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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 fiber type is chemically unique, fibers can be classified according to their solubility, viscosity, and fermentability in order to better understand their physiological effects. (More information)
  • Soluble gel-forming fibers, such as psyllium and fibers found in oat products, can lower serum LDL cholesterol concentrations and normalize blood glucose and insulin responses. (More information)
  • Large/coarse insoluble fibers (e.g., wheat bran) and nonfermentable, soluble gel-forming fibers (e.g., psyllium) can have a potential laxative effect. (More information)
  • Large prospective cohort studies consistently report inverse associations between consumption of diets rich in fiber and risks of cardiovascular disease and type 2 diabetes mellitus. (More information)
  • A review of the most recent meta-analyses of observational studies suggests that dietary fiber consumption is inversely associated with the risk of cancer of the esophagus, stomach, colon, pancreas, ovary, and breast. (More information)
  • Both consumption of fiber-rich diets and supplementation with soluble gel-forming fibers could help improve glycemic control in individuals with type 1 or type 2 diabetes mellitus. (More information)
  • The Adequate Intake (AI) recommendation for total daily fiber intake is 38 g/day for men and 25 g/day for women. However, the average American consumes only about 17 g/day of dietary fiber, and dietary fiber intake might be closer to 10 g/day in those following a low-carbohydrate diet. (More information)
  • 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. (More information)

Introduction

All dietary fibers are resistant to digestion in the small intestine, meaning they arrive intact in the large intestine (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 β-glycosidic bonds, which explains why amylose, a glucose polymer with α-1,4 glycosidic bonds, is digestible by human enzymes, while cellulose, a glucose polymer with β-1,4 glycosidic bonds, is indigestible (Figure 1).

Figure 1. Chemical Structures of Amylose, Cellulose, and beta-Glucan.

[Figure 1 - Click to Enlarge]

 

Definitions of Fiber

Although nutritional scientists and clinicians generally agree that a healthy 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, polydextrose, wheat dextrin) and 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 (now the National Academy 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 β-1,4 glycosidic bonds found in all plant cell walls (Figure 1) (6).
  • β-Glucans: β-Glucans are glucose polymers with a mixture of β-1,4 glycosidic bonds and β-1,3 glycosidic bonds (Figure 1). Oats and barley are particularly rich in β-glucans (7).
  • Hemicelluloses: Hemicelluloses are a diverse group of polysaccharides (sugar polymers) containing different types of sugar monomers, including glucose, xylose, arabinose, mannose, galactose, rhamnose, or pentose. Glucomannan is a hemicellulose containing about 60% of mannose and 40% of glucose bonded together by β-1,4 glycosidic linkages. Like cellulose, hemicelluloses are found in plant cell walls.
  • Pectins: Pectins are linear polysaccharides made of 300 to 1,000 monosaccharides, primarily galacturonic acid residues linked together by α-1,4 glycosidic bonds (8). Pectins are soluble viscous fibers that are particularly abundant in berries and other fruit (4).
  • Gums: Gums are viscous polysaccharides often found in seeds (4). Guar gum extracted from guar beans is a galactomannan polysaccharide composed of mannose and galactose residues. It is used in the food industry for its thickening and stabilizing properties. The viscosity of guar gum is lost when guar gum is partially hydrolyzed to derive partially hydrolyzed guar gum (PHGG).
  • Inulin and oligofructose: Inulin is a mixture of fructose chains that vary in length and often terminate with a glucose molecule (9). 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). It is fermented by bacteria in the colon. 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, gel-forming mucilage, which is isolated from the outer coat (husk) of psyllium seeds — known in India as ispaghula husk — from the plant Plantago ovata or blond psyllium (4).
  • Chitin and chitosan: Chitin is a polysaccharide polymer extracted from the exoskeletons of crustaceans, such as crab and lobster. It is a polymer of more than 5,000 acetylated glucosamine units linked by β-1,4 glycosidic bonds. Deacetylated chitin or chitosan thus consists in unbranched chains of glucosamine (8).
  • Fructooligosaccharides: Fructooligosaccharides are short, synthetic fructose chains terminating with a glucose unit. They are used as food additives (9).
  • Galactooligosaccharides: Galactooligosaccharides are produced through the enzymatic conversion of lactose and are classified as prebiotics (10).
  • Polydextrose and polyols: Polydextrose and polyols are synthetic carbohydrates. Polydextrose is made of glucose and sorbitol (a sugar alcohol) and may be used as bulking agent in food. Polyols are non-sugar molecules containing multiple hydroxyl groups (-OH). Polydextrose and polyols are used as sugar substitutes in food (4).
  • Resistant dextrins: Resistant dextrins, also called resistant maltodextrins, are synthetic 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

