Nutrition

Insoluble Fiber: What It Does and the Best Food Sources

Q: Is psyllium husk soluble or insoluble?

Soluble: psyllium is a gel-forming soluble fiber.[1][11] It's my first choice of fiber supplement for constipation, though it isn't the best fit if you bloat easily.[23]

Q: Is wheat dextrin soluble or insoluble?

Soluble: wheat dextrin is a soluble, fermentable fiber.[1] It doesn't have a laxative effect, and one review lists it among the fibers that can be constipating.[11]

What insoluble fiber is and how it keeps your gut moving, the foods richest in it with USDA fiber numbers per serving, how much of each food's fiber is the insoluble kind, and how to add more without the bloating.

By Dr. Josh Axe, DC, DNM, CNS·20-min read

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Evidence-based44 sources citedUpdated October 18, 2026

Published Oct 18, 2026Last updated Oct 18, 2026Editorial policyMedical review policyFact-checking policy

Coarse bran, one of the most concentrated sources of insoluble fiber.

Insoluble fiber at a glance

Insoluble fiber is the part of plant fiber that doesn't dissolve in water, mainly cellulose, lignin and some hemicelluloses, and it adds the bulk that helps a slow gut move.1 2

The richest everyday sources are beans and lentils, whole grains, vegetables, fruit eaten with its skin, and nuts and seeds; half a cup of cooked navy beans carries 9.6 grams of total fiber3
Plant foods carry both kinds of fiber in different amounts: cooked split peas' fiber is 99.2% insoluble, regular oatmeal's 74.5% and an apple's with the skin 69.7%4
In a randomized trial of 51 adults with constipation, whole-grain rye bread shortened transit 23% and added 1.4 bowel movements a week compared with white bread5
I recommend at least 30 grams of total fiber a day for most people, about 10 to 15 grams at each meal6 7
Add it a little at a time and drink water with it, so your body gets used to the change8

Insoluble fiber is the sturdy, structural part of plant foods: the bran on a whole grain, the skin on a peach, the cell walls that give a cooked lentil its shape. Your digestive enzymes can't break it down, so it travels through your stomach and small intestine and reaches your colon largely intact.2 1

Americans as a whole eat far less fiber than they need. In 2017–18, they averaged 8.1 grams of fiber per 1,000 calories, 58% of the recommended 14 grams.9

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The list below ranks the best insoluble-fiber foods by food group, with USDA fiber numbers per realistic serving and, where a laboratory analysis measured it, how much of each food's fiber is the insoluble kind.3 4 My approach to fiber is food first: a wide variety of plants, with grains and beans prepared the traditional way, soaked, sprouted or fermented.10 6

Insoluble fiber is like cleaning out your plumbing. It's like cleansing your colon.Dr. Josh Axe, DC, DNM, CNSFrom 7 Habits That Age You Faster & Cost Nothing to Fix, The Dr. Josh Axe Show (2026), at 24:03

What is insoluble fiber?

Quick answer

Insoluble fiber is the fiber in plant foods that doesn't dissolve in water: cellulose, lignin, some hemicelluloses and some pectins, materials that make up much of a plant's cell walls.1 2 Your enzymes can't digest it, so it reaches your colon largely intact, and because gut bacteria barely ferment its cellulose and lignin, much of it passes on into your stool.1 11

By the Institute of Medicine's definition, dietary fiber is the nondigestible carbohydrate and lignin that's intrinsic and intact in plants.2 It comes in two broad kinds. Soluble fiber, such as pectin, the beta-glucans in oats and the gel-forming fiber in psyllium husk, dissolves in water, and insoluble fiber doesn't.1 For a side-by-side look at the two, see soluble vs. insoluble fiber.

Plant foods usually carry both kinds, in proportions that vary from food to food. In a USDA laboratory analysis of 70 commonly eaten foods, the share of insoluble fiber varied across food groups and even within a single group, which led the researchers to recommend judging foods case by case.4 You'll get insoluble fiber from vegetables and whole grains for the most part, as I've explained on the show, and it's the fiber that adds bulk to your stool.12

Resistant starch is a related fiber: starch that escapes digestion in the upper gut, is counted as dietary fiber and is fermented by bacteria in the colon.1 The how-to section below covers a simple way to get more of it.

