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Fiber


Summary

  • Dietary fiber is a diverse group of compounds, including lignin and complex carbohydrates, which cannot be digested by human enzymes in the small intestine. (More Information)
  • Although each class of fiber is chemically unique, scientists have tried to classify fibers on the basis of their solubility, viscosity, and fermentability in order to better understand their physiological effects. (More Information)
  • Viscous fibers, such as those found in oat products and legumes, can lower serum LDL cholesterol levels and normalize blood glucose and insulin responses. (More Information)
  • High-fiber intakes promote bowel health by preventing constipation and diverticular disease. (More Information)
  • Large prospective cohort studies provide strong and consistent evidence that diets rich in fiber from whole grains, legumes, fruits, and nonstarchy vegetables can reduce the risk of cardiovascular disease and type 2 diabetes. (More Information)
  • Although the results of case-control studies suggested that colorectal cancer was more prevalent in people with low-fiber intakes, more recent findings from large prospective cohort studies and four clinical intervention trials do not support an association between fiber intake and the risk of colorectal cancer. (More Information)
  • Observational studies on dietary fiber intake and breast cancer incidence have reported inconsistent findings. (More Information)
  • Numerous controlled clinical trials in people with type 1 and type 2 diabetes have found that increasing fiber intake improves blood glucose (glycemic) control and serum lipid profiles. (More Information)
  • In 2001, the Food and Nutrition Board of the Institute of Medicine established an Adequate Intake (AI) recommendation for total daily fiber intake. For adults who are 50 years of age and younger, the AI recommendation for total fiber intake is 38 g/day for men and 25 g/day for women. For adults over 50 years of age, the recommendation is 30 g/day for men and 21 g/day for women. (More Information)

Introduction

All dietary fibers are resistant to digestion in the small intestine, meaning they arrive at the colon intact (1). Although most fibers are carbohydrates, one important factor that determines their susceptibility to digestion by human enzymes is the conformation of the chemical bonds between sugar molecules (glycosidic bonds). Humans lack digestive enzymes capable of hydrolyzing (breaking apart) most beta-glycosidic bonds, which explains why amylose, a glucose polymer with alpha-1,4 glycosidic bonds, is digestible by human enzymes, while cellulose, a glucose polymer with beta-1,4 glycosidic bonds, is indigestible (Figure).

Definitions of Fiber

Although nutritional scientists and clinicians generally agree that a healthful diet should include plenty of fiber-rich foods, agreement on the actual definition of fiber has been more difficult to achieve (2-4). In the 1970s, dietary fiber was defined as remnants of plant cells that are resistant to digestion by human enzymes (5). This definition includes a component of some plant cell walls called lignin, as well as indigestible carbohydrates found in plants. However, this definition omits indigestible carbohydrates derived from animal sources (e.g., chitin) and synthetic (e.g., fructooligosaccharides) and digestible carbohydrates that are inaccessible to human digestive enzymes (e.g., resistant starch) (6). These compounds share many of the characteristics of fiber present in plant foods.

Institute of Medicine: Dietary, Functional, and Total Fiber

Before establishing intake recommendations for fiber in 2001, a panel of experts convened by the Institute of Medicine developed definitions of fiber that made a distinction between fiber that occurs naturally in plant foods (dietary fiber) and isolated or synthetic fibers that may be added to foods or used as dietary supplements (functional fiber) (4). However, these distinctions are controversial, and there are other classification systems for dietary fiber (see Other Classification Systems below).

Dietary Fiber

  • Lignin: Lignin is not a carbohydrate; rather, it is a polyphenolic compound with a complex three-dimensional structure that is found in the cell walls of woody plants and seeds (7).
  • Cellulose: Cellulose is a glucose polymer with beta-1,4 glycosidic bonds found in all plant cell walls (see Figure) (6).
  • Beta-Glucans: Beta-glucans are glucose polymers with a mixture of beta-1,4 glycosidic bonds and beta-1,3 glycosidic bonds (see Figure). Oats and barley are particularly rich in beta-glucans (7).
  • Hemicelluloses: Hemicelluloses are a diverse group of polysaccharides (sugar polymers) containing six-carbon sugars (hexoses) and five-carbon sugars (pentoses) (6). Like cellulose, hemicelluloses are found in plant cell walls.
  • Pectins: Pectins are viscous polysaccharides that are particularly abundant in fruits and berries (4).
  • Gums: Gums are viscous polysaccharides often found in seeds (4).
  • Inulin and oligofructose: Inulin is a mixture of fructose chains that vary in length and often terminate with a glucose molecule (8). Oligofructose is a mixture of shorter fructose chains that may terminate in glucose or fructose. Inulin and oligofructose occur naturally in plants, such as onions and Jerusalem artichokes.
  • Resistant starch: Naturally occurring resistant starch is sequestered in plant cell walls and is therefore inaccessible to human digestive enzymes (4). Bananas and legumes are sources of naturally occurring resistant starch. Resistant starch may also be formed by food processing or by cooling and reheating.