Fibers can be classified into four clinically meaningful categories according to their physiochemical properties, i.e., their solubility, viscosity, and fermentability (reviewed in 11):

Soluble, viscous/gel forming, readily fermented fibers

E.g., β-glucans from oats and barley, raw guar gum

Soluble fibers dissolve in water, while insoluble fibers do not. Viscous fibers thicken in the presence of water, forming very viscous solutions or even visco-elastic gels. Fermentable fibers are readily metabolized by the gut microbiota (i.e., bacteria that normally colonize the large intestine). Fermentation of fiber 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. Fermentation of fiber is estimated to contribute up to 10% of daily energy intake (12). Fibers that are fermentable and can stimulate the growth and/or activity of beneficial gut bacteria are called prebiotic fibers (13). Fibers that are soluble, viscous, and fermentable have been shown to improve glycemic control and to lower blood cholesterol concentration. However, their water-holding capacity is lost when they are fermented in the colon such that they have no laxative effect (see Biological Activities).

Soluble, viscous/gel forming, nonfermented fibers

E.g., psyllium

These fibers can dissolve in water and form viscous gels. They can improve glycemic control and lower blood cholesterol concentration. In addition, they retain their water-holding/gel-forming capacity in the large intestine since they are resistant to fermentation. As a consequence, they can exert a stool-normalizing effect, preventing constipation or softening hard stool as well as firming loose/liquid stool in diarrhea and fecal incontinence (see Biological Activities).

Soluble, nonviscous, readily fermented fibers

E.g., inulin, wheat dextrin, oligosaccharides, resistant starches

Although these fibers can dissolve in water, they cannot provide any health benefits associated with fiber viscosity. They are fully fermented and thus do not exert a laxative effect. They can nonetheless exert a prebiotic effect by influencing the composition of the gut microbiota. In vitro studies have shown inulin to selectively stimulate the proliferation of beneficial bacteria and limit the growth of potentially pathogenic bacteria ) (see Isolated fibers and supplements) (reviewed in 14). However, no health benefit is currently associated with this fiber-driven prebiotic effect.

Insoluble, poorly fermented fibers

E.g., wheat bran, cellulose, lignin

These fibers do not dissolve in water, do not trap water, and are poorly fermented. Large/coarse fiber particles can have a laxative effect. They can irritate the large intestine mucosa and trigger the secretion of mucus and water, which increases the water content of stools. Small, insoluble fiber particles (e.g., finely ground wheat bran) have no laxative effect and can actually have a constipating effect by adding only to the dry stool mass (see Improving regularity in stool elimination).

Biological Activities

Lowering serum cholesterol

Some, but not all, fibers can improve blood lipid abnormalities observed in conditions like dyslipidemia, overweight/obesity, type 2 diabetes mellitus, and metabolic syndrome. Only supplementation with highly viscous fibers (i.e., gel-forming soluble fibers), such as high molecular weight (MW) β-glucan (found in oat bran), raw guar gum, and psyllium, has been shown to decrease total and LDL-cholesterol concentrations when compared to appropriate controls (e.g., fiber-free supplement, low-fiber supplement, or supplementation with insoluble fiber) (15). A 2009 meta-analysis that combined the results of 21 randomized controlled trials in 1,717 participants with hypercholesterolemia found dose- and time-dependent reductions in total and LDL-cholesterol concentrations with psyllium supplementation (3.0-20.4 g/day for >2 weeks) (16). Additionally, results from two trials showed that, compared to statins alone, combining psyllium and statins resulted in larger reductions in LDL-cholesterol concentrations in individuals with hypercholesterolemia (17). Another more recent meta-analysis of 28 randomized controlled trials examined the cholesterol-lowering effect of psyllium fiber on LDL-cholesterol, non-HDL-cholesterol, and apolipoprotein B (apo B) (18) — non-HDL-cholesterol and apo B appear to better predict cardiovascular events than LDL-cholesterol (19). Supplementation with psyllium at a median dose of 10.2 g/day and for a median of eight weeks to participants with or without hypercholesterolemia reduced LDL-cholesterol by 0.33 mmol/L (28 trials; 1,924 participants), non-HDL-cholesterol by 0.39 mmol/L (27 trials; 1,899 participants), and apo B by 0.05 g/L (9 trials; 895 participants) (18).