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What does insoluble fiber do in your body?

Quick answer

It adds bulk and water to your stool, and when your gut runs slow, it shortens the time food takes to pass through.13 The Institute of Medicine's fiber report says that fibers that are poorly fermented, as cellulose and lignin are, are known to improve stool bulk and regularity and ease constipation.2 1

Here's how insoluble fiber moves things along, step by step:

1Your enzymes can't break it down

Cellulose, lignin and some hemicelluloses pass through your stomach and small intestine without being digested.1

2Gut bacteria barely ferment it

Cellulose and lignin are classed as non-fermentable, so much of this fiber passes through your colon intact and ends up in your stool.1 11

3It adds bulk to your stool

In intervention studies, each gram of poorly fermented fiber added about 2.88 grams to daily stool weight, against 0.89 grams for readily fermented fiber.13

4Coarse pieces prompt water and mucus

Large, coarse particles such as unground wheat bran rub against the lining of the colon, which responds with water and mucus that soften the stool. Finely ground bran loses that effect and can even be constipating.11

5A slow gut speeds up

When transit took 48 hours or longer, each extra gram of cereal, fruit or vegetable fiber shortened it by about 30 minutes. Transit that was already under 48 hours didn't change.13

Those stool and transit numbers come from a review of intervention studies in apparently healthy adults. By source, each gram of cereal fiber added about 2.17 grams to daily stool weight, vegetable fiber 2.03 and fruit fiber 0.88; a breakfast-cereal maker sponsored the review, and three of its employees were among the authors.13 In general, cereal fibers are the most effective at increasing stool weight, and wheat bran is considered the standard for bulking.1

Which foods are highest in insoluble fiber?

Quick answer

Beans, lentils and split peas lead the list: half a cup of cooked navy beans carries 9.6 grams of total fiber, split peas 8.1 and lentils 7.8.3 Whole and sprouted grains, vegetables, fruit eaten with its skin, and nuts and seeds follow.

Each card covers one food group, with its best picks and how to prepare them. Every fiber number is total dietary fiber per serving from USDA FoodData Central, worked out from each record's per-100-gram value and its own serving weight; half-cup values are half the record's 1-cup weight.3 Food labels and USDA records list total fiber, so the insoluble share appears only where a laboratory analysis measured it.4 Each card's badge grades the human research behind that food group.

Beans, lentils and split peas

Moderate evidenceHuman studyReview of 41 randomized trials of pulses · USDA analysis of 70 foods

Legumes are some of the richest fiber foods you can eat, and most of their fiber is insoluble. Half a cup of cooked navy beans carries 9.6 grams of total fiber, split peas 8.1, lentils 7.8, pinto beans 7.7, black beans 7.5, kidney beans 6.5 and chickpeas 6.2.3 In the USDA laboratory analysis, the fiber in cooked split peas was 99.2% insoluble, lentils 92.5% and canned pinto beans 85.1%.4

Beans also steady blood sugar. In a review of 41 randomized trials, pulses eaten on their own (11 trials) lowered fasting blood sugar and insulin, and pulses as part of high-fiber diets (11 trials) lowered fasting blood sugar and longer-term markers such as HbA1c, with results that varied a good deal from trial to trial.14

The Biblio Diet, my book with Jordan Rubin, lists lentils and black, kidney, navy and pinto beans among its legumes and says soaked or pressure-cooked beans are best.10 As Dr. Will Cole and I explain in Heal Your Cells, soaking, sprouting and fermenting lower phytic acid, a compound that holds onto minerals, so more of the iron, zinc, calcium and magnesium becomes available.15 The human studies agree: in a review of 42 of them, phytate-rich meals reduced iron and zinc absorption in 13 of 17 feeding studies, and removing the phytate improved it in 11 of 14.16 If you're working through SIBO, the safety section covers when to hold off on beans.