Functional Fiber

According to the Institute of Medicine’s definition, functional fiber “consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans (4).” Functional fibers may be nondigestible carbohydrates that have been isolated or extracted from a natural plant or animal source, or they may be manufactured or synthesized. However, designation as a functional fiber by the Institute of Medicine requires the presentation of sufficient evidence of physiological benefit in humans. Fibers identified as potential functional fibers by the Institute of Medicine include:

  • Isolated or extracted forms of the dietary fibers listed above.
  • Psyllium: Psyllium refers to viscous mucilage, which is isolated from the husks of psyllium seeds. The husks are usually isolated from the seeds of Plantago ovata or blond psyllium. Psyllium is also known as ispaghula husk (4).
  • Chitin and chitosan: Chitin is a nondigestible carbohydrate extracted from the exoskeletons of crustaceans, such as crabs and lobsters. It is a long polymer of acetylated glucosamine units linked by beta-1,4 glycosidic bonds. Deacetylation of chitin is used to produce chitosan, a nondigestible glucosamine polymer (9).
  • Fructooligosaccharides: Fructooligosaccharides are short, synthetic fructose chains terminating with a glucose unit. They are used as food additives (8).
  • Polydextrose and polyols: Polydextrose and polyols are synthetic polysaccharides used as bulking agents and sugar substitutes in foods (4).
  • Resistant dextrins: Resistant dextrins, also called resistant maltodextrins, are indigestible polysaccharides formed when starch is heated and treated with enzymes. They are used as food additives (4).

Total Fiber

Total fiber is defined by the Institute of Medicine as “the sum of dietary fiber and functional fiber (4).”

Other Classification Systems

Viscous and Nonviscous Fiber

Some fibers form very viscous solutions or gels in water. This property is linked to the ability of some fibers to slow the emptying of the stomach, delay the absorption of some nutrients in the small intestine, and lower serum cholesterol. Viscous fibers include pectins, beta-glucans, some gums (e.g., guar gum), and mucilages (e.g., psyllium). Cellulose, lignin, and some hemicelluloses are nonviscous fibers (6, 7).

Fermentable and Nonfermentable Fiber

Some fibers are readily fermented by bacteria that normally colonize the colon. In addition to increasing the amount of bacteria in the colon, fermentation results in the formation of short-chain fatty acids (acetate, propionate, and butyrate) and gases (1). Short-chain fatty acids can be absorbed and metabolized to produce energy. Interestingly, the preferred energy source for colonocytes (epithelial cells that line the colon) is butyrate. Pectins, beta-glucans, guar gum, inulin, and oligofructose are readily fermented, while cellulose and lignin are resistant to fermentation in the colon (6, 7). Foods that are rich in fermentable fibers include oats and barley, as well as fruits and vegetables. Cereal fibers that are rich in cellulose, such as wheat bran, are relatively resistant to bacterial fermentation (1).

Soluble and Insoluble Fiber

"Soluble fiber" originated as an analytical term (10). Soluble fibers are dispersible in water, while insoluble fibers are not. Originally, the solubility of fiber was thought to predict its physiological effects. For example, it was thought that soluble fibers were more likely to form viscous gels and were more easily fermented by colonic bacteria. Further research has revealed that solubility does not reliably predict the physiological effects of fiber. However, the terms “soluble” and “insoluble” fiber are still used by many nutrition and health care professionals, as well as the U.S. Food and Drug Administration (FDA) for nutrition labeling. Beta-glucans, gums, mucilages (e.g., psyllium), pectins, and some hemicelluloses are soluble fibers, while cellulose, lignin, some pectins, and some hemicelluloses are insoluble fibers (10). Oat products and legumes (dry beans, peas, and lentils) are rich sources of soluble fiber.

Biological Activities

Lowering Serum Cholesterol

Numerous controlled clinical trials have found that increasing the intake of viscous dietary fibers, particularly from legumes (dry beans, peas, and lentils) (11-13) and oat products (14-19), decreases serum total and LDL cholesterol. Such findings led the FDA to approve health claims like the following on labels of foods containing at least 0.75 g/serving of soluble fiber from whole oats: “Soluble fiber from foods such as oat bran, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease” (20). Supplementation with viscous fibers, such as pectin, guar gum, and psyllium, has also been found to decrease total and LDL cholesterol levels when compared to low-fiber placebos (17, 21-26). Although many of these studies examined relatively high-fiber intakes, a meta-analysis that combined the results of 67 controlled trials found that even a modest 10-g per day increase in viscous fiber intake resulted in reductions in LDL-cholesterol averaging 22 mg/dL (0.57 mmol/L) and reductions in total cholesterol averaging 17 mg/dL (0.45 mmol/L) (17).

