fiber, insoluble

What Is It?
Health Benefits
Dosage Information

Guidelines for Use

General Interaction
Possible Side Effects
Evidence Based Rating Scale 

What Is It?

Insoluble fiber is a subclass of dietary fiber. Like its soluble cousin, insoluble fiber differs from starch because the chemical bonds that join individual sugar units cannot be digested by enzymes in the human gastrointestinal tract. For this reason, insoluble fiber is considered a "noncarbohydrate carbohydrate”.  Insoluble fiber is made up of compounds called lignins, cellulose, and hemicelluloses, which form the structural parts of plants: they do not readily dissolve in water and they are not metabolized by intestinal bacteria. Bran fiber is rich in hemicelluloses, while a cotton ball is pure cellulose.

Insoluble fiber is important because it provides mass to the stool, helping to ease elimination. The fiber absorbs water and holds onto it in the intestine rather than allowing it to be excreted in the urine. With adequate fiber intake, the water-retaining property helps to enlarge and soften the stool. As a result, less pressure is required to move the bowels.

Insoluble fibers, especially certain types of hemicelluloses, are the best fibers for softening and adding bulk to the stool. Bran, which is the fibrous covering of grain kernels, is rich in hemicelluloses. Bran layers form the outer covering of all grains, so whole grains are good sources of insoluble fiber. Wheat products are especially beneficial in increasing fecal bulk, while brown rice is useful in decreasing intestinal transit time. Dietary fiber from fruits and vegetables has demonstrated the most protective effect in human studies. High fluid intake is also important with a high fiber intake in order to obtain the bulk and consistency of stool that can move efficiently through the colon.

Health Benefits

As a result of its ability to increase fecal bulk and decrease intestinal transit time, insoluble fiber decreases the risk for gastrointestinal problems and diseases of the colon, including colon cancer. Additionally, evidence indicates that insoluble fiber may have a protective effect against heart disease and obesity.

Specifically, insoluble fiber may help to:

Protect against cancers. Evidence suggests that dietary fiber serves a major protective function in cancers of the breast, colon, and prostate. Researchers hypothesize that carcinogens are diluted by fluid, attracted and bound to fiber, and then more quickly excreted as the fiber passes through the gastrointestinal tract for elimination. Dietary sources containing lignans (distinct from lignins), fiber compounds found in flax and other seeds, grains, and legumes, have been shown to positively affect these hormone-related cancers. Lignans appear to have anti-estrogen effects, resulting in the ability to inhibit growth of hormone-dependent cancer cells such as those in many breast cancers. (1-4) Early large-scale population studies have also shown protective effects of crude fiber from green vegetables. In one study examining dietary intake in 834 women in two Chinese populations at low risk for breast cancer, the dietary intake of fiber, carotene, and vitamin C, along with low fat intake, showed a significant, inverse association with breast cancer risk. Women who had consumed the lowest amount of fiber and highest amount of fat had a 2.9-fold higher risk for cancer compared to those with the highest fiber and lowest fat intake. (5) In another population study of 543 women, the same protective effect was found for fibrocystic breast disease, a painful condition caused by noncancerous breast lumps. (6) Two recent meta-analyses of several prospective studies found a significant inverse association between fiber intake and breast cancer risk, but only when there was a large difference between the groups studied in the amount of fiber consumed. (7, 8)

Population research has shown that intake of dietary lignans is associated with a decreased risk of developing prostate cancer. (9) Studies involving flaxseed as the primary source of lignans are somewhat conflicting and inconclusive, however. Preliminary research indicates that dietary supplementation with 30 grams of flaxseed daily, along with a fat-restricted diet, appears to lower prostate specific antigen (PSA) levels in men with prostatic intraepithelial neoplasia (PIN), a precancerous proliferation of prostatic cells. (10) However, in men with prostate cancer awaiting prostatectomy (the removal of the prostate gland), supplementation with 30 grams of flaxseed with or without a fat-restricted diet did not significantly reduce PSA, even though it did seem to lower testosterone levels and reduce tissue proliferation in these men. (11, 12) When evaluating the role of whole grains and prostate cancer risk, a large prospective study of more than 26,000 middle-age Danish men showed no association between higher intakes of total or specific whole-grain products and risk of prostate cancer. (13) More research is needed in this area.