Whole grains: sprouted, fermented and ancient

Strong evidenceHuman studyTrials of 24 and 51 adults with constipation · review of 25 cohort studies

A cup of cooked spelt carries 7.6 grams of fiber, quinoa 5.2, regular oatmeal 4.0 and brown rice 3.2, and a slice of sprouted wheat bread carries 2.0.3 Brown rice's fiber is 86.8% insoluble.4 Wheat bran, the outer layer of the wheat kernel, is the most concentrated source on this page: 42.8 grams per 100 grams, or 6.2 grams in a quarter cup.3

Cereal fiber has been studied for regularity more than any other kind.13 In a placebo-controlled trial of 24 adults with chronic constipation, wheat bran improved bowel frequency more than the placebo, and in a five-group randomized trial of 51 constipated adults, whole-grain rye bread shortened transit compared with white bread.17 5 In long-term studies, people who eat the most cereal fiber and whole grains have less type 2 diabetes and colorectal cancer; the sections below have the numbers.18 19

I rank whole-grain sourdough made from an ancient grain such as einkorn as the best bread, and 100% sprouted whole-grain bread, the style known as Ezekiel bread, second.10 My grain list keeps wheat, rye and barley to their sprouted forms, includes oats when they're organic and slow-cooked, and puts instant oatmeal and dried cereal on the avoid side.10 On insoluble fiber, the lab bears out the oatmeal choice: regular oatmeal's fiber was 74.5% insoluble, instant oatmeal's 44.0%.4 If you eat grains, make sure they're 100% organic and carry good fiber; otherwise they'll spike your blood sugar.7 Einkorn, spelt and kamut are ancient wheats that still contain gluten; the safety section covers celiac disease.10

Vegetables, steamed or baked

Moderate evidenceHuman studyReview of intervention studies in healthy adults · USDA analysis of 70 foods

A medium cooked artichoke carries 6.8 grams of fiber, a cup of cooked green peas 8.8, a cup of cooked collard greens 7.6, a cup of baked sweet potato 6.6 and a cup of cooked kale 4.7. Half a cup of cooked broccoli has 2.6, and a cup of chopped raw carrots 3.6.3 Raw broccoli's fiber is 87.4% insoluble, raw carrots' 83.0% and raw spinach's 75.9%, and a baked potato eaten with its skin is 73.6% insoluble.4 In a review of intervention studies in healthy adults, vegetable fiber bulked stool nearly as much as cereal fiber, gram for gram.13

My fiber list on the show starts with steamed vegetables, cabbage and broccoli among them, and includes artichokes.7 Gentle cooking also makes vegetables easier on the gut, and steaming or sautéing can raise the availability of certain nutrients.15

Fruit, with the skin on

Moderate evidenceHuman studyCohort of 50,019 women · USDA analysis of 70 foods

Berries carry the most fiber of the fruits here: a cup of raspberries has 8.0 grams and a cup of blackberries 7.6.3 A medium pear carries 5.5 grams, a medium apple with its skin 4.4 and half a cup of cubed avocado 5.0.3 The fiber in a pear with its skin is 71.0% insoluble, a Red Delicious apple's 69.7% and a Hass avocado's 63.4%.4 Keep the skin on when you can: peaches eaten with their skin carried 1.54 grams of insoluble fiber per 100 grams, against 1.16 peeled.4

In a large U.S. study that followed women for 24 years, those who ate more whole fruit, apples and pears among them, had less diverticulitis; the diverticular section has the numbers.20 Berries, apples and pomegranates are the fruits on that same fiber list.7 At breakfast, I aim for protein and fiber: one morning that meant one big apple, a lot of mornings it's a pumpkin smoothie, and for fiber I add a granola that's mostly chia and flax.12

Nuts and seeds

Emerging evidenceHuman studyRandomized trial of 90 adults with constipation (flaxseed)

An ounce of chia seeds carries 9.8 grams of fiber, an ounce of almonds 3.5 and an ounce of pumpkin seeds 1.7, and a tablespoon of ground flaxseed 1.9.3 Flaxseed has a constipation trial behind it: in a randomized trial of 90 adults, flaxseed flour with meals raised weekly bowel movements as much as a common laxative did, or a little more; the constipation section has the numbers.21

Chia and flax are on my fiber list too.7 I suggest keeping nuts to about a handful a day, and soaking makes almonds more digestible.10

Total fiber per serving in 10 high-fiber foods from this list

Source: USDA Agricultural Research Service · FoodData Central, SR Legacy [3]Total dietary fiber per serving, from each record's per-100-gram value and serving weight.