Lowering Postprandial Glycemia (Blood Sugar)

The addition of viscous dietary fiber (27, 28) and isolated viscous fibers (29-32) to a carbohydrate-containing meal has been found to result in significant improvements in blood glucose and insulin responses in numerous controlled clinical trials (33). Large, rapid increases in blood glucose levels are potent signals to the beta-cells of the pancreas to increase insulin secretion. Over time, recurrent elevations in blood glucose and excessive insulin secretion are thought to increase the risk of developing type 2 diabetes mellitus (DM), as well as cardiovascular disease (see Disease Prevention below). When the carbohydrate content of two meals is equal, the presence of fiber, particularly viscous fiber, generally results in smaller but more sustained increases in blood glucose and thus significantly lower insulin levels (33).

Softening Stool

Increasing intakes of dietary fibers and fiber supplements can prevent or ameliorate constipation by softening and adding bulk to stool and by speeding its passage through the colon (34). Wheat bran and fruits and vegetables are the fiber sources that have been most consistently found to increase stool bulk and shorten transit time (35). Fiber supplements that have been found to be effective in treating constipation include cellulose and psyllium (4). Sufficient fluid intake is also required to maximize the stool-softening effect of increased fiber intake (36). In addition to increasing fiber intake, drinking at least 64 ounces (~ 2 liters or 2 quarts) of fluid daily is usually recommended to help prevent and treat constipation (37).

Disease Prevention

Observational studies that have identified associations between high-fiber intakes and reductions in chronic disease risk have generally assessed only fiber-rich foods, rather than fiber itself, making it difficult to determine whether observed benefits are related to fiber or other nutrients and phytochemicals commonly found in fiber-rich foods. In contrast, intervention trials often use isolated fibers to determine whether a specific fiber component has beneficial health effects.

Cardiovascular Disease

Prospective cohort studies have consistently found that high intakes of fiber-rich foods are associated with significant reductions in coronary heart disease (CHD) risk (38-48) and cardiovascular-related mortality (48-51). A pooled analysis of ten prospective cohort studies of dietary fiber intake in the U.S. and Europe found that each 10 g/day increase in total dietary fiber intake was associated with a 14% decrease in the risk of coronary events, such as myocardial infarction (MI), and a 24% decrease in deaths from CHD (51). This inverse association between fiber intake and CHD death was particularly high for cereal fiber and fruit fiber. Three large prospective cohort studies (42, 43, 45) found that dietary fiber intakes of approximately 14 g per 1,000 kcal of energy were associated with substantial (16%-33%) decreases in the risk of CHD; these results are the basis for the Institute of Medicine’s Adequate Intake (AI) recommendation for fiber (see Intake Recommendations below) (4).

Although the cholesterol-lowering effect of viscous dietary fibers and fiber supplements probably contributes to the cardioprotective effects of dietary fiber (see Lowering Serum Cholesterol), other mechanisms are likely to play a role. Beneficial effects of fiber consumption on blood glucose and insulin responses may also contribute to observed reductions in CHD risk (52). Low-fiber, high-glycemic load diets are associated with higher serum triglyceride levels and lower HDL cholesterol levels, two risk factors for cardiovascular disease (53, 54). Fiber-rich diets that are rich in certain micronutrients like magnesium and potassium may also help lower blood pressure, another important risk factor for cardiovascular disease. Some observational studies have found inverse associations between dietary fiber intake and blood pressure (55) or hypertension (56). A meta-analysis of 24 randomized, placebo-controlled trials found that dietary fiber supplementation (average of 11.5 g/day) lowered systolic blood pressure (SBP) by 1.13 mm Hg and diastolic blood pressure (DBP) by 1.26 mm Hg (57). Similarly, another meta-analysis of 25 randomized controlled trials found that an increase in dietary fiber (median increase of 10.7 g/day compared to the control group) was associated with a 1.15 mm Hg reduction in SBP and a 1.65 mm Hg reduction in DBP (58). Both analyses reported that the reductions in DBP, but not SBP, were statistically significant. Additionally, recent studies have indicated that higher consumption of dietary fiber may lower levels of C-reactive protein (59, 60), a biomarker of inflammation that is strongly associated with the risk of cardiovascular events, such as MI and stroke (61). Thus, several mechanisms might contribute to the cardioprotective effect of dietary fiber. Although viscous dietary fibers and fiber supplements appear to be most effective in lowering LDL-cholesterol levels, large epidemiological studies provide strong and consistent evidence that diets rich in all fiber from whole grains, legumes, fruits, and nonstarchy vegetables can significantly reduce CHD risk (62).