Despite considerable heterogeneity across studies, the cholesterol-lowering efficacy of other highly viscous fibers, including β-glucans from oat or barley and glucomannan (a hemicellulose), was also reported in recent meta-analyses of trials conducted by one research team (20-22). The cholesterol-lowering effect of soluble fibers, such as psyllium and β-glucan, is directly linked to their high viscosity. Reduction of the gel-forming capacity of these fibers with pressure and/or heat during processing leads to the loss of their cholesterol lowering capacity (23). Accordingly, low-viscosity soluble fibers (e.g., gum arabic/acacia gum, methylcellulose, low MW β-glucan), nonviscous soluble fermentable fibers (e.g., inulin, fructooligosaccharides, wheat dextrin), and insoluble fibers (e.g., wheat bran) do not decrease serum cholesterol at physiologic levels (15).

Highly viscous fibers can trap bile that is released in the small intestine in response to a meal to assist the digestion and absorption of fatty acids. The main mechanism underlying the cholesterol-lowering effect of these fibers is thus linked to their ability to prevent the reabsorption of bile in the terminal ileum and facilitate its elimination in the stool. In order to maintain sufficient bile for digestion, hepatocytes must increase LDL-cholesterol clearance to synthesize more bile acids as cholesterol is a component of bile (15).

These findings led the FDA to approve health claims in relation to the prevention of heart disease on labels of low cholesterol/saturated fat foods containing ≥0.75 g/serving of β-glucan from whole oat or barley or ≥1.7 g/serving of psyllium (see also Cardiovascular disease) (24).

Improving glycemic control

As with cholesterol lowering, the efficacy of fiber on glycemic control is dependent on viscosity. A 2017 review of 14 randomized controlled trials showed that none of the supplemented soluble, nonviscous, fermentable fibers examined (i.e., inulin, fructooligosaccharide, galactooligosaccharide, and oligofructose) could lead to reductions in postprandial and/or fasting blood glucose concentrations (15). Supplementation with insoluble fiber also failed to improve glycemic control in subjects with elevated fasting blood glucose concentrations (25). In contrast, the capacity of dietary viscous fiber (26, 27) and isolated viscous fibers (28-31) to improve glycemic control has been demonstrated in numerous controlled clinical trials conducted over three decades (15). A 2015 review of the efficacy of psyllium showed evidence of reductions in postprandial blood glucose concentration following a single meal in people with type 2 diabetes mellitus (6 studies) as well as in nondiabetic/euglycemic subjects (11 studies) (32). Supplementation with psyllium also resulted in reductions in postprandial blood insulin concentrations in subjects without type 2 diabetes (6 studies) but not in those with type 2 diabetes (3 studies). Longer-term studies of psyllium supplementation also found reductions in mean fasting glucose (4 studies) and mean glycated hemoglobin (HbA1c; 3 studies) in subjects with type 2 diabetes. Finally, while there was no effect of long-term exposure to psyllium on fasting glucose concentration in healthy individuals with euglycemia (14 studies), the glycemic benefit of psyllium was found to increase proportionally with the increase of baseline fasting glucose concentration (reviewed in 32).

The role of soluble viscous fibers on glycemic control is related to their ability to increase chyme viscosity, thereby slowing the degradation of complex nutrients and allowing the absorption of nutrients, including glucose, along the entire small intestine rather than in the upper small intestine. Absorption of nutrients in the distal ileum has been associated with a reduction in gastric emptying and intestinal transit — through a distal to proximal feedback mechanism — which in turn reduces hunger and food intake (reviewed in 15). Nutrient delivery in the distal ileum also triggers the release of short-lived glucagon-like peptide 1 (GLP-1) into the circulation. GLP-1 improves insulin secretion by pancreatic β-cells in response to glucose absorption and is involved in the regulation of food intake at the central nervous system level (33).

Lowering blood pressure

An analysis of 2004-2014 US National Health and Nutrition Examination Survey (NHANES) data from 18,433 participants found inverse associations between either total, cereal, or vegetable fiber intake and the odds of hypertension (defined according to new 2017 American College of Cardiology/American Heart Association clinical practice guidelines; 34). A 2018 meta-analysis of 22 randomized control trials that examined the effect of isolated soluble fiber supplements or diets enriched with soluble viscous fiber in either normotensive or hypertensive participants found an overall 1.59 mm Hg reduction in systolic blood pressure and 0.39 mm Hg reduction in diastolic blood pressure (35). Further analyses showed that, among all the soluble viscous fiber under examination (β-glucan, guar gum, konjac glucomannan, pectin, and psyllium), only psyllium could reduce systolic blood pressure (mean reduction of 2.39 mm Hg) (35). It is not yet understood how soluble viscous fiber would induce a lowering of blood pressure, but this effect may be indirect and dependent on established benefits of these fiber on other cardiometabolic parameters (see the above sections).