How much of each food's fiber is insoluble?

Quick answer

It varies from food to food: in a USDA laboratory analysis, the fiber in cooked brown rice was 86.8% insoluble, regular oatmeal 74.5%, a pear with its skin 71.0% and instant oatmeal 44.0%.4 So the foods you eat give you some of each.

The table comes from a USDA Agricultural Research Service analysis of 70 commonly eaten foods, bought in North Carolina and measured as eaten, in grams per 100 grams.4 The share is the insoluble grams divided by the two kinds combined.

Food	Soluble fiber (g/100 g)	Insoluble fiber (g/100 g)	Share insoluble
Split peas, cooked	0.09	10.56	99.2%
Lentils, cooked	0.44	5.42	92.5%
Broccoli, raw	0.44	3.06	87.4%
Brown rice, cooked	0.44	2.89	86.8%
Pinto beans, canned	0.99	5.66	85.1%
Carrots, raw	0.49	2.39	83.0%
Red kidney beans, canned	1.36	5.77	80.9%
Spinach, raw	0.77	2.43	75.9%
Oatmeal, regular, cooked	0.42	1.23	74.5%
Potato, baked with skin	0.61	1.70	73.6%
Pear with skin	0.92	2.25	71.0%
Apple (Red Delicious) with skin	0.67	1.54	69.7%
Avocado (Hass)	2.03	3.51	63.4%
Broccoli, microwaved	1.85	2.81	60.3%
Oatmeal, instant, cooked	1.45	1.14	44.0%

Cooking can shift the balance. Microwaved broccoli's fiber was 60.3% insoluble and microwaved carrots' 59.2%, both well below their raw shares, yet per 100 grams microwaved broccoli carried 4.66 grams of total fiber, against 3.50 raw, so cooked vegetables still bring plenty.4

How much of each food's fiber is the insoluble kind

Source: Li, Andrews and Pehrsson · Journal of Food Composition and Analysis, 2002 [4]Regular oatmeal is about three-quarters insoluble; instant oatmeal is less than half.

Does insoluble fiber help constipation?

Quick answer

Yes, especially when your gut runs slow. In a placebo-controlled trial of 24 adults, wheat bran improved bowel frequency more than the placebo, and in a review of 16 randomized trials of fiber supplements in 1,251 adults with chronic constipation, 66% of people taking fiber improved in the trials that counted responders, against 41% on a comparison.17 22

That review pooled fiber supplements of several kinds, at 4 to 40 grams a day for 2 to 8 weeks. Psyllium raised stool frequency, as did pectin and wheat bran, the last in a single trial; inulin-type fibers, galacto-oligosaccharides and polydextrose didn't. Doses above 10 grams a day and courses of at least 4 weeks worked best, and the longer courses added about 1.33 bowel movements a week.22 Natural constipation relief covers the other steps that help.

Bran's trial was small and tightly controlled. In 24 adults with chronic constipation, each person took 20 grams of bran a day for 4 weeks and a placebo for 4 weeks, with neither the people nor the researchers knowing which was which. Bran improved bowel frequency and transit more than the placebo, and transit returned to normal in those whose slowdown was in the colon, though their other constipation symptoms didn't differ between the two.17

Whole-grain bread works too. In a trial that randomly assigned 51 constipated adults to one of five groups for 3 weeks, whole-grain rye bread, at least 240 grams a day, shortened transit 23% compared with white wheat bread, added 1.4 bowel movements a week, softened stools and made them easier to pass, with no more digestive side effects. Against white bread, it also raised stool butyrate 63%, and compared with the people's usual laxatives, it shortened transit 41%.5

Flaxseed has a trial of its own. In 90 adults with functional constipation randomly assigned for 4 weeks, 50 grams of flaxseed flour a day with meals raised median bowel movements from 2 to 7 a week, against 2 to 6 on lactulose, a common laxative, and constipation scores fell from 14 to 6.5 on flax and from 15 to 9 on lactulose.21

If you want a supplement on top of food, psyllium husk is my first choice for constipation, though it isn't the best fit if you bloat easily. For a sensitive gut, I lean toward gentler prebiotic fibers, such as partially hydrolyzed guar gum at about 5 to 7 grams a day, or acacia fiber.23 24 Psyllium itself is a gel-forming soluble fiber that holds water in the stool, a different route to the same soft, easy-to-pass result.11

Does insoluble fiber help blood sugar?