Type 2 Diabetes Mellitus

Increasing intakes of refined carbohydrates and decreasing intakes of fiber in the U.S. have paralleled the increasing prevalence of type 2 diabetes mellitus (DM) to near epidemic proportions (63). Numerous prospective cohort studies have found that that diets rich in fiber, particularly cereal fiber from whole grains, are associated with significant reductions in the risk of developing type 2 DM (64-74). Although no intervention trials have evaluated the effect of increasing dietary fiber intake alone on type 2 DM prevention, two important intervention trials found that a combination of lifestyle modifications that included increasing fiber intake decreased the risk of developing type 2 DM in adults with impaired glucose tolerance (75, 76). Although multiple factors, including obesity, inactivity, and genetics, increase the risk of developing type 2 DM, the results of observational studies and intervention trials indicate that fiber-rich diets improve glucose tolerance and decrease the risk of type 2 DM, particularly in high-risk individuals.

Cancer

Colorectal Cancer

The majority of case-control studies conducted prior to 1990 found the incidence of colorectal cancer was lower in people with higher fiber intakes (77, 78). A recent nested case-control study found an inverse association between dietary fiber intake and risk of colorectal cancer when fiber intake was assessed by food diaries but not when assessed by food-frequency questionnaires (79). To date, most prospective cohort studies have not found significant associations between measures of dietary fiber intake and colorectal cancer risk (80-89). A pooled analysis of 13 prospective cohort studies, which analyzed data from 725,628 adults, did not find high dietary fiber intake to be protective against colorectal cancer when other dietary factors were taken into account (90). However, the largest prospective study on diet and cancer to date, which included 519,978 men and women participating in the European Prospective Investigation into Cancer and Nutrition (EPIC) project, found that dietary fiber from foods was protective against colon cancer development (91). This EPIC study was not included in the earlier pooled analysis mentioned above that reported no association between dietary fiber intake and colorectal cancer (90).

In addition, four controlled clinical trials have failed to demonstrate a protective effect of fiber consumption on the recurrence of colorectal adenomas (precancerous polyps). The rate of recurrence of colorectal adenomas over a 4-year period was not significantly different between those who consumed about 33 g/day of fiber from a fruit and vegetable-rich, low-fat diet and those in a control group who consumed about 19 g/day of fiber (92). In another trial, there was no significant difference in the rate of colorectal adenoma recurrence over a 3-year period between those supplemented with 13.5 g/day of wheat-bran fiber and those supplemented with 2 g/day of wheat-bran fiber (93). More recently, a 4-year intervention trial found that supplementation with 7.5 g/day of wheat bran had no effect on colorectal adenoma recurrence (94). Surprisingly, in another intervention trial, supplementation with 3.5 g/day of psyllium for three years resulted in a significant increase in adenoma recurrence compared to placebo (95).

The reasons for the discrepancies between the findings of case-control studies with those of most prospective cohort studies and recent intervention trials have generated considerable debate among scientists. Potential reasons for the lack of a protective effect by dietary fiber observed in these studies include the possibility that the type or the amount of fiber consumed by most people in these studies was inadequate to prevent colorectal cancer (4), or that other dietary factors like fat may interact with fiber, influencing its effects on colorectal cancer (1, 96). The methods used to assess fiber intake in observational studies may also contribute to the disparate results (79). Clearly, more research is needed to sort out the complex effects of dietary fiber and fiber supplements on colorectal cancer risk and progression.

Breast Cancer

A number of early case-control studies found significant inverse associations between dietary fiber intake and breast cancer incidence (97-100), but many prospective cohort studies have not found dietary fiber intake to be associated with significant reductions in breast cancer risk (101-108). Three studies have reported a protective effect of dietary fiber on breast cancer risk. A prospective cohort study in the UK found that dietary fiber intake was inversely associated with risk of breast cancer in premenopausal women but not in postmenopausal women (109). Additionally, a prospective cohort study in Sweden found that postmenopausal women with the highest fiber intakes (averaging about 26 g/day) had a risk of breast cancer that was 40% lower than women with the lowest fiber intakes (averaging about 13 g/day) (110). Women with the highest fiber and lowest fat intakes had the very lowest risk of breast cancer. More recently, a prospective study in a cohort of more than 185,000 U.S. postmenopausal women found that those with the highest intakes of dietary fiber (median, 26 g/day) had a 13% lower risk of all forms of breast cancer and a 44% lower risk of hormone receptor-negative tumors (ER-/PR-) compared to those with the lowest intakes of dietary fiber (median, 11 g/day) (111). A 2011 meta-analysis of ten prospective cohort studies found a modest, 11% lower risk of breast cancer in women with the highest intakes of dietary fiber (112). The results of small, short-term intervention trials in premenopausal and postmenopausal women suggest that low-fat (10%-25% of energy), high-fiber (25-40 g/day) diets could decrease circulating estrogen levels by increasing the excretion of estrogens and by promoting the metabolism of estrogens to less estrogenic forms (113, 114). However, it is not known whether fiber-associated effects on endogenous estrogen levels have a clinically significant impact on breast cancer risk (4). Overall, observational studies examining dietary fiber intake and breast cancer incidence have reported mixed results. In such studies, it is possible that the association may be confounded by consumption of fiber-rich foods, such as fruits and vegetables.

