Insoluble fiber is the part of plant food that does not dissolve in water and that human digestive enzymes cannot break down. Its best-known examples are cellulose and lignin, the structural material of plant cell walls, and it is concentrated in wheat bran, whole grains, seeds, nuts and bean skins. It is one of the two kinds that make up dietary fiber, next to soluble fiber. Labs measure it with an enzyme-and-weighing method that splits fiber into an insoluble and a soluble part, and US labels may list it as a line under dietary fiber.
What it does is mostly mechanical. Coarse insoluble particles such as wheat bran pass through the gut largely intact, hold water and add bulk to stool, and a 2017 review says they also irritate the lining of the large bowel so that it secretes water and mucus. That is the basis of two European Union health claims for wheat bran fiber. The same review says insoluble fibers such as wheat bran lack the gel-forming viscosity that lowers cholesterol and blood sugar in soluble fibers such as psyllium.
Most readers do not know how modest bran is as a stool bulker. In a 1978 feeding study of 19 healthy volunteers, about 20 g a day of concentrated bran fiber raised stool weight by 12%, against 59% for carrot fiber and 69% for cabbage fiber. The other surprise is bran’s reputation in irritable bowel syndrome: in the largest trial, 10 g a day of bran did not ease symptoms more than placebo, and the people who dropped out early were mostly those whose symptoms had worsened.
Health benefits of insoluble fiber
The best-supported finding is the oldest: wheat bran fiber increases stool weight and speeds transit, which the EU recognizes in two authorized claims, and a pooled analysis of 65 studies found 3.7 g more stool a day for each extra gram of wheat fiber. It works less well in people who are already constipated. Beyond the bowel the evidence is thinner or points the other way: bran did not beat placebo for irritable bowel syndrome, a fiber-supplement trial in diverticular disease helped only constipation, and 25 g a day of wheat bran did not prevent new colon polyps. For heart disease, type 2 diabetes and colorectal cancer the support is observational, and cereal fiber comes packaged with the other parts of whole grains. Most trials tested wheat bran, so results are for bran, not for insoluble fiber as a class, and each card says what was tested. Here is what 22 studies and reports found, strongest evidence first, including the ones that found nothing. Seven more studies on forms of insoluble fiber are under Forms of insoluble fiber below.
The EU lets foods high in wheat bran fiber claim that it contributes to an increase in faecal bulk and to an acceleration of intestinal transit
Both claims are authorized in Regulation (EU) No 432/2012, based on a 2010 opinion of the European Food Safety Authority. Each may be used only on food that is high in that fiber, meaning at least 6 g of fiber per 100 g or at least 3 g per 100 kcal, and the transit claim also needs the consumer to be told that the effect comes with a daily intake of at least 10 g of wheat bran fiber. The register has related bowel claims for other cereal fibers: oat grain fibre and barley grain fibre contribute to an increase in faecal bulk, and rye fibre contributes to normal bowel function. These are rules for what a food label may say, not trial results.
Across 65 intervention studies, each extra gram of wheat fiber a day raised stool weight by 3.7 g a day
Systematic review with weighted regression, in generally healthy people. Dry stool weight rose by 0.75 g a day and stool frequency by 0.004 times a day per gram, and among people whose transit time was over 48 hours it fell by 0.78 hours per gram. Too few studies tested other cereal fibers to pool them. The review was funded by the Kellogg Company, which paid the authors for the review and the writing.
In a review of 6 trials in chronic constipation, the evidence for insoluble fiber was conflicting, while soluble fiber helped
Four of the randomized trials used soluble fiber and two used insoluble fiber, against placebo or no treatment. Soluble fiber improved global symptoms (86.5% against 47.4%) and raised stools from 2.9 to 3.8 a week. The authors did not pool the trials because of concerns about their quality, and data on adverse events were limited.
In 275 people with irritable bowel syndrome, 10 g a day of bran did not ease symptom severity more than placebo, while psyllium did
A 12-week trial in general practice gave 10 g a day of psyllium (soluble, 85 people), bran (insoluble, 97) or rice flour placebo (93). After 3 months symptom severity had fallen by 90 points on psyllium (P = 0.03 against placebo), 58 on bran (P = 0.61) and 49 on placebo. Bran beat placebo only in the third month (57% against 32% responders), and not in the worst-case analysis. Early dropout was most common on bran, mainly because symptoms worsened.
Show 18 more studies
In 58 people with diverticular disease, bran crispbread did no more than relieve constipation
Randomized, double-blind crossover trial of bran crispbread, an ispaghula (psyllium) drink and placebo, each for four months. Pain, lower bowel and total symptom scores were no better on fiber than on placebo, but constipation symptoms were relieved, and both fiber regimens changed stool weight, consistency and frequency as expected. The authors conclude that fiber supplements at the usual doses do no more than relieve constipation.
A systematic review found no high-quality evidence that a high-fiber diet treats diverticular disease
It included three randomized trials and one case-control study. One trial found no difference. A second trial and the case-control study favored a high-fiber diet for symptomatic disease, and the third trial favored methylcellulose, a fiber supplement, and also showed a placebo effect. No study of preventing recurrent diverticulitis with fiber met the inclusion criteria. Guidelines still recommend a high-fiber diet.