Improving regularity in stool elimination

The benefit of fiber on the regularity with which bulky, soft, easy-to-pass stools are eliminated is best assessed by an increase in stool output and an increase in the water content of stool. There are two mechanisms that support the laxative effects of certain fiber types: (i) large/coarse insoluble fiber (e.g., wheat bran) has an irritating effect on the colonic mucosa, which stimulates the secretion of water and mucus (unlike finely ground wheat bran that has a stool-hardening/constipating effect); and (ii) the presence of soluble viscous, gel-forming fiber (e.g., psyllium) helps stool to resist dehydration in the colon (15). Therefore, only fibers that remain relatively intact during the transit throughout the length of the colon (i.e., that resists bacterial fermentation) and are thus found in the stool can have a potential laxative effect. A 2016 review of fiber interventions conducted by McRorie and Chey (36) examined the potential laxative effect of fermentable fiber; the main findings of this review are summarized in Table 1.

Table 1. Interventions Examining the Laxative Effect of Fermentable Fiber (adapted from McRorie and Chey  [36])
Fiber Type of Fiber Number of Studies Doses Laxative Effects Adverse Effects
β-Glucan, guar gum, xanthan gum
Soluble, viscous, fermentable 5 studies: all conducted in healthy participants; 3 studies with β-glucan and 2 studies (guar gum, xanthan gum) 87-100 g/day (β-glucan) and 15 g/day (guar gum, xanthan gum)

• Stool hardening effect and minimal increase in stool output (β-glucan)

• No effect on stool output and stool consistency (guar gum, xanthan gum)

Not reported
Inulin Soluble, nonviscous, fermentable 11 studies: 3 studies in people with constipation and 8 studies in healthy people 5-20 g/day • No effect on colonic transit time, stool consistency, stool water content, or stool output

Abdominal pain, bloating, flatulence, and borborygmus (1 study)

Polydextrose Soluble, nonviscous, fermentable 6 studies: all conducted in healthy participants 8-30 g/day • No effect on stool output, consistency, bowel movement frequency, or colonic transit time Flatulence and borborygmus (1 study)
Resistant starch (incl. resistant dextrin) Soluble, nonviscous, fermentable 6 studies: all conducted in healthy participants 7.5-15 g/day

• Stool hardening effect and reduction in stool output (2 studies)

• No effect on consistency, stool water content, stool output, or bowel movement frequency (4 studies)

Not reported

In summary, although the bulk of evidence comes primarily from studies in healthy subjects, supplementation with fermentable fibers appears unlikely to exert a laxative effect in people suffering from constipation (Table 1). In contrast, a 2016 meta-analysis of seven randomized controlled trials identified psyllium and nonprebiotics as effectively able to increase stool frequency and improve stool consistency in participants affected by chronic idiopathic constipation (see also Chronic idiopathic 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 the 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 reported associations between high intakes of fiber-rich foods and low risks of coronary heart disease (CHD) and total cardiovascular disease (CVD). Three large prospective cohort studies (38-40) 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) (4). A 2013 meta-analysis of 17 prospective cohort studies found each 7 g/day increase in total dietary fiber intake to be associated with a 9% decrease in risk of coronary or total cardiovascular events (41). The most recent meta-analysis that included 18 prospective studies, with a total of 672,408 participants, found a 7% lower risk of CHD and a 17% lower risk of CHD-related mortality with the highest versus lowest intakes of total dietary fiber (42). Subgroup analyses by type or source of dietary fiber showed evidence of inverse associations between cereal, fruit, or soluble fiber intake and the risk of CHD (42).

The US FDA has approved health claims like the following on the labels of foods containing at least 0.75 g/serving of β-glucan soluble fiber: "Diets low in saturated fat and cholesterol that include at least 3 g/day of β-glucan soluble fiber from either whole oats or barley or a combination of both may reduce the risk of coronary heart disease" (24). Similarly, the FDA has approved health claims on the labels of foods containing at least 1.7 g/serving of psyllium: "Diets low in saturated fat and cholesterol that include at least 7 g/day of soluble fiber from psyllium seed husk may reduce the risk of heart disease" (24).