Diverticular Disease

Some observational studies have associated high-fiber intakes with a decreased risk of diverticulosis, a relatively common condition that is characterized by the formation of small pouches (diverticula) in the colon (115, 116). However, a recent cross-sectional study of 2,104 adults, found that those with the highest fiber intakes, measured by food frequency questionnaires, had a higher prevalence of diverticula (assessed by colonoscopy) compared to those with the lowest fiber intakes (117). Although most people with diverticulosis experience no symptoms, about 15%-20% may develop pain or inflammation, known as diverticulitis (118). In a large prospective cohort study, men with the highest insoluble fiber intakes (median, 22.7 g/day) had a 37% lower risk of developing symptomatic diverticular disease compared to men with the lowest insoluble fiber intakes (median, 10.1 g/day). The protective effect of dietary fiber against diverticular disease was strongest for cellulose and lignin (119). More research is needed to clarify the association between dietary fiber and diverticular disease.

Weight Control

In addition to providing less energy, there is some evidence that higher fiber intakes can help to prevent weight gain or promote weight loss by extending the feeling of fullness after a meal (satiety) (120). Observational studies have found that adults with higher intakes of dietary fiber are leaner (121, 122) and less likely to be obese than adults with low-fiber intakes (123, 124). One large prospective cohort study found that women whose intake of high-fiber foods increased by an average of 9 g/day over a 12-year period were half as likely to experience a major weight gain of at least 55 lb (25 kg) than those whose intake of high-fiber foods decreased by an average of 3 g/day (125). The results of short-term clinical trials examining the effect of increased fiber intake on weight loss have been mixed. Overall, a systematic review of clinical trials conducted prior to 2001 found that increasing fiber intake from foods or supplements by 14 g/day resulted in a 10% decrease in energy intake and weight losses averaging about 4 1b (1.9 kg) over four months (120). However, some more recent clinical trials did not find fiber-rich cereal (126) or fiber supplements (127) enhanced weight loss. A 2011 systematic review of 61 randomized controlled trials examined the effect of different fiber types on body weight (128). This analysis found that dextrins and marine polysaccharides reduced body weight in all of the studies, while chitosan, arabinoxylans, and fructans reduced body weight in at least two-thirds of the studies. Average weight reductions were greatest for the fructans and marine polysaccharides groups (~1.3 kg or 2.8 lb/4 weeks for a 79 kg person in both groups). For all fiber types combined, the average weight reduction was only 0.3 g (0.7 lb) per 4 weeks for a 79-kg person (128). Although people with higher intakes of fiber-rich foods, particularly whole grains, appear more likely to maintain a healthy body weight, the role of fiber alone in long-term weight control is not yet clear. Effects on body weight might depend on the specific type of dietary fiber.

All-cause Mortality

Several prospective cohort studies have found higher intakes of dietary fiber to be associated with a lower risk of mortality from all causes. A recent report from the NIH-AARP Diet and Health Study, which followed 388,122 older adults for an average of nine years, found that men and women in the highest quintiles of dietary fiber intake had a 22% lower risk of mortality when compared to the lowest quintiles of dietary fiber intake (50). Smaller prospective studies have also reported an inverse association between total fiber intake and all-cause mortality (44, 48, 129), and an inverse association between cereal fiber intake and all-cause mortality was found in the Nurses’ Health Study, which included more than 50,000 participants (130). However, no association was found between total fiber intake or soluble fiber intake and mortality from all causes in the National Health and Nutrition Examination Survey (NHANES) I Epidemiologic Follow-Up Study—a prospective study that assessed fiber intake by a single, 24-hour dietary recall method (46).