In 411 people after polyp removal, 25 g a day of wheat bran did not prevent new colorectal adenomas
Australian randomized, partly double-blind trial that tested lower dietary fat (25% of calories), wheat bran (25 g a day) and beta carotene (20 mg a day). Outcome data came from 390 people at 24 months and 306 at 48 months. No intervention prevented total new adenomas, and bran gave a small, non-significant fall in large adenomas (odds ratio 0.8 at both times). The finding of no large adenomas in the low-fat plus bran group (P = .03) rests on small numbers, and the authors urge caution. Larger fiber trials in people with polyps are on the fiber page.
In 24 people with chronic constipation, 20 g a day of wheat bran beat placebo for bowel frequency and transit time
Double-blind crossover trial with two 4-week periods after a 2-week baseline, with water and fiber intake standardized. Transit and frequency improved on both treatments compared with baseline, more on bran. Transit time became normal only in people with slow colonic transit, not in those with slow rectal transit, and the accompanying symptoms did not differ from placebo.
In 153 people who ate little fiber, 14 days of a wheat bran cereal improved self-rated bowel function and bloating
An open study with no control group: each person was compared with their own 14-day baseline. Participants ate under 15 g of fiber a day and then a bowl of cereal with at least 5.4 g of fiber, 3.5 g of it from wheat bran. Ratings of ease of defecation, bloating and constipation improved, and gains grew with the amount eaten. The Kellogg Company funded the study and two authors work for it.
In 17 overweight or obese women, 3 days of purified insoluble oat fiber improved insulin sensitivity
Randomized, single-blind crossover trial. White bread enriched with 31.2 g a day of insoluble fiber or plain white bread was eaten for 72 hours, with energy intake matched. Glucose uptake in the clamp test was 6.56 against 6.07 mg per minute per kg (P = 0.043), and 6.85 against 6.06 in the 13 people whose breath hydrogen rose, which suggests they ate the fiber. Body weight and blood fats did not change. The fiber was purified oat fiber, not bran, and 3 days is short.
In 18 people with symptomatic diverticular disease, a high-fiber diet gave more relief than the control diet over 3 months
Double-blind controlled trial. The high-fiber group had significantly greater symptom relief despite a marked placebo effect early on, and the benefit grew over the three months. It is a very small trial, and the larger crossover trial of 58 people above found no benefit beyond relieving constipation.
In people given radioactive iron, phytate was the main reason bran blocked iron absorption
Human absorption study using two radioiron tracers to follow non-heme iron from a meal. Washing bran with hydrochloric acid, but not with water, removed the inhibition, and putting phytate back restored most of it. Breaking down the phytate in bran with the bran’s own enzyme raised absorption significantly, and adding potassium and magnesium phytates in the amounts found in bran gave the same inhibition. Other factors in bran play a smaller part.
In 43,881 male health professionals, insoluble fiber went with a relative risk of diverticular disease of 0.63, and cellulose with 0.52
Prospective cohort of men aged 40 to 75 with no diverticular disease, polyps, colitis or cancer at the start, comparing the highest fifth of intake with the lowest. Total fiber gave a relative risk of about 0.60 whichever laboratory method defined it (AOAC, Southgate or Englyst). Cohorts cannot rule out other differences between men who eat more and less fiber.
In 22 cohort publications, insoluble fiber and cereal fiber were linked to lower risk of coronary heart disease
Dose-response meta-analysis of cohorts followed for at least 3 years. Total fiber gave a risk ratio of 0.91 per 7 g a day for both cardiovascular disease and coronary heart disease. Insoluble fiber and fiber from cereal and vegetable sources were linked to lower risk of both, and fruit fiber to lower risk of cardiovascular disease. Support for one author came from the Kellogg Marketing and Sales Company (UK).
In a German cohort and a meta-analysis of 9 cohorts, higher cereal fiber went with lower type 2 diabetes risk (0.67 in the meta-analysis), but fruit and vegetable fiber did not
In the Potsdam cohort, which recorded 844 new cases of diabetes, the highest fifth of cereal fiber intake had a relative risk of 0.72 against the lowest. In the meta-analysis, extreme categories gave 0.67 for cereal fiber, 0.96 for fruit fiber and 1.04 for vegetable fiber. Observational data cannot show that cereal fiber itself is the cause.
In 25 cohort studies, each extra 10 g a day of cereal fiber went with a relative risk of colorectal cancer of 0.90
Dose-response meta-analysis of prospective studies. Total fiber also gave 0.90 per 10 g a day, while fruit fiber (0.93) and vegetable fiber (0.98) were not significant. Three daily servings of whole grains gave 0.83. Cereal fiber and whole grains come with many other grain components, and observational data cannot separate them.
In 4,517 colorectal cancer cases in a European cohort, only fiber from cereals was linked to lower rectal cancer risk
Prospective cohort with a mean follow-up of 11.0 years and adjustment for body mass index, smoking, alcohol, red and processed meat and calcium, among others. Total fiber gave a hazard ratio of 0.87 per 10 g a day. Fiber from cereals and from fruit and vegetables were similarly linked to colon cancer, but for rectal cancer only cereal fiber was.
In 27,082 men followed for 8 years, each extra 20 g a day of bran went with 0.36 kg less weight gain
Prospective cohort of US men aged 40 to 75, with weight reported by the men themselves. Each 40 g a day more whole grain went with 0.49 kg less gain, and changes in cereal fiber and fruit fiber were also inversely linked to weight gain. Refined grains and added germ were not. The whole grain link remained after accounting for bran and fiber, so other grain components matter too.