While the cholesterol-lowering effect of viscous/gel-forming soluble fibers is recognized as a major contributor to the cardioprotective effects of fiber (see Lowering serum cholesterol), other mechanisms are likely to be involved. Findings from pooled analyses of prospective cohort studies found some evidence of an inverse association between the risks of CHD and total CVD and intakes of insoluble fiber (41, 42). In addition, a cross-sectional analysis of 2005-2010 National Health and Nutrition Examination Survey (NHANES) data found dietary fiber intake to be inversely associated with serum LDL-cholesterol concentration, but also with blood pressure, body mass index (BMI), and serum insulin concentration (43). Beneficial effects of fiber-rich diets or isolated fiber consumption on blood glucose and insulin responses and on blood pressure may also likely contribute to observed reductions in CHD risk (see Biological Activities).

Type 2 diabetes mellitus

A 2014 meta-analysis of prospective cohort studies (488,293 participants) found intakes of total fiber (12 studies), cereal fiber (10 studies), fruit fiber (8 studies), and insoluble fiber (3 studies) to be inversely associated with the risk of developing type 2 diabetes mellitus (44). A dose-response data analysis reported a nonlinear relationship between total fiber intake and diabetes risk, with evidence of risk reduction with total fiber intakes ≥25 g/day. A linear dose-response relationship between cereal fiber intake and diabetes risk indicated a 6% reduction in diabetes risk for each 2 g-increment in daily cereal fiber intake. There was no evidence of an inverse association between either vegetable fiber (9 studies) or soluble fiber (3 studies) and the risk of type 2 diabetes (44). Similar findings were reported in another meta-analysis of 18 prospective cohort studies in 617,968 participants (45). Higher versus lower intakes of total fiber were found to be associated with a 15% lower risk of type 2 diabetes (17 studies); the risk of type 2 diabetes was inversely related to the intake of cereal fiber (13 studies) and insoluble fiber (3 studies) but not related to the intake of fruit fiber (11 studies), vegetable fiber (11 studies), or soluble fiber (3 studies) (45). However, only randomized controlled interventions can establish whether there is a link of causality between an exposure (fiber intake) and an outcome (type 2 diabetes). For a discussion of the difference between observational studies and intervention studies, see the article "Epidemiological Studies" in the Spring/Summer LPI 2016 Research Newsletter.

Whole-grain cereals contain insoluble fibers, including cellulose, hemicellulose, and lignin. To date, intervention studies examining the effect of cereal fiber supplementation on the risk of type 2 diabetes are limited (46). In a randomized controlled study of 61 adults with metabolic syndrome, the consumption of a diet based on several whole-grain cereal products for 12 weeks had no effect on fasting plasma concentrations of glucose, insulin, or lipids, or on measures of insulin resistance compared with a refined grain-based diet (47). There was only some weak evidence of an effect of whole-grain cereals on postprandial insulin response (47). A recent 24-month, randomized, double-blind, placebo-controlled trial examined the effect of daily supplementation with 15 g of primarily insoluble fiber on glycemic control in 180 adults with impaired glucose tolerance who were counselled to adopt more healthy lifestyle habits. Insoluble fiber supplementation failed to improve fasting glucose concentration, measures of insulin sensitivity, HbA1c concentration (a marker of glycemic control), as well as glucose and insulin responses after a glucose load compared to placebo (25). Although observational data suggest a protective association of cereal fiber against type 2 diabetes, the current (yet limited) evidence from intervention trials does not support a role for insoluble fiber in glycemic control in individuals at risk of type 2 diabetes.

Fiber-related benefits on glucose homeostasis have been linked to the viscosity of certain soluble fibers (e.g., psyllium, β-glucan, raw guar gum) (see Improving glycemic control). Soluble viscous fibers in cereal, particularly β-glucans, rather than insoluble fiber (and soluble nonviscous fiber), are thus more likely to be involved in a protective effect of cereal intake against type 2 diabetes (48). Apart from fiber, other bioactive compounds in cereal, like magnesium, might contribute to improving glycemic control in people with impaired glucose tolerance (see the article on Magnesium) (49).

The current position of the American Diabetes Association is to encourage people at risk of type 2 diabetes to achieve the daily Adequate Intake (AI) of 14 g/1,000 kcal for dietary fiber (see Intake Recommendations) (50, 51). Adherence to a Mediterranean-style diet, the composition of which intends to meet the AI for fiber, has been associated with a lower risk of developing type 2 diabetes (52, 53).