Disease Treatment

Diabetes Mellitus

Numerous controlled clinical trials in people who have type 1 or type 2 diabetes mellitus (DM) have found that increasing fiber intake from foods (131, 132) or viscous fiber supplements (133-135) improves markers of glycemic control, particularly postprandial glucose levels and serum lipid profiles. A meta-analysis that combined the results of 23 clinical trials comparing the effects of high-fiber diets (>20 g/1,000 kcal) with those of low-fiber diets (<10 g/1,000 kcal) in diabetic patients found that high-fiber diets lowered postprandial blood glucose concentrations by 13%-21%, serum LDL cholesterol concentrations by 8%-16%, and serum triglyceride concentrations by 8%-13% (136). Based on the evidence from this meta-analysis, the authors recommended a dietary fiber intake of 25-50 g/day (15-25 g/1,000 kcal) for individuals with diabetes, which is consistent with the recommendations of many international diabetes organizations of at least 25-35 g/day (137-139). In general, the results of controlled clinical trials support recommendations that people with diabetes aim for high-fiber intakes by increasing consumption of whole grains, legumes, nuts, fruits, and nonstarchy vegetables. Since there is little evidence from clinical trials that increasing nonviscous fiber alone is beneficial (140), individuals with diabetes should avoid increasing fiber intake exclusively from nonviscous sources, such as wheat bran (136).

Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a functional disorder of the intestines, characterized by episodes of abdominal pain or discomfort associated with a change in bowel movements, such as constipation or diarrhea (141). Although people diagnosed with IBS are often encouraged by health care providers to increase dietary fiber intake, the results of controlled clinical trials of psyllium, methylcellulose, and wheat bran have been mixed (142-144). A systematic review and meta-analysis of 12 randomized controlled trials (RCTs) found a beneficial effect of fiber that was limited to ispaghula husk (psyllium) (143). More recently, a 3-month randomized, placebo-controlled trial in 275 patients with IBS found that supplementation with psyllium (10 g/day) improved symptoms of abdominal pain or discomfort in the first two months of supplementation and also improved symptom severity after three months’ supplementation (145). Compared to placebo, supplementation with insoluble bran fiber (10 g/day) improved abdominal pain or discomfort only after three months’ supplementation and had no effect on symptom severity (145). Additionally, a systematic review of 17 RCTs of fiber supplements in IBS patients found that supplementation with soluble fiber, mainly from psyllium, significantly improved a global measure of IBS symptoms, while supplementation with insoluble fiber, such as corn bran or wheat bran, did not improve IBS symptoms (146). In general, fiber supplements improved constipation in IBS patients, but did not improve IBS-associated abdominal pain. Thus, the results of randomized controlled trials suggest that increasing soluble or viscous fiber intake gradually to 12-30 g/day may be beneficial for patients in whom constipation is the predominant symptom of IBS (147). However, fiber supplements could actually exacerbate symptoms in those whom diarrhea predominates (148). A few clinical trials have found that partially hydrolyzed guar gum (5 g/day), a water-soluble, non-gelling fiber, may improve IBS symptoms in patients with diarrhea and in those with constipation-predominant IBS (149). IBS patients should be advised to increase fiber intake gradually, since increasing intake of viscous, readily fermented fibers could increase gas production and bloating.

Sources

Food Sources

Dietary fiber intakes in the U.S. average from 16-18 g/day for men and 12-14 g/day for women—well below recommended intake levels (4) (see Intake Recommendations below). Good sources of dietary fiber include legumes, nuts, whole grains, bran products, fruits, and nonstarchy vegetables. Legumes, whole grains, and nuts are generally more concentrated sources of fiber than fruits and vegetables. All plant-based foods contain mixtures of soluble and insoluble fiber (10). Oat products and legumes are rich sources of soluble and viscous fiber. Wheat bran and whole grains are rich sources of insoluble and nonviscous fiber. The total fiber content of some fiber-rich foods is presented in the table below. Some strategies for increasing dietary fiber intake include increasing fruit and nonstarchy vegetable intake, increasing intake of legumes, eating whole-grain cereal or oatmeal for breakfast, substituting whole grains for refined grains, and substituting nuts or popcorn for less healthy snacks. For more information about the fiber content of specific foods, search the USDA National Nutrient Database.