In 45 studies, 3 daily servings (90 g) of whole grains went with a relative risk of death from any cause of 0.83
Meta-analysis of prospective studies of whole grains, a mixture of bran, starch and other parts, so the result cannot be credited to insoluble fiber alone. For the same 90 g, coronary heart disease risk was 0.81, cardiovascular disease 0.78 and cancer deaths 0.85. Whole grain bread, whole grain breakfast cereals and added bran were also linked to lower risk of cardiovascular disease or death, while there was little evidence for refined grains or white rice.
In 170,776 women followed for 26 years, fiber from cereals, whole grains or legumes did not change the risk of Crohn’s disease
Nurses’ Health Study. The highest fifth of total fiber intake (median 24.3 g a day) had a hazard ratio for Crohn’s disease of 0.59 against the lowest, and the fall was greatest for fruit fiber. Neither total fiber (0.82) nor fiber from any source was significantly linked to ulcerative colitis.
Of 100 people with irritable bowel syndrome who had tried bran, 55% said it made them worse and 10% that it helped
A questionnaire study of consecutive new referrals to one hospital clinic. Every symptom was made worse, bowel habit most often, then abdominal distension and pain. Other kinds of fiber, except fruit, were less harmful, and proprietary fiber supplements were found helpful. People referred to a specialist clinic may not represent everyone with the condition, and there was no comparison group.
In rats given a carcinogen, adding wheat bran to a raw potato starch diet suppressed tumor formation
Rats ate a basic low-fiber diet, a diet with raw potato starch (a resistant starch) as 20% of its carbohydrate, or the potato starch diet plus 10% wheat bran fiber. The potato starch diet gave more and larger tumors, and wheat bran suppressed this by acting on early precancer foci, although it did not stop the increase in cell growth. It is a rat model of colon cancer, not a human result.
Foods high in insoluble fiber
Insoluble fiber sits in the structural parts of plants: bran and seed coats, bean skins, nuts and seeds, and the skins and flesh of vegetables and fruit. The ranking below covers 22 foods with insoluble fiber values, taken from matched USDA FoodData Central Foundation records or, where there is none, from the Danish food database Frida. Per serving, black beans lead with 10.7 g in 1 cup (172 g), followed by chia seeds (10.1 g in 1 oz), chickpeas (8.86 g in 1 cup), pears (4.98 g in 1 medium pear, 178 g), avocados (4.15 g in half an avocado) and blueberries (3.26 g in 1 cup). Per 100 g the top is chia seeds (35.6 g), then sesame seeds (9.95 g), almonds (9.05 g), sunflower seeds (8.05 g) and hazelnuts (7.9 g). Per 100 kcal the leaders are watery vegetables and fruit: broccoli (7.74 g), chia seeds (7.31 g), zucchini (6.4 g), pears (4.91 g), black beans (4.72 g) and carrots (4.63 g).
Wheat bran is not in the ranking, because only 22 foods have values. In FoodData Central, crude wheat bran has 42.8 g of total dietary fiber per 100 g, which counts all its fiber, not only the insoluble part. In a meta-analysis of cohorts, whole grain bread, whole grain breakfast cereals and added bran went with lower risks of cardiovascular disease or death, while refined grains and white rice showed little evidence, and the Dietary Guidelines for Americans 2025-2030 tell people to prioritize fiber-rich whole grains. Different databases measure fiber with different methods, and the 1988 interlaboratory study that tested the AOAC method for insoluble and soluble fiber said the separate measurement needed further study, so treat a gap of a gram or so between two foods with care (our reading). No daily need is set for insoluble fiber, so the percentages in the ranking compare each food with the top food, not with a target.
Per serving. No daily amount is set, so bars compare each food with the top food.
Per 100 g. No daily amount is set, so bars compare each food with the top food.
Per 100 kcal: the same calories of each food, for a fair comparison. Bars compare each food with the top food.
Food labels have no Daily Value or Reference Intake for insoluble fiber, so the % compares each food with the top food.
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Sources
- U.S. Department of Agriculture, Agricultural Research Service. FoodData Central. Foundation Foods records matched to each food, and the entry for wheat bran, crude (FDC 169722, SR Legacy). Values for the ranking as shown above.
- Langwagen M, Jacobsen J, Christensen T, Poulsen A. The Danish Food Composition Database, Frida, version 5.5. National Food Institute, Technical University of Denmark. 2025.
- Prosky L, Asp NG, Schweizer TF, et al. Determination of insoluble, soluble, and total dietary fiber in foods and food products: interlaboratory study. J Assoc Off Anal Chem. 1988.
- Aune D, Keum N, Giovannucci E, et al. Whole grain consumption and risk of cardiovascular disease, cancer, and all cause and cause specific mortality: systematic review and dose-response meta-analysis of prospective studies. BMJ. 2016.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2025-2030.
How insoluble fiber works
Insoluble fiber is not absorbed, so what it does comes from its physical form and from how little of it gut bacteria use. Most of the evidence in this section is gut physiology from small studies and reviews, and the trials of health outcomes are in the cards above. Three ideas cover most of it: it resists enzymes and, mostly, fermentation; it holds water and adds bulk, with the source of the fiber mattering; and particle size changes the result. Here is each one, and what has been seen in people.
It resists enzymes and, mostly, fermentation
That is the definition. FDA defines dietary fiber as non-digestible carbohydrates plus lignin that are intrinsic and intact in plants, and the 1976 paper that introduced the term described dietary fiber as the residue of plant food resistant to human digestive enzymes. In the large bowel, a 2015 review of 65 studies attributes the stool-bulking effect of wheat bran largely to its high resistance to fermentation by colon bacteria, together with its water binding: 1 g of fiber binds about 3 g of water. The review contrasts wheat bran with more easily fermented fibers such as those from oats and barley.