Quick answer

It can. In a trial of 19 men with type 2 diabetes who each tried both, 15 grams of raw wheat bran eaten before breakfast cut the meal's peak blood-sugar rise by 15.80%.25 In a review of 17 cohort studies that followed people over time, those who ate the most insoluble fiber, in the studies that measured it, had a 25% lower risk of type 2 diabetes.18

In that breakfast trial, the bran carried 5.8 grams of insoluble fiber and shrank the total blood-sugar rise over 2 hours by 23.14%; the researchers credit raw bran's ability to slow starch-digesting enzymes and note that heated bran, as in bread or pasta, lacks it.25 In 17 overweight or obese women without blood-sugar problems who each tried both, whole-body glucose uptake, a measure of insulin sensitivity, was 6.56 mg/kg/min after 3 days of white bread enriched with 31.2 grams a day of purified insoluble oat fiber, against 6.07 after plain white bread, about 8% higher.26

The longest trial ran 2 years. In 180 adults with impaired glucose tolerance who were also in a lifestyle program, neither the people nor the researchers knew who got an insoluble oat-fiber supplement, 15 grams a day, and who got a placebo. After the first year, 2-hour glucose had fallen about 14 mg/dL on fiber and 8 on placebo, and over the 2 years, 9 of 89 people on fiber developed diabetes against 16 of 91 on placebo; neither difference reached statistical significance. HbA1c, a three-month blood-sugar marker, held steady on fiber and rose about 0.1 percentage point on placebo, a statistically significant difference, and among the women, 2-hour glucose fell about 16 mg/dL on fiber against 4 on placebo, also a significant difference.27

Long-term studies point the same way. A review of 17 cohort studies of 488,293 people pooled the studies that measured each kind of fiber: those who ate the most insoluble fiber had a 25% lower risk of type 2 diabetes than those who ate the least, and those who ate the most cereal fiber had a 23% lower risk.18 In a pooled analysis of 19 cohorts, the 12 that measured cereal fiber showed 25% lower risk for every 10 grams of it a day.28

On my show, I've called protein and fiber the biggest needle movers for insulin resistance.7 My Insulin Resistance & Prediabetes Protocol lists fiber, 30 grams or more a day, among the things your blood sugar runs on, because it slows how fast sugar enters the blood.29 For the numbers to aim for, see normal blood sugar.

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Does insoluble fiber protect against diverticular disease?

Quick answer

It's linked with lower risk. In 43,881 U.S. men followed over time, those who ate the most insoluble fiber had a 37% lower risk of symptomatic diverticular disease, and those who ate the most cellulose 48% lower.30 An international consensus published in 2026 states that high dietary fiber intake is protective.31

Diverticula are small pouches in the wall of the colon. Diverticular disease means they cause symptoms or complications.31 The link with fiber shows up as a steady slope. In 47,033 mostly health-conscious British adults followed for an average of 11.6 years, with 812 cases, relative risk across five levels of fiber intake ran 1.00, 0.86, 0.76, 0.72 and 0.59, from the lowest intake (under 14 grams a day) to the highest. That top group, at 25.5 grams a day or more for women and 26.1 for men, had 41% lower risk.32

More fiber, fewer cases of diverticular disease

Source: Crowe et al. · BMJ, 2011 (EPIC-Oxford) [32]The lowest fifth ate under 14 grams of fiber a day; the highest ate at least 25.5 grams (women) or 26.1 grams (men).