Food Serving Fiber (g)
Legumes
Navy beans, cooked from dried ½ cup 9.6
Split peas, cooked from dried ½ cup 8.1
Lentils, cooked from dried ½ cup 7.8
Kidney beans, canned ½ cup 6.8
Refried beans, canned ½ cup 6.1
Cereals and grains
100% (wheat) Bran Cereal ½ cup 12.5
Oats ½ cup 8.3
Bulgur, cooked ½ cup 4.1
Pearled barley, cooked ½ cup 3.0
Oat bran, cooked ½ cup 2.9
Quinoa, cooked ½ cup 2.6
Instant oatmeal, cooked ½ cup 2.0
Rice, long-grained brown, cooked ½ cup 1.8
Vegetables
Artichoke hearts, cooked ½ cup 7.2
Spinach, frozen, cooked ½ cup 3.5
Brussel sprouts, frozen, cooked ½ cup 3.2
Winter squash, cooked 1 cup 2.9
Mushrooms, white, cooked from fresh 1 cup 1.7
Fruits
Prunes, uncooked ½ cup, pitted 6.2
Guava, fresh ½ cup 4.5
Asian pear 1 small pear 4.4
Raspberries, fresh ½ cup 4.0
Blackberries, fresh ½ cup 3.8
Nuts and Seeds
Almonds 1 ounce (23 kernels) 3.5
Pistachio nuts 1 ounce (49 kernels) 2.9
Pecans 1 ounce (19 halves) 2.7
Peanuts 1 ounce 2.4

 

Isolated Fibers and Supplements

Beta-Glucans

Beta-glucans are viscous, easily fermented, soluble fibers found naturally in oats, barley, mushrooms, yeast, bacteria, and algae (151). Beta-glucans extracted from oats, mushrooms, and yeast are available in a variety of nutritional supplements without a prescription.

Pectin

Pectins are viscous fibers, most often extracted from citrus peels and apple pulp. Pectins are widely used as gelling agents in foods but are also available as dietary supplements without a prescription (9).

Inulins and Oligofructose

Inulins and oligofructose, extracted from chicory root or synthesized from sucrose, are used as food additives (8). Isolated inulin is added to replace fat in products like salad dressing, while sweet-tasting oligofructose is added to products like fruit yogurts and desserts. Inulins and oligofructose are highly fermentable fibers that are also classified as prebiotics because of their ability to stimulate the growth of potentially beneficial Bifidobacteria species in the human colon (152). Encouraging the growth of Bifidobacteria could promote intestinal health by suppressing the growth of pathogenic bacteria known to cause diarrhea or by enhancing the immune response (153). Although a number of dietary supplements containing inulins and oligofructose are marketed as prebiotics, the health benefits of prebiotics have not yet been convincingly demonstrated in humans (154, 155).

Guar gum

Guar gum is a viscous, fermentable fiber derived from the Indian cluster bean (4). It is used as a thickener or emulsifier in many food products. Dietary supplements containing guar gum have been marketed as weight-loss aids, but a meta-analysis that combined the results of 11 randomized controlled trials found that guar gum supplements were not effective in reducing body weight (156).

Psyllium

Psyllium, a viscous, soluble fiber isolated from psyllium seed husks, is available without a prescription in laxatives, ready-to-eat cereals, and dietary supplements (9). The FDA has approved health claims like the following on the labels of foods containing at least 1.7 g/serving of soluble fiber from psyllium: “Diets low in saturated fat and cholesterol that include 7 g/day of soluble fiber from psyllium may reduce the risk of heart disease (20).”

Chitosan

Chitosan is an indigestible glucosamine polymer derived from chitin. When administered with food, chitosan decreased fat absorption in animal studies (157). Consequently, chitosan has been marketed as a dietary supplement to promote weight loss and lower cholesterol. Controlled clinical trials in humans have not generally found chitosan supplementation to be more effective than placebo in promoting weight loss (158). While some clinical trials in humans have found chitosan supplementation to result in modest reductions in total and LDL cholesterol levels compared to placebo (159, 160), others found no improvement (161, 162). Chitosan is available as a dietary supplement without a prescription in the U.S.

Note: All fiber supplements should be taken with sufficient fluids. Most clinicians recommend taking fiber supplements with at least 8 ounces (240 ml) of water and consuming a total of at least 64 ounces (~2 liters or 2 quarts) of fluid daily (163, 164).

Safety

Adverse Effects

Dietary Fiber

Some people experience abdominal cramping, bloating, or gas when they abruptly increase their dietary fiber intakes (163, 164). These symptoms can be minimized or avoided by increasing intake of fiber-rich foods gradually and increasing fluid intake to at least 64 oz/day (~2 liters or 2 quarts/day). There have been rare reports of intestinal obstruction related to large intakes of oat bran or wheat bran, usually in people with impaired intestinal motility or difficulty chewing (165-168). The Institute of Medicine has not established a tolerable upper intake level (UL) for dietary or functional fiber (4).

Isolated Fibers and Fiber Supplements

Gastrointestinal symptoms: The following fibers have been found to cause gastrointestinal distress, including abdominal cramping, bloating, gas, and diarrhea: guar gum, inulin and oligofructose, fructooligosaccharides, polydextrose, resistant starch, and psyllium (4). Use of a guar gum-containing supplement for weight loss has been associated with esophageal and small bowel obstruction (169). Additionally, several cases of intestinal obstruction by psyllium have been reported when taken with insufficient fluids or by people with impaired swallowing or gastrointestinal motility (170, 171).