Four steps by which coarse wheat bran adds to stool, as described in a 2015 review and a 2017 review. A diagram, not to scale.
It adds bulk, and the source of the fiber matters
Wheat bran is the classic stool bulker, yet it is not the strongest one per gram. In a 1978 controlled feeding study, 19 healthy volunteers added about 20 g a day of concentrated fiber from carrot, cabbage, apple, bran or guar gum to the same basic diet. Stool weight rose by 12% on bran, against 69% on cabbage, 59% on carrot, 40% on apple and 20% on guar gum. The rises tracked how much pentose-containing polysaccharide each fiber supplied, and people differed a lot in how much their stool weight changed. A 2015 pooled analysis of wheat fiber trials found 3.7 g of extra stool a day for each extra gram, so the bran effect is real, and its size depends on what it is compared with.
Rise in stool weight in 19 healthy volunteers, drawn to scale from zero. The teal bar is wheat bran; the gray bars are the other fibers.
Particle size changes the result
Coarse bran bulks stool more than fine bran, but fine bran still bulks it. In 21 healthy people, 20 g a day of coarse bran gave a mean stool weight of 219.4 g a day against 199 g for the same bran milled fine, and the coarse bran held nearly twice as much water. Two later crossover trials found that bran ground as fine as 50 micrometers still raised stool bulk, with no significant difference between particle sizes. The 2017 review says that large, coarse particles mechanically irritate the gut lining, and that fine, smooth bran particles can be constipating. We found no trial in which fine bran constipated people. Shape and size matter in a more basic way too: in 18 volunteers, swallowed plastic flakes shaped like bran cut whole-gut transit time by 24%, while small plastic granules did not change transit significantly.
Coarse and fine bran at 20 g a day in 21 healthy people. Each pair of bars is drawn to scale from zero, and the two pairs use different units.
A little is fermented, more when the bran is ground fine
The finest bran in the crossover trials gave higher fecal butyrate and breath methane than coarser bran, which suggests that bacteria ferment more of it. A month of finely milled wheat bran at 20 g a day raised fasting blood short-chain fatty acids in 14 obese adults, but it changed no health marker the researchers measured, including stool output, transit, cholesterol, triglycerides and C-reactive protein, so more fermentation did not bring a measurable benefit there.
What has been seen in people
The clearest effects are in the bowel: more stool, faster transit, and a smaller effect in people who are already constipated. In irritable bowel syndrome the 2009 primary-care trial gave a different picture for soluble and insoluble fiber, shown in the chart. For cholesterol and blood sugar the 2017 review finds that insoluble fibers such as wheat bran lack the viscosity that soluble gel-forming fibers use, and the one short insulin trial here used purified oat fiber at 31.2 g a day for 3 days. The lower risks of diverticular disease, heart disease, diabetes and colorectal cancer come from cohorts, where cereal fiber is hard to separate from whole grains as a whole.
Fall in symptom severity score in a 12-week trial of 275 people with irritable bowel syndrome, drawn to scale from zero. The teal bar differed significantly from placebo; the gray bars did not (bran, P = 0.61).
Sources
- de Vries J, Miller PE, Verbeke K. Effects of cereal fiber on bowel function: A systematic review of intervention trials. World J Gastroenterol. 2015.
- 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.
- Trowell H. Definition of dietary fiber and hypotheses that it is a protective factor in certain diseases. Am J Clin Nutr. 1976.
- U.S. Food and Drug Administration. Questions and answers on dietary fiber.
- Cummings JH, Branch W, Jenkins DJ, et al. Colonic response to dietary fibre from carrot, cabbage, apple, bran. Lancet. 1978.
- Brodribb AJ, Groves C. Effect of bran particle size on stool weight. Gut. 1978.
- Jenkins DJ, Kendall CW, Vuksan V, et al. The effect of wheat bran particle size on laxation and colonic fermentation. J Am Coll Nutr. 1999.
- Lewis SJ, Heaton KW. Roughage revisited: the effect on intestinal function of inert plastic particles of different sizes and shape. Dig Dis Sci. 1999.
- Deroover L, Vázquez-Castellanos JF, Vandermeulen G, et al. Wheat bran with reduced particle size increases serum SCFAs in obese subjects without improving health parameters compared with a maltodextrin placebo. Am J Clin Nutr. 2021.
- Bijkerk CJ, de Wit NJ, Muris JW, et al. Soluble or insoluble fibre in irritable bowel syndrome in primary care? Randomised placebo controlled trial. BMJ. 2009.
- Weickert MO, Möhlig M, Schöfl C, et al. Cereal fiber improves whole-body insulin sensitivity in overweight and obese women. Diabetes Care. 2006.
How much insoluble fiber do you need?
No daily need is set for insoluble fiber on its own. What covers it is the target for total fiber: adults need about 25 to 38 g of fiber a day in the US, which is 14 g per 1,000 kcal, and at least 25 g a day in the EU and by WHO. We found no source that splits the target between insoluble and soluble fiber. On a 2,000 kcal diet, 14 g per 1,000 kcal is 28 g, which is also the FDA Daily Value for fiber (our arithmetic). US labels may list insoluble fiber voluntarily, and must list it when a label makes a claim about it, but FDA’s Daily Value is set for dietary fiber as a whole.