A pooled analysis of 5 cohort studies with 865,829 people found 26% lower risk for every 10 grams of fiber a day, and 41% lower risk at 30 grams a day than at 7.5 grams; cereal fiber and fruit fiber were both tied to lower risk.33 In 50,019 U.S. women aged 43 to 70 followed from 1990 to 2014, those who ate the most fiber had a 14% lower rate of diverticulitis. Fiber from fruit and cereals went with lower risk, while vegetable fiber didn't, and each daily serving of whole fruit went with 5% lower risk, with apples, pears and prunes among the fruits tied to lower risk.20

The 2026 consensus adds that in symptomatic, uncomplicated diverticular disease, dietary fiber may help relieve symptoms.31 If you're in the middle of a diverticulitis flare, the safety section covers what to do first.

Does insoluble fiber lower colorectal cancer risk?

Higher fiber intake, from cereal fiber and whole grains especially, is linked with lower colorectal cancer risk in long-term studies of people's diets, which show a link rather than proof of cause. On my show, I've pointed to research in the British Medical Journal linking each additional 10 grams of fiber a day to a 10% lower colorectal cancer risk.7 That review pooled 25 prospective studies, 16 of them on total fiber; cereal fiber showed the same 10% per 10 grams, and every three daily servings of whole grains went with 17% lower risk.19

A meta-analysis of observational studies that split fiber by type found insoluble fiber tied to 23% lower risk and soluble fiber to 22% lower, comparing the highest intakes with the lowest.34 A trial tested wheat-bran cereal directly: in 1,429 adults who'd had a colon polyp removed, randomly assigned to 13.5 or 2 grams of wheat-bran fiber a day, at least one new polyp turned up within about 3 years in 47.0% of the high-fiber group and 51.2% of the low-fiber group among the 1,303 who finished, a difference too small to rule out chance.35

Whole grains carry a clear signal. In 478,994 U.S. adults followed for 16 years, those who ate the most whole grains had 16% lower colorectal cancer risk, and fiber from grains was tied to lower colon and rectal cancer risk, while total fiber from all sources went with only 4% lower risk, a difference small enough to be chance. The researchers concluded that guidance should focus on whole grains as a source of fiber.36 Part of the reason may be butyrate, the short-chain fatty acid your gut bacteria make from fiber, which protects the lining of the colon.7 Whole grains bring fermentable fiber along with the insoluble kind, and whole-grain rye bread raised stool butyrate 63% in the constipation trial above.5

How much insoluble fiber do you need a day?

Quick answer

There's no separate target for insoluble fiber; recommendations cover total fiber, and I recommend at least 30 grams a day for most people.2 6 The Institute of Medicine set an adequate intake of 38 grams a day for young men and 25 for young women.2

The Institute of Medicine based those figures on the intake that protected against coronary heart disease.2 The federal benchmark works out to 14 grams for every 1,000 calories you eat.9

I put the average at around 14 to 15 grams a day, and most people should get at least 30, so at least double it.6 I'd aim for about 40 grams a day, which is 10 to 15 grams at each meal.7

On 7 Habits That Age You Faster & Cost Nothing to Fix, at 22:44, I put it this way: "Higher fiber intake is linked with lower cardiovascular disease, lower mortality, better cholesterol, lower blood pressure, and more weight loss."12 A Lancet review of 185 prospective studies and 58 clinical trials found 15% to 30% lower death from all causes and from cardiovascular disease, and less coronary heart disease, stroke, type 2 diabetes and colorectal cancer, in people who ate the most fiber. Risk reduction was greatest at 25 to 29 grams a day, and the trend suggested that eating more could add further benefit.37 The 2025–2030 Dietary Guidelines for Americans say to prioritize fiber-rich whole grains and note that high-fiber foods support a diverse microbiome.38

Here's what a day at about 44 grams can look like, adding up the USDA values:3

Meal	Foods	Fiber (g)
Breakfast	1 cup cooked regular oatmeal, 1 cup raspberries, 1 tablespoon ground flaxseed	13.9
Lunch	½ cup cooked lentils over 1 cup cooked collard greens	15.4
Dinner	Wild salmon with 1 cup baked sweet potato and ½ cup cooked broccoli	9.2
Snack	1 medium pear	5.5
Total		44.0

For more ways to build your day, see how much fiber per day and the high-fiber foods list.

How do you add insoluble fiber without the bloating?