Colorectal adenomas: One randomized controlled trial in patients with a history of colorectal adenomas (precancerous polyps) found that supplementation with 3.5 g/day of psyllium for three years resulted in a significant increase in colorectal adenoma recurrence compared to placebo (see Colorectal Cancer above) (95).

Allergy and anaphylaxis: Since chitin and chitosan may be isolated from the exoskeletons of crustaceans, such as crabs and lobsters, people with shellfish allergies should avoid taking chitin or chitosan supplements (9). Anaphylaxis has been reported after intravenous (IV) administration of inulin (172), as well as ingestion of margarine containing inulin extracted from chicory (173). Anaphylaxis has also been reported after the ingestion of cereals containing psyllium, and asthma has occasionally been reported in people with occupational exposure to psyllium powder (174).

Drug Interactions

Psyllium may reduce the absorption of lithium, carbamazepine (Tegretol), digoxin (Lanoxin), and warfarin (Coumadin) when taken at the same time (9). Guar gum may slow the absorption of digoxin, acetaminophen (Tylenol), and bumetanide (Bumex) and decrease the absorption of metformin (Glucophage), penicillin, and some formulations of glyburide (Glynase) when taken at the same time (175). Pectin may decrease the absorption of lovastatin (Mevacor) when taken at the same time (176). Concomitant administration of kaolin-pectin has been reported to decrease the absorption of clindamycin, tetracyclines, and digoxin, but it is not known whether kaolin or pectin is responsible for the interaction (9). In general, medications should be taken at least one hour before or two hours after fiber supplements.

Nutrient Interactions

The addition of cereal fiber to meals has generally been found to decrease the absorption of iron, zinc, calcium, and magnesium in the same meal, but this effect appears to be related to the phytate present in the cereal fiber rather than the fiber itself (177). In general, dietary fiber as part of a balanced diet has not been found to adversely affect the calcium, magnesium, iron, or zinc status of healthy people at recommended intake levels (4). Evidence from animal studies and limited research in humans suggests that inulin and oligofructose may enhance calcium absorption (178, 179). The addition of pectin and guar gum to a meal significantly reduced the absorption of the carotenoids beta-carotene, lycopene, and lutein from that meal (180, 181).

Intake Recommendations

The Adequate Intake (AI) for Total Fiber

In light of consistent evidence from prospective cohort studies that fiber-rich diets are associated with significant reductions in cardiovascular disease risk, the Food and Nutrition Board of the Institute of Medicine established its first recommended intake levels for fiber in 2001 (4). The Adequate Intake (AI) recommendations for total fiber intake are based on the findings of several large prospective cohort studies that dietary fiber intakes of approximately 14 g for every 1,000 calories (kcal) consumed were associated with significant reductions in the risk of coronary heart disease (CHD) (42, 43, 45), as well as type 2 diabetes (66, 67). For adults who are 50 years of age and younger, the AI recommendation for total fiber intake is 38 g/day for men and 25 g/day for women. For adults over 50 years of age, the recommendation is 30 g/day for men and 21 g/day for women. The AI recommendations for males and females of all ages are presented in the table below (4).

Adequate Intake (AI) for Total Fiber

Life Stage  Age  Males (g/day)  Females (g/day) 
Infants  0-6 months   ND* ND
Infants  7-12 months   ND ND
Children  1-3 years   19 19
Children  4-8 years   25 25
Children  9-13 years   31 26
Adolescents  14-18 years   38 26
Adults  19-50 years   38 25
Adults 51 years and older   30 21
Pregnancy  all ages   - 28
Breast-feeding  all ages   - 29

* Not determined

Some Suggestions for Increasing Fiber Intake

  • Eat at least five servings of fruits and vegetables daily (See Fruits and Vegetables).
  • Substitute whole grains for refined grains (see Whole Grains).
  • Eat oatmeal, whole-grain cereal, or bran cereal for breakfast.
  • Eat beans, split peas, or lentils at least once weekly (see Legumes).
  • Substitute nuts or popcorn for less healthful snacks like potato chips or candy (see Nuts).

References


Written in December 2005 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in April 2012 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in April 2012 by:
David M. Klurfeld, Ph.D.
National Program Leader, Human Nutrition
USDA Agricultural Research Service

Copyright 2005-2014  Linus Pauling Institute


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The Linus Pauling Institute Micronutrient Information Center provides scientific information on the health aspects of dietary factors and supplements, foods, and beverages for the general public. The information is made available with the understanding that the author and publisher are not providing medical, psychological, or nutritional counseling services on this site. The information should not be used in place of a consultation with a competent health care or nutrition professional.

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