The US value comes from the National Academies and is an adequate intake, based on studies of protection against cardiovascular disease rather than on clinical studies of fiber, as the American Dietetic Association position paper notes. It adds that the usual US intake is only 15 g a day. The 2025-2030 Dietary Guidelines for Americans set no gram goal for fiber in their guidance pages but say to prioritize fiber-rich whole grains, with 2 to 4 servings of whole grains a day, adjusted to calorie needs.
| Source | Group | Value | Notes |
|---|---|---|---|
| National Academies 2005, adequate intake (as reported in the position paper) | Adults | 14 g per 1,000 kcal; 25 g (women), 38 g (men) a day | Total fiber. Based on studies of protection against cardiovascular disease. Source 1. |
| Dietary Guidelines for Americans 2020-2025, Appendix 1, Table A1-2 | Ages 2 and older, by calorie level | 14 g per 1,000 kcal | The table lists 28 g for women aged 19 to 30 and 34 g for men aged 19 to 30 at its calorie levels. Dietary fiber is called a dietary component of public health concern. Source 2. |
| Dietary Guidelines for Americans 2025-2030 | Adults | No gram goal for fiber | Says to prioritize fiber-rich whole grains; goal of 2 to 4 servings of whole grains a day, adjusted to calorie needs. Source 3. |
| EFSA 2010, adequate intake | Adults from 18 years | 25 g a day | Children: 10 g (1 to 3 years), 14 g (4 to 6), 16 g (7 to 10), 19 g (11 to 14), 21 g (15 to 17). Source 4. |
| WHO 2023 | Adults | At least 25 g a day of naturally occurring dietary fiber | Children: at least 15 g (2 to 5 years), 21 g (6 to 9), 25 g (10 and older). Source 5. |
| FDA Daily Value | Adults and children aged 4 and older | 28 g of dietary fiber a day | Label value for fiber as a whole. Declaring insoluble fiber is voluntary unless a claim is made about it. Sources 6, 7. |
Low insoluble fiber intake: signs and who is at risk
Low intake does not cause a classic deficiency disease, and there is no blood test for fiber status because fiber is not absorbed. Most people sit below the targets: usual US intake of fiber is only about 15 g a day, and the Dietary Guidelines list fiber among the dietary components of public health concern because low intakes are linked to health problems. Constipation is the symptom people link to low fiber, but the trials of insoluble fiber conflict, and bran helped bowel frequency and transit in 24 constipated people but not their other symptoms. A hard stool is not proof of a bran shortfall.
Sources
- Slavin JL. Position of the American Dietetic Association: health implications of dietary fiber. J Am Diet Assoc. 2008.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 9th edition. 2020.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2025-2030.
- European Food Safety Authority. Summary of Dietary Reference Values for the EU population (version 4). 2017. The values for dietary fibre come from the opinion of the EFSA NDA Panel, 2010.
- World Health Organization. WHO updates guidelines on fats and carbohydrates. 17 July 2023. Full guideline: Carbohydrate intake for adults and children.
- U.S. Food and Drug Administration. Daily Value on the Nutrition and Supplement Facts Labels.
- U.S. Food and Drug Administration. Nutrition labeling of food, 21 CFR 101.9, paragraph (c)(6)(i). Electronic Code of Federal Regulations.
- Suares NC, Ford AC. Systematic review: the effects of fibre in the management of chronic idiopathic constipation. Aliment Pharmacol Ther. 2011.
- Badiali D, Corazziari E, Habib FI, et al. Effect of wheat bran in treatment of chronic nonorganic constipation. A double-blind controlled trial. Dig Dis Sci. 1995.
Forms of insoluble fiber: absorption and effects
Insoluble fiber is not absorbed, so “absorption” here means what reaches the colon and how it behaves on the way. Forms differ in particle size, in how much of the fiber bacteria can ferment and in how well they have been tested. The honest bottom line is that wheat bran has the trials, mostly for stool and bowel habit and with mixed results in constipation and irritable bowel syndrome, while purified cellulose and lignin have little human outcome evidence of their own; their pages say what exists. The American Dietetic Association position paper notes that few fiber supplements have been studied for physiological effectiveness and advises getting fiber from foods.