Go slowly and give your gut time to adjust. If you have gut symptoms such as bloating or IBS, my rule is simple: start low and slow, because sudden increases in fiber or resistant starch can overwhelm an unbalanced gut.15

Add one fiber-rich food at a time. A little at a time lets your body get used to the change.8
Drink water with it. Drinking water and other liquids can help fiber work better.8
Soak, sprout or pressure-cook your beans and grains. It lowers phytic acid so more of their minerals become available.10 15
Steam or sauté your vegetables. Gentle cooking makes them easier on the gut, and cooked vegetables still bring plenty of fiber.15 4
Keep the skins on. A peach eaten with its skin carries more insoluble fiber than a peeled one.4
Rotate your plants. The average person rotates about 10 plants a week, and you want 20 to 30: an apple one morning, oatmeal or berries the next, broccoli and cauliflower one day and carrots and asparagus another.6
Cook potatoes and let them cool. I recommend eating cooked potatoes later that day or the next, because that type of resistant starch is the number one starch for supporting butyrate production.6

Building a higher-fiber week?

Ask Dr. Axe AI. Its answers draw on this library and show their sources.

Who should be careful with insoluble fiber?

Expect some gas while you raise your fiber. Your gut is adjusting to break the extra fiber down, and as I've explained on the show, that's normal and healthy when you're improving your diet, while ongoing gas and bloating isn't.39 In the constipation trials, people on fiber reported more flatulence than those on a comparison.22 I also suggest not going over about 50 grams of fiber a day, because past that it can become too much.7

Take extra care with fiber if any of these apply:

IBS: Start with gentler, soluble fibers and build slowly. In a review of 14 trials in 906 adults with IBS, people on soluble fiber were 17% less likely to still have symptoms, while bran wasn't significantly better than the comparison.40 15
SIBO: My SIBO Protocol limits beans and lentils, wheat and rye, and apples and pears early on, because they feed fermentation, then reintroduces them one at a time in Phase 3.41
Gastroparesis: When the stomach empties too slowly, the National Institute of Diabetes and Digestive and Kidney Diseases recommends low-fiber foods and avoiding high-fiber ones.42
Major gut issues: A temporary low-fiber approach may be necessary, under supervision.15
Celiac disease: Einkorn, spelt and kamut contain gluten. Choose naturally gluten-free whole grains such as brown rice and quinoa.10
Thyroid medication: Keep taking it exactly as prescribed, on an empty stomach and at least four hours apart from fiber supplements, as my Hashimoto's Protocol lays out.43
Diverticulitis flare: Follow the treatment plan you've been given for that episode. The fiber guidance on this page is for the long run.
When to see a doctor right away: Constipation with bleeding from your rectum, blood in your stool, constant pain in your abdomen, inability to pass gas, vomiting, fever, lower back pain or losing weight without trying.44

The bottom line

Why insoluble fiber belongs on your plate every day

Insoluble fiber is your gut's broom. It bulks and softens your stool and moves a slow gut along.13 11 In long-term studies, people who eat the most fiber have less diverticular disease, type 2 diabetes and colorectal cancer.33 18 19 I value the traditional ways of preparing these foods, soaking, sprouting and fermenting, because they make the minerals in grains and beans easier to absorb.15 16

Build your plate around beans and lentils, sourdough or sprouted whole grains, steamed vegetables, fruit with its skin and a handful of nuts and seeds. Aim for at least 30 grams of fiber a day, raise it gradually and drink water with it. The free Healing Leaky Gut protocol shows how to build fiber in gently, step by step.

Give your gut the fiber it was designed to run on, and it can do its work every day.

Insoluble fiber: frequently asked questions

What foods are high in insoluble fiber?

Beans, lentils and split peas, whole grains such as spelt, quinoa and brown rice, vegetables such as artichokes, green peas and collard greens, fruit eaten with its skin, and nuts and seeds.3 Half a cup of cooked navy beans carries 9.6 grams of total fiber, and the fiber in cooked split peas is 99.2% insoluble.3 4

What does insoluble fiber do?

It adds bulk and water to your stool and helps a slow gut move faster.13 Your enzymes can't digest it, so it reaches your colon largely intact, and because gut bacteria barely ferment its cellulose and lignin, much of it passes on into your stool.1 11

What is the difference between soluble and insoluble fiber?

