedExtremely viscous soluble fiber that absorbs many times its weight in water, slowing gastric emptying. | 1 g before meals with a full glass of water | Low |
Psyllium is viscous, gel-forming, and poorly fermented, which is why it lowers LDL, normalizes stool, and causes relatively little gas. It carries an FDA-authorized health claim for coronary heart disease. Methylcellulose forms a gel without being fermented: a gas-free laxative without psyllium's cholesterol effect. Inulin and wheat dextrin are soluble but non-viscous; they are fermented, do not bulk stool, and do not lower LDL. Partially hydrolyzed guar gum ferments slowly and is often better tolerated than inulin in IBS. Glucomannan is highly viscous; its weight-loss effects are small and inconsistent, and tablets taken without enough water can lodge in the esophagus.
Typical Dosing
| Group | Adequate Intake (AI) |
|---|---|
| Men 19-50 | 38 g |
| Men 51+ | 30 g |
| Women 19-50 | 25 g |
| Women 51+ | 21 g |
| Upper limit (UL) | None established |
The AI is based on 14 g per 1,000 kcal. Increase supplemental fiber gradually and take it with plenty of water.
Food sources: beans and lentils, whole grains, vegetables, fruit, nuts and seeds.
What to Look For
The panel lists grams of fiber per serving; the ingredient list tells you which fiber, and that matters more. A "fiber gummy" usually provides 2-5 g of inulin or polydextrose, which is neither a meaningful dose nor a gel-forming fiber.
Start low, 3-5 g a day, and increase by a few grams each week, with a full glass of water per dose. Take fiber at least two hours apart from medications, since it can reduce their absorption. Flavored psyllium products often contain added sugar; plain husk or sugar-free versions avoid it.
Red flags: "prebiotic" blends providing 1-3 g, cholesterol claims attached to non-viscous fibers, and glucomannan capsules sold for weight loss.
Psyllium Husk
The most studied supplemental fiber. It lowers LDL and normalizes stool in both directions, and it improves glycemic control in proportion to how poor that control is.
What to look for: Plain psyllium husk or powder with grams of psyllium per serving, sugar-free for daily use. Start at 3-5 g.
Find on iHerb Affiliate linkMethylcellulose
A gas-free laxative, and nothing more.
What to look for: Useful when psyllium causes bloating. No cholesterol or glucose effect.
Find on iHerb Affiliate linkPartially Hydrolyzed Guar Gum
Fermentable but slow, so it produces less gas than inulin. Small IBS trials favor it.
What to look for: Sunfiber is the most studied ingredient. About 5-6 g a day.
Find on iHerb Affiliate linkProducts on Titrate
2,168 of the 214,745 labels in the NIH Dietary Supplement Label Database list fiber as an active ingredient (1,232 on market). 322 are sold as fiber products by name.
Top on-market labels, ranked by label completeness and simplicity:
Fiber does not have its own search category yet. Each product above links to its full label in the NIH database.
Related Tools
- Fiber Diversity Score
- Macro Calculator
- Methylcellulose vs Psyllium Husk
- Partially Hydrolyzed Guar Gum vs Psyllium Husk
- Glucomannan vs Psyllium Husk
- Inulin vs Partially Hydrolyzed Guar Gum
- Inulin vs Psyllium Husk
- Methylcellulose vs Wheat Dextrin
- Partially Hydrolyzed Guar Gum vs Wheat Dextrin
- Psyllium Husk vs Wheat Dextrin
- All fiber form comparisons
- All supplement guides
References
Institute of Medicine. National Academies Press, 2005. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids.
Moayyedi P, et al. The effect of fiber supplementation on irritable bowel syndrome: a systematic review and meta-analysis. Am J Gastroenterol. 2014;109(9):1367-1374.
Gibb RD, et al. Psyllium fiber improves glycemic control proportional to loss of glycemic control: a meta-analysis of data in euglycemic subjects, patients at risk of type 2 diabetes mellitus, and patients being treated for type 2 diabetes mellitus. Am J Clin Nutr. 2015;102(6):1604-1614.
McRorie JW Jr, 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.
Jovanovski E, et al. Effect of psyllium (Plantago ovata) fiber on LDL cholesterol and alternative lipid targets, non-HDL cholesterol and apolipoprotein B: a systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2018;108(5):922-932.
McDonald D, et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems. 2018;3(3):e00031-18.
Reynolds A, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393(10170):434-445.
Wastyk HC, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137-4153.
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