| Form | What it is | What studies show | Source |
|---|---|---|---|
| Wheat bran (coarse) | Insoluble cereal fiber from the outer layers of the wheat kernel; resists fermentation | The EU allows claims for faecal bulk and for faster transit (at least 10 g of wheat bran fiber a day for the transit claim). Each extra gram of wheat fiber raised stool weight by 3.7 g a day in 65 studies, in a review funded by Kellogg. Crude wheat bran has 42.8 g of total fiber per 100 g in FoodData Central. It did not beat placebo for irritable bowel syndrome symptom severity, and 25 g a day did not prevent new colon adenomas. | 2, 3, 4, 5, 6 |
| Fine wheat bran | Wheat bran milled to small particles | Coarse bran raised stool weight more than fine bran (219.4 against 199 g a day in 21 people), but bran at 50 micrometers still bulked stool in two crossover trials and made more butyrate. A 2017 review says fine, smooth bran particles can be constipating; we found no trial showing it. In obese adults, finely milled bran raised blood short-chain fatty acids but no health marker. | 7, 8, 9, 10 |
| Oat, barley and rye grain fiber | Fiber from the whole grain, with its beta-glucan and other fibers | The EU allows claims that oat grain fibre and barley grain fibre contribute to an increase in faecal bulk and that rye fibre contributes to normal bowel function, each only for food high in that fibre. Oat and barley beta-glucan, a soluble fiber, has its own page: beta-glucan. | 2 |
| Cellulose and powdered cellulose | The main structural fiber of plant cell walls, also sold as an isolated fiber | FDA names cellulose among the seven isolated fibers it accepts as meeting its fiber definition. In a cohort of men, cellulose from food went with a relative risk of diverticular disease of 0.52. We found no trial of purified cellulose in this section; see cellulose. | 1, 11 |
| Lignin | A woody polymer in cell walls that is not a carbohydrate but is counted as fiber | FDA’s definition counts lignin that is intrinsic and intact in plants, and the 1976 paper that introduced the term listed lignin with cellulose and hemicellulose as dietary fiber. We found no trial of purified lignin as a fiber; see lignin. | 1, 12 |
| Purified oat insoluble fiber | An isolated insoluble fiber added to bread | 31.2 g a day for 3 days improved insulin sensitivity in 17 overweight or obese women in a crossover trial. The trial was short and tested no other dose. | 13 |
| Resistant starch (type 2) | Starch that resists digestion; counted as a fiber and a related carbohydrate | The EU allows a claim for a smaller glucose rise after a meal when resistant starch is at least 14% of total starch. In 15 insulin-resistant people, 40 g a day of high-amylose maize resistant starch raised muscle glucose uptake by 65%. In rats, raw potato starch gave more tumors and wheat bran suppressed them. See starch. | 2, 14, 15 |
| Sugar beet fiber | Fiber from sugar beet | Bulked stool about as much as wheat bran at 20 g in 17 healthy men, with more water and less dry matter in the stool. | 16 |
Here are the form-specific studies, strongest evidence first, 7 in all.
The EU authorizes a claim that replacing digestible starch with resistant starch contributes to a smaller blood glucose rise after a meal
Regulation (EU) No 432/2012, based on a 2011 EFSA opinion: “Replacing digestible starches with resistant starch in a meal contributes to a reduction in the blood glucose rise after that meal.” It may be used only for food in which digestible starch has been replaced by resistant starch so that the final content of resistant starch is at least 14% of total starch. It is a claim about one meal, not about long-term diabetes risk. Resistant starch is a related fiber; see starch.
In 21 healthy people, coarse bran raised stool weight more than the same bran milled fine (219.4 g against 199 g a day)
Twenty-eight investigations in 21 people given 20 g a day (dry weight) of bran, as large particles or after milling. The difference was 20.4 g a day and significant. Coarse bran also held more water (7.3 g of water per gram against 3.9 g) and took up 2.5 times the volume of fine bran for a given weight, so people who take bran by the spoonful get less fiber from the fine kind.
In two crossover trials, wheat bran ground as fine as 50 micrometers still increased stool bulk
Healthy volunteers (23 in a metabolic study, 24 in a breakfast cereal study) took an extra 19 g a day of fiber from bran in bread, with mean particle size 50 or 758 micrometers, or in cereal, with 692 or 1,158 micrometers, against a low-fiber control. All brans raised stool bulk, with no significant difference between particle sizes and no difference in stool water. The finest bran gave higher fecal butyrate, butyrate output and breath methane, which suggests more bacterial fermentation.
In 18 volunteers, swallowed plastic flakes shaped like bran cut whole-gut transit time by 24%, and small granules did not
Randomized crossover study of 24 g a day of inert plastic, as branlike flakes or small granules, for 10 to 12 days with a two-week washout. The flakes also cut orocecal transit time by 19%, with matching changes in stool form and weight. Granules did not change either transit time significantly, although stool output rose. Plastic is not fiber, so this physiology experiment shows that the size and shape of solid particles can by themselves drive part of the laxative effect. It is not a treatment.
In 17 healthy men, 20 g of sugar beet fiber bulked stool as much as wheat bran, through a different mix of dry matter and water
Randomized crossover trial. Both fibers were milled to similar particle sizes and added to the men’s usual diets. Transit times were similar, and wet stool weight was similar on the two fibers. Dry weight was greater and moisture lower with wheat bran, and fecal pH was lower with wheat bran.
In 14 obese and 36 normal-weight adults, a month of finely milled wheat bran raised blood short-chain fatty acids in the obese group but changed no health marker
The adults took 20 g a day of wheat bran with reduced particle size, or a maltodextrin placebo. Fasting acetate and total short-chain fatty acids rose in the obese group (P < 0.05). Stool output, transit, stool consistency, gut permeability, cholesterol, triglycerides, C-reactive protein and the make-up of gut bacteria did not change.
In 15 people with insulin resistance, 40 g a day of resistant starch for 8 weeks raised muscle glucose uptake by 65%
Randomized crossover trial of HAM-RS2, a resistant starch from high-amylose maize that the authors describe as an insoluble dietary fiber, against a matched placebo. Fatty acid suppression in fat tissue also improved. There was no effect on the insulin sensitivity of the liver or on blood fats. It is a small trial of one resistant starch product.
Sources
- U.S. Food and Drug Administration. Questions and answers on dietary fiber.
- Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. Official Journal of the European Union. 2012. Opinions: wheat bran, EFSA J 2010;8(10):1817; oat and barley grain fibre, 2011;9(6):2249; rye fibre, 2011;9(6):2258; resistant starch, 2011;9(4):2024.
- de Vries J, Miller PE, Verbeke K. Effects of cereal fiber on bowel function: A systematic review of intervention trials. World J Gastroenterol. 2015.