Evidence regarding the protective effects of adding fiber as a dietary supplement in colon cancer has been conflicting. In a 1995 study of 390 patients with prior colorectal adenomas/polyps (benign tumors), patients consuming both a low-fat diet and a supplement of 25 grams of wheat bran daily had no large (10 mm or larger) polyps after two and four years—a statistically significant effect compared to the groups doing only one or the other. (14) In the 2000 Polyp Prevention Trial involving 1,905 middle-age men and women who had had one or more polyps removed from the colon within the previous six months, adopting a diet low in fat and high in fiber from barley, oat bran, oats, rice bran, wheat bran, fruits, and vegetables did not influence the risk of developing additional polyps. (15) But a later study found that those participants who had strictly adhered to the low-fat, high-fiber, high-fruit and -vegetable intervention for four years – showed a 35% reduced risk of recurrent adenomas compared to the control group. (16)

Prevent diverticulosis. The cellulose in insoluble fiber may help to soften stools, increase bulk and speed transit time in the colon, thus guarding against the development of diverticulosis, a condition in which small pouches bulge from the side of the intestinal wall and may become infected. In a 1998 prospective cohort study of more than 43,800 healthy male health professionals ages 40 to 75, the insoluble component of fiber was significantly associated with a decreased risk of diverticular disease. This inverse association was particularly strong for cellulose, which helps to soften stools, increase bulk, and speed transit time. (17) However, conflicting evidence exists. A 2001 cross-sectional study of 2,104 participants, ages 30 to 80 with asymptomatic diverticulosis, found that a high-fiber diet and increased frequency of bowel movements was actually associated with a greater, rather than lower, prevalence of diverticulosis. Risk actually increased when calculated based on intake of total fiber, fiber from grains, soluble fiber, and insoluble fiber. (18)

Treat irritable bowel syndrome (IBS). High-fiber diets seem to have a strong beneficial impact on diseases of the colon, such as irritable bowel syndrome. A 2002 multi-center, randomized, open study comparing a wheat bran diet with guar gum in treating IBS showed that taking wheat bran may reduce abdominal pain and improve bowel function in patients with mild to moderate IBS. However, it may not be as effective as guar gum, a soluble dietary fiber often used as a bulk laxative. (19) Conflicting studies have shown that insoluble fibers may actually exacerbate symptoms. In a 2004 review of 17 studies evaluating different types of fiber in the treatment of IBS, insoluble fibers like corn and wheat bran, in some individual cases, worsened symptoms, but there was no significant difference compared with placebo. The review did, however, indicate that fiber, both soluble and insoluble, led to marginal improvements in constipation in patients with IBS. (20) Several small studies have shown that taking psyllium husk, which contains both soluble and insoluble fiber and acts as a bulk laxative, seems to improve symptoms of IBS, including relieving constipation and improving abdominal pain, diarrhea, and overall well-being. (21-24) However, conflicting evidence shows no benefit. (25)

Lower cholesterol. A high-fiber diet has been shown to reduce total serum cholesterol and triglyceride levels while raising serum HDL (or "good") cholesterol levels in preliminary studies. A 2010 double-blind, randomized, placebo-controlled study of 88 participants with high cholesterol showed that consuming insoluble fiber for four weeks reduced total cholesterol by 17% and LDL (or "bad") cholesterol by 22%, while at the same time improving the LDL to HDL ratio by 26% and lowering triglycerides by 16%. (26) More research is needed.

Decrease risk of heart disease. The protective effects of a high-fiber, low-fat diet can help to prevent the build-up of arterial plaque and even reverse it, thus slowing, stopping, or even reversing the risk of heart disease. In addition to preventing plaque build-up, fiber also seems to protect against heart disease by decreasing inflammation as indicated by levels of C-reactive protein (CRP), a marker for heart disease. A high-fiber, low-fat diet seems to lower the risk of elevated CRP. (27) One six-year cohort study of more than 43,700 healthy male health professionals found that a 10 gram increase in fiber intake was associated with a 20% reduction in relative risk for myocardial events, such as heart attack. (28) Fiber from grains has been shown to be most strongly related to a reduced risk of heart attack and stroke, compared to vegetable and fruit intake. (28-29) In a 14-year prospective study of 58,730 Japanese men and women ages 40 to 79, dietary fiber intake, both insoluble and soluble, and especially fruit and grain fibers, was inversely associated with risk of mortality from coronary heart disease. A higher intake of insoluble fiber reduced the risk of CHD in men by 48% and in women by 49%. (30)

Promote weight loss and reduce obesity. Some studies and human case reports indicate that high fiber intake leads to increased post-meal satiety and decreased subsequent hunger. Eating less overall thus leads to weight loss. A 2001 review of studies found that increasing insoluble fiber and soluble fiber intake by an additional 14 grams daily is associated with a 10% decrease in caloric intake and an average weight loss of 4.2 pounds over 3.8 months. Furthermore, when fiber intake was increased, obese individuals ate less and lost more weight than lean individuals. Obese individuals consumed 12% fewer calories and lost 3.5 more pounds than lean participants. (31) More research is needed to confirm or refute efficacy.