Soluble fiber dissolves in water, and some kinds, such as psyllium, form a gel; insoluble fiber, mainly cellulose, lignin and some hemicelluloses, doesn't dissolve.1 Plant foods usually carry both kinds in different proportions.4 See soluble vs. insoluble fiber for the full comparison.

Is oatmeal soluble or insoluble fiber?

Both: regular oatmeal has more insoluble fiber, while instant oatmeal has more soluble. In a USDA laboratory analysis, cooked regular oatmeal's fiber was 74.5% insoluble, and instant oatmeal's was 44.0%.4 A cup of cooked regular oatmeal carries 4.0 grams of total fiber.3

Are beans soluble or insoluble fiber?

Mostly insoluble: the fiber in canned pinto beans was 85.1% insoluble, canned red kidney beans 80.9% and cooked lentils 92.5%.4 Beans carry some soluble fiber too.

Are apples soluble or insoluble fiber?

Both, with more insoluble: a Red Delicious apple with its skin was 69.7% insoluble fiber.4 A medium apple with its skin carries 4.4 grams of total fiber.3

Is avocado soluble or insoluble fiber?

Both: a Hass avocado's fiber was 63.4% insoluble and 36.6% soluble.4 Half a cup of cubed avocado carries 5.0 grams of total fiber.3

Is psyllium husk soluble or insoluble?

Soluble: psyllium is a gel-forming soluble fiber.1 11 It's my first choice of fiber supplement for constipation, though it isn't the best fit if you bloat easily.23

Is wheat dextrin soluble or insoluble?

Soluble: wheat dextrin is a soluble, fermentable fiber.1 It doesn't have a laxative effect, and one review lists it among the fibers that can be constipating.11

Does insoluble fiber help with constipation?

Yes, especially for a slow gut: in a placebo-controlled trial of 24 adults with chronic constipation, wheat bran improved bowel frequency more than the placebo, and in a randomized trial of 51 constipated adults, whole-grain rye bread shortened transit 23% and added 1.4 bowel movements a week compared with white bread.17 5

Can too much fiber cause bloating or constipation?

It can cause gas and bloating if you add it too fast, and some fibers, such as wheat dextrin, can be constipating.39 11 I suggest raising fiber a little at a time with water and staying under about 50 grams a day.8 7

How much insoluble fiber should you eat a day?

No separate target exists; the Institute of Medicine's adequate intake for total fiber is 38 grams a day for young men and 25 for young women.2 I recommend at least 30 grams of total fiber a day for most people, about 10 to 15 grams at each meal.6 7

Is insoluble fiber good for IBS?

With IBS, gentler soluble fibers are the better place to start: in 14 trials in 906 adults with IBS, people on soluble fiber were 17% less likely to still have symptoms.40 Start low and slow and build from there.15

Does cooking destroy insoluble fiber?

No. In a USDA laboratory analysis, microwaved broccoli still carried 2.81 grams of insoluble fiber per 100 grams, against 3.06 raw, though more of its fiber was the soluble kind: 60.3% insoluble, against 87.4% raw.4

Freshness & method note

October 18, 2026 — Rewrote the page in Dr. Axe's voice, replacing the 2025 version by Jillian Levy ("25 Insoluble Fiber Foods & Surprising Benefits Beyond Constipation Relief"). Replaced the 25-food list with five food groups ranked with USDA FoodData Central fiber per serving, plus a USDA laboratory analysis of how much of each food's fiber is insoluble. Corrected the earlier statement that soluble fiber ferments in the stomach (gut bacteria ferment fiber in the colon), and removed uncited claims that insoluble fiber sweeps carcinogens out of the gut and lowers the risk of SIBO. Added the research on constipation, blood sugar, diverticular disease and colorectal cancer, three charts, a diagram of how insoluble fiber works, a sample day of meals, one safety section and an FAQ, with 44 numbered references.

References

Dr. Josh Axe, DC, DNM, CNS

Doctor of Natural Medicine · Founder, DrAxe.com

Dr. Axe is a certified doctor of natural medicine, clinical nutritionist, and multiple New York Times bestselling author. He has spent 20+ years in clinical practice helping people heal at the root cause and founded one of the most-visited natural-health platforms in the world.

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