- U.S. Department of Agriculture, Agricultural Research Service. FoodData Central. Wheat bran, crude (FDC 169722, SR Legacy).
- Bijkerk CJ, de Wit NJ, Muris JW, et al. Soluble or insoluble fibre in irritable bowel syndrome in primary care? Randomised placebo controlled trial. BMJ. 2009.
- MacLennan R, Macrae F, Bain C, et al. Randomized trial of intake of fat, fiber, and beta carotene to prevent colorectal adenomas. J Natl Cancer Inst. 1995.
- Brodribb AJ, Groves C. Effect of bran particle size on stool weight. Gut. 1978.
- Jenkins DJ, Kendall CW, Vuksan V, et al. The effect of wheat bran particle size on laxation and colonic fermentation. J Am Coll Nutr. 1999.
- 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.
- Deroover L, Vázquez-Castellanos JF, Vandermeulen G, et al. Wheat bran with reduced particle size increases serum SCFAs in obese subjects without improving health parameters compared with a maltodextrin placebo. Am J Clin Nutr. 2021.
- Aldoori WH, Giovannucci EL, Rockett HR, et al. A prospective study of dietary fiber types and symptomatic diverticular disease in men. J Nutr. 1998.
- Trowell H. Definition of dietary fiber and hypotheses that it is a protective factor in certain diseases. Am J Clin Nutr. 1976.
- Weickert MO, Möhlig M, Schöfl C, et al. Cereal fiber improves whole-body insulin sensitivity in overweight and obese women. Diabetes Care. 2006.
- Robertson MD, Wright JW, Loizon E, et al. Insulin-sensitizing effects on muscle and adipose tissue after dietary fiber intake in men and women with metabolic syndrome. J Clin Endocrinol Metab. 2012.
- Young GP, McIntyre A, Albert V, et al. Wheat bran suppresses potato starch–potentiated colorectal tumorigenesis at the aberrant crypt stage in a rat model. Gastroenterology. 1996.
- Lampe JW, Wetsch RF, Thompson WO, et al. Gastrointestinal effects of sugarbeet fiber and wheat bran in healthy men. Eur J Clin Nutr. 1993.
- Slavin JL. Position of the American Dietetic Association: health implications of dietary fiber. J Am Diet Assoc. 2008.
Is insoluble fiber safe? Bran, side effects and who should watch it
Insoluble fiber from food is safe for healthy people at the amounts eaten in the studies above. General limits and side effects of fiber, such as gas, bloating and swallowing problems with swelling supplements, are on the fiber page. These points are specific to bran and other insoluble fibers.
- Irritable bowel syndrome. In the largest trial, early dropout was most common on 10 g a day of bran, mainly because symptoms worsened, and in a clinic survey 55% of 100 people with the condition said bran made them worse. If you have irritable bowel syndrome, soluble fiber such as psyllium did better than bran in that trial.
- Iron and other minerals. Phytate in bran was the main reason bran blocked iron absorption in a human tracer study. A review concluded that raising fiber would not be expected to harm mineral absorption if protein and the acids in fruit and vegetables rise too, but it advised against excessive phytate for people whose mineral needs are high.
- Thyroid medicine. In hypothyroid patients who needed unusually high doses of levothyroxine, a lower fiber intake went with a lower need, and wheat bran adsorbed levothyroxine in lab tests. Ask your doctor or pharmacist about the timing of fiber and thyroid tablets.
- Constipation that does not improve. Bran made transit normal only in people with slow colonic transit, not in those with slow rectal transit. If more bran does not help, see a doctor, because the cause may not be a lack of fiber.
How to get more insoluble fiber
- Start with beans and seeds. In our ranking 1 cup of black beans gives 10.7 g of insoluble fiber, 1 oz of chia seeds 10.1 g and 1 cup of chickpeas 8.86 g.
- Choose whole grains over refined grains. In a meta-analysis, whole grain bread, whole grain breakfast cereals and added bran went with lower risks of cardiovascular disease or death, and the Dietary Guidelines for Americans 2025-2030 say to prioritize fiber-rich whole grains and cut refined carbohydrates such as white bread.
- Add bran slowly and in a coarse form. A cereal with 5.4 g of fiber (3.5 g from wheat bran) eaten daily for 14 days improved self-rated bowel function in people who ate little fiber, in a study funded by the cereal maker Kellogg. Coarse bran bulked stool more than fine bran in one trial.
Sources
- Francis CY, Whorwell PJ. Bran and irritable bowel syndrome: time for reappraisal. Lancet. 1994.
- Hallberg L, Rossander L, Skånberg AB. Phytates and the inhibitory effect of bran on iron absorption in man. Am J Clin Nutr. 1987.
- Torre M, Rodriguez AR, Saura-Calixto F. Effects of dietary fiber and phytic acid on mineral availability. Crit Rev Food Sci Nutr. 1991.
- Liel Y, Harman-Boehm I, Shany S. Evidence for a clinically important adverse effect of fiber-enriched diet on the bioavailability of levothyroxine in adult hypothyroid patients. J Clin Endocrinol Metab. 1996.
- Badiali D, Corazziari E, Habib FI, et al. Effect of wheat bran in treatment of chronic nonorganic constipation. A double-blind controlled trial. Dig Dis Sci. 1995.
- Bijkerk CJ, de Wit NJ, Muris JW, et al. Soluble or insoluble fibre in irritable bowel syndrome in primary care? Randomised placebo controlled trial. BMJ. 2009.