  • Capsule
  • Food
  • Powder
  • Tablet 

Dosage Information 

The American Dietetic Association recommends adults get 20 - 35 grams per day of dietary fiber, preferably from foods rather than supplements. Read the ingredient list on labels to determine the source and amount of dietary fiber in food. Whole-grain breads and cereals are high in insoluble fiber. 

Guidelines for Use 

Drink 8 ounces of water with fiber supplements and a total of 6 - 8 full glasses of water throughout the day to avoid constipation.  

The United States Department of Agriculture (USDA) 2011 Guidelines suggest that at least half of all daily grains should be whole grains. From the integrative medicine perspective, this is a rather conservative recommendation: it would be preferable for all grain consumed to be whole grain unless there is a specific medical reason to avoid it. (32) 

General Interaction 

In general, fiber supplements may reduce or delay the body's absorption of certain medications. Try to take medications at least 1 hour before or 2 - 4 hours after taking fiber. 

Consult a physician before taking fiber supplements, as they can have side effects or interactions when taken in conjunction with other medications. Some of these interactions include: 

  • Antidepressant medications - Dietary fiber may lower the blood levels and effectiveness of tricyclic antidepressants, such as amitriptyline (Elavil), doxepin (Sinequan), and imipramine (Tofranil)

  • Diabetes medications - While fiber supplements may help to regulate blood sugar levels, they can also reduce absorption of some medications, such as glyburide (Diabeta) and metformin (Glucophage).

  • Digoxin - Fiber supplements may reduce the body's ability to absorb digoxin (Lanoxin), a medication used to regulate heart function. Do not take fiber supplements at the same time as digoxin. 

Possible Side Effects 

Fiber can cause gas and bloating. 


Taking fiber supplements without enough water may cause the supplement to swell and could cause choking. People with esophageal stricture (narrowing of the esophagus) or any other narrowing or obstruction of the gastrointestinal tract should not take fiber supplements. 

If chest pain, vomiting or difficulty swallowing or breathing occur after taking fiber supplements, seek immediate medical attention.  

Consult a physician before taking fiber supplements to treat impacted stool or complications of constipation. 