- U.S. Department of Agriculture, Agricultural Research Service. FoodData Central. Values as shown in the ranking above.
- Aune D, Keum N, Giovannucci E, et al. Whole grain consumption and risk of cardiovascular disease, cancer, and all cause and cause specific mortality: systematic review and dose-response meta-analysis of prospective studies. BMJ. 2016.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2025-2030.
- Lawton CL, Walton J, Hoyland A, et al. Short term (14 days) consumption of insoluble wheat bran fibre-containing breakfast cereals improves subjective digestive feelings, general wellbeing and bowel function in a dose dependent manner. Nutrients. 2013.
- Brodribb AJ, Groves C. Effect of bran particle size on stool weight. Gut. 1978.
History of insoluble fiber
Fiber was long measured as “crude fiber”, the residue left after plant food is extracted with dilute acid and then dilute alkali. In 1976 Hugh Trowell pointed out that this understates what people eat: whole wheatmeal had about 11% dietary fiber but only about 2% crude fiber. He described dietary fiber as the residue of plant food resistant to human digestive enzymes and said it is made of cellulose, hemicellulose and lignin.
The idea that this residue protects health took shape between 1966 and 1972 around the surgeon Denis Burkitt, according to a 2018 history, and in 1972 Burkitt, Walker and Painter published their Lancet paper on the effect of dietary fiber on stools and transit times. The split into insoluble and soluble fiber came from laboratory methods. In 1985 a collaborative study led to the adoption of an enzyme-and-weighing method for total dietary fiber as an official AOAC method, and a 1988 interlaboratory study extended it to measure insoluble and soluble fiber separately, while noting that the separate measurement needed further study.
Doubts about the split followed. A 2008 position paper noted that solubility was once thought to decide physiological effect but that fermentability or viscosity may matter more, and a 2017 review argued that viscosity and fermentation, not solubility, explain the effects of fiber on cholesterol, blood sugar and stool. That is why this page is about what each insoluble fiber does, mostly wheat bran, rather than about the label.
Sources
- Trowell H. Definition of dietary fiber and hypotheses that it is a protective factor in certain diseases. Am J Clin Nutr. 1976.
- Cummings JH, Engineer A. Denis Burkitt and the origins of the dietary fibre hypothesis. Nutr Res Rev. 2018.
- Burkitt DP, Walker AR, Painter NS. Effect of dietary fibre on stools and the transit-times, and its role in the causation of disease. Lancet. 1972.
- Prosky L, Asp NG, Furda I, et al. Determination of total dietary fiber in foods and food products: collaborative study. J Assoc Off Anal Chem. 1985.
- Prosky L, Asp NG, Schweizer TF, et al. Determination of insoluble, soluble, and total dietary fiber in foods and food products: interlaboratory study. J Assoc Off Anal Chem. 1988.
- Slavin JL. Position of the American Dietetic Association: health implications of dietary fiber. J Am Diet Assoc. 2008.
- 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.
Frequently asked questions about insoluble fiber
What is insoluble fiber?
It is dietary fiber that does not dissolve in water and that human enzymes cannot digest, mainly the cellulose and lignin of plant cell walls. It is found in wheat bran, whole grains, seeds, nuts and bean skins. FDA’s definition of dietary fiber counts lignin that is intrinsic and intact in plants, and US labels may declare insoluble fiber voluntarily.
What foods are highest in insoluble fiber?
Per serving, black beans lead our ranking with 10.7 g in 1 cup, followed by chia seeds (10.1 g in 1 oz), chickpeas (8.86 g in 1 cup), pears (4.98 g in 1 medium pear) and avocados (4.15 g in half an avocado). Wheat bran is not in the ranking, but crude wheat bran has 42.8 g of total fiber per 100 g in FoodData Central.
Does insoluble fiber help constipation?
Sometimes. A pooled analysis of 65 studies found 3.7 g more stool a day for each extra gram of wheat fiber, and 20 g a day of bran improved bowel frequency and transit time in 24 constipated people. A review of 6 trials in chronic constipation found the evidence for insoluble fiber conflicting, and bran worked less well in constipated people than in healthy ones.
What is the difference between soluble and insoluble fiber?
Soluble fiber dissolves in water and some forms a gel, such as psyllium and oat beta-glucan; insoluble fiber does not dissolve and adds bulk. A 2017 review found that gel-forming soluble fibers lowered cholesterol and improved blood sugar control, while wheat bran did not. Solubility is a rough guide only; see soluble fiber and fiber for the comparison.
Is insoluble fiber bad for irritable bowel syndrome?
It often does not help, and it can make symptoms worse. In 275 people, 10 g a day of bran did not reduce symptom severity more than placebo, and the early dropouts were mostly people whose symptoms worsened. Psyllium did better. In a clinic survey, 55% of 100 people with the condition said bran made them worse.
Does wheat bran prevent colon cancer?
Cohort studies suggest cereal fiber might: each extra 10 g a day went with a relative risk of 0.90 in 25 studies. The one trial of bran we found did not show it. In 411 people with a removed polyp, 25 g a day of wheat bran did not prevent new adenomas, so the cohort link may reflect whole grains or other habits.
Does insoluble fiber lower cholesterol or blood sugar?
Not reliably. A 2017 review says insoluble fibers such as wheat bran lack the viscosity that gives soluble fibers their effect on cholesterol and glucose. The one trial we found, in 17 overweight women, gave purified insoluble oat fiber for only 3 days and improved insulin sensitivity. That is too short to tell us about daily bran.