1. Adlercreutz H, Fotsis T, Bannwart C, et al. Determination of urinary lignans and phytoestrogen metabolites, potential antiestrogens and anticarcinogens, in urine of women on various habitual diets. J Steroid Biochem. 1986;25:791-7.
2. Adlercreutz H. Diet, breast cancer, and sex hormone metabolism. Ann NY Acad Sci. 1990;595:281-90.
3. Lampe JW, Martini MC, Kurzer MS, et al. Urinary lignan and isoflavonoid excretion in premenopausal women consuming flaxseed powder. Am J Clin Nutr. 1994;60:122-8.
4. Wang C, Makela T, Hase T, et al. Lignans and flavonoids inhibit aromatase enzyme in human preadipocytes. J Steroid Biochem Mol Biol. 1994;50:205-12.
5. Yuan JM, Wang QS, Ross RK, et al. Diet and breast cancer in Shanghai and Tianjin, China. Br J Cancer. 1994;71:1353-1358.
6. Baghurst PA, Rohan TE. Dietary fiber and risk of benign proliferative epithelial disorders of the breast. Int J Cancer. 1995;63:481-85.
7. Dong JY, He K, Wang P, Qin LQ. Dietary fiber intake and risk of breast cancer: a meta-analysis of prospective cohort studies. Am J Clin Nutr. 2011 Sep;94(3):900-5.
8. Aune D, Chan DS, Greenwood DC, Vieira AR, et al. Dietary fiber and breast cancer risk: a systematic review and meta-analysis of prospective studies. Ann Oncol. 2012 Jan 10.
9. Heald CL, Ritchie MR, Bolton-Smith C, et al. Phyto-oestrogens and a risk of prostate cancer in Scottish men. Br J Nutr. 2007;98:388-96.
10. Demark-Wahnefried W, Robertson CN, Walther PJ, et al. Pilot study to explore effects of low-fat, flaxseed-supplemented diet on proliferation of benign prostatic epithelium and prostate-specific antigen. Urology. 2004;63:900-4.
11. Demark-Wahnefried W, Price DT, Polascik TJ, et al. Pilot study of dietary fat restriction and flaxseed supplementation in men with prostate cancer before surgery: exploring the effects on hormonal levels, prostate-specific antigen, and histopathologic features. Urology. 2001;58:47-52.
12. Demark-Wahnefried W, Polascik TJ, George SL, et al. Flaxseed supplementation (not dietary fat restriction) reduces prostate cancer proliferation rates in men presurgery. Cancer Epidemiol Biomarkers Prev. 2008;17:3577-87.
13. Egeberg R, Olsen A, Christensen J, et al. Intake of whole-grain products and risk of prostate cancer among men in the Danish Diet, Cancer and Health cohort study. Cancer Causes Control. 2011 Aug;22(8):1133-9.
14. MacLennan R, Macrae F, Bain C, et al. Randomized trial of intake of fat, fiber, and beta carotene to prevent colorectal adenomas: the Australian Polyp Prevention Project. J Natl Cancer Inst. 1995;87:1760-6.
15. Schatzkin A, Lanza E, Corle D, et al. Lack of effect of a low-fat, high-fiber diet on the recurrence of colorectal adenomas. Polyp Prevention Trial Study Group. N Engl Med. 2000;342:1149-55.
16. Sansbury LB, Wanke K, Albert PS, et al. The effect of strict adherence to a high-fiber, high-fruit and -vegetable, and low-fat eating pattern on adenoma recurrence. Am J Epidemiol. 2009 Sep 1;170(5):576-84.
17. Aldoori WH, Giovannucci EL, Rockett HR, et al. A prospective study of dietary fiber types and symptomatic diverticular disease in men. J Nutr. 1998 Apr;128(4):714-9.
18. Peery AF, Barrett PR, Park D, et al. A High-fiber diet does not protect against asymptomatic diverticulosis. Gastroenterology. 2011 Nov 4.
19. Parisi GC, Zilli M, Miani MP, et al. High-fiber diet supplementation in patients with irritable bowel syndrome (IBS): a multicenter, randomized, open trial comparison between wheat bran diet and partially hydrolyzed guar gum (PHGG). Dig Dis Sci. 202;47:1697-704.
20. Bijkerk CJ, Muris JW, Knottnerus JA, et al. Systematic review: the role of different types of fibre in the treatment of irritable bowel syndrome. Ailment Pharmacol Ther. 2004 Feb 1;19(3):245-51.
21. Kumar A, Kumar N, Vij JC, et al. Optimum dosage of ispaghula husk in patients with irritable bowel syndrome: correlation of symptom relief with whole gut transit time and stool weight. Gut 1987;28:150-5.
22. Prior A, Whorwell PJ. Double blind study of ispaghula in irritable bowel syndrome. Gut. 1987;28:1510-3.
23. Misra SP, Thorat VK, Sachdev GK, Anand BS. Long-term treatment of irritable bowel syndrome: results of a randomized controlled trial. Q J Med. 1989;73:931-9.
24. Jalihal A, Kurian G. Ispaghula therapy in irritable bowel syndrome: improvement in overall well-being is related to reduction bowel dissatisfaction. J Gastroenterol Hepatol. 1990;5:507-13.
25. Longstreth GF, Fox DD, Youkeles L, et al. Psyllium therapy in the irritable bowel syndrome. A double-blind trial. Ann Intern Med. 1981;95:53-6.
26. Ruiz-Roso B, Quintela JC, de la Fuente E, et al. Insoluble carob fiber rich in polyphenols lowers total and LDL cholesterol in hypercholesterolemic subjects. Plant Foods Hum Nutr. 2010 Mar;65(1):50-6.
27. King DE, Egan BM, Geesey ME. Relation of dietary fat and fiber to elevation of C-reactive protein. Am J Cardiol. 2003;92:1335-9.
28. Rimm EB, Ascherio A, Giovannucci E, et al. Vegetable, fruit and cereal fiber intake and risk of coronary heart disease among men. JAMA. 1996 Feb 14;275(6):447-51.
29. Erkkilä AT, Lichtenstein AH. Fiber and cardiovascular disease risk: how strong is the evidence? J Cardiovasc Nurs. 2006 Jan-Feb;21(1):3-8.
30. Eshak ES, Iso H, Date C, et al. Dietary fiber intake is associated with reduced risk of mortality from cardiovascular disease among Japanese men and women. J Nutr. 2010 Aug;140(8):1445-53.
31. Howarth NC, Saltzman E, Roberts SB. Dietary fiber and weight regulation. Nutr Rev. 2001 May;59(5):129-39.
32. United States Department of Agriculture-2011 Nutritional Guidelines. Available at Accessed January 12, 2012.

Evidence Based Rating Scale

The Evidence Based Rating Scale is a tool that helps consumers translate the findings of medical research studies and what our clinical advisors have found to be efficacious in their personal practice into a visual and easy to interpret format. This tool is meant to simplify the information on supplements and therapies that demonstrate promise in the treatment of certain conditions.








Several population studies have shown efficacy to decrease the incidence of cancers of the breast, colon and prostate with high intake of fiber. More research is needed. (1-16)


  Date Published: 04/18/2005
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