Metabolic syndrome

Metabolic syndrome is estimated to affect nearly 35% of US adults and half of those older than 60 years (54). The term metabolic syndrome refers to a cluster of metabolic disorders that increase the risk of cardiovascular disease and type 2 diabetes mellitus. The diagnosis in an individual is based on the presence at once of at least three of the following metabolic risk factors: abdominal obesity, dyslipidemia, hypertension, impaired glucose tolerance/insulin resistance, and decreased blood HDL-cholesterol concentration. Two recent meta-analyses of observational studies have reported an inverse association between total fiber intake and odds of metabolic syndrome in cross-sectional studies but not in prospective cohort studies (55, 56).

Cancer

Numerous observational studies have examined the relationship between consumption of fiber and risk of cancer at various sites. A 2014 review compiled published analyses of data from the large, multicenter European Prospective Investigation into Cancer and Nutrition (EPIC) prospective cohort (>500,000 participants). This umbrella review reported evidence of inverse associations between higher versus lower daily total fiber intakes (≥28.5 g/day versus ≤16.4 g/day) and the risk of colorectal, liver, and breast cancers (57). Total fiber intakes were not associated with the risk of cancer at other sites, i.e., the biliary tract, endometrium, prostate, kidney, or bladder. In addition, further analyses suggested that the consumption of cereal fiber was inversely related to the risk of colorectal, stomach, and liver cancers, and vegetable fiber intake was inversely associated with breast cancer. Yet, associations between the consumption of specific fiber types were not examined in relation to the risk of endometrial, kidney, or bladder cancer in EPIC participants (57).

Pooling information from individual observational studies can be helpful to draw conclusions regarding the potential associations between dietary fiber consumption and cancer risk. Results from the most recent meta-analyses of observational studies are reported in Table 2.

Table 2. Dietary Fiber Intake and Cancer Risk: Meta-analyses of Observational Studies
Type of Cancer Type of Observational Studies Risk Ratio [RR] or Odds Ratio [OR]* (95% Confidence Interval) Risk Ratio [RR] in Subgroup Analyses (e.g., by fiber type, study type, and cancer subtype) References
Breast cancer 16 prospective cohort studies RR: 0.93 (0.89-0.98) RR: 0.95 (0.86-1.06) in studies reporting on fruit fiber
RR: 0.99 (0.92-1.07) in studies reporting on vegetable fiber
RR : 0.96 (0.90-1.02) in studies reporting on cereal fiber
RR: 0.91 (0.84-0.99) in studies reporting on soluble fiber
RR: 0.95 (0.89-1.02) in studies reporting on insoluble fiber
Aune et al. (2012) (75)
20 prospective cohort and 4 case-control studies RR: 0.88 (0.83-0.93) RR: 0.91 (0.87-0.95) in cohort studies only
RR: 0.75 (0.47-1.02) in case-control studies only
Chen et al. (2016) (76)
Colorectal adenoma 4 prospective cohort and 16 case-control studies RR: 0.72 (0.63-0.83) RR: 0.84 (0.76-0.94) in studies reporting on fruit fiber
RR: 0.93 (0.84-1.04) in studies reporting on vegetable fiber
RR : 0.76 (0.62-0.92) in studies reporting on cereal fiber
RR: 0.92 (0.76-1.10) in cohort studies only
RR: 0.66 (0.56-0.77) in case-control studies only
Ben et al. (2014) (58)
Colorectal cancer 11 prospective cohort studies RR: 0.86 (0.78-0.95) for proximal colorectal cancer RR: 0.93 (0.72-1.14) in women only
RR: 0.79 (0.71-0.87) in men only
Ma et al. (2018) (60)
RR: 0.79 (0.71-0.87) for distal colorectal cancer RR: 0.70 (0.52-0.87) in women only
RR: 0.85 (0.74-0.95) in men only
13 prospective cohort and 8 case-control studies RR: 0.74 (0.67-0.84) RR: 0.81 (0.74-0.89) in cohort studies only
RR: 0.58 (0.50-0.68) in case-control studies only
Gianfredi et al. (2018) (59)
Endometrial cancer 3 prospective cohort and 11 case-control studies RR: 0.86 (0.73-1.02) RR: 1.22 (1.00-1.49) in cohort studies only
OR: 0.76 (0.64-0.89) in case-control studies only
RR: 1.26 (1.03-1.52) in studies reporting on cereal fiber
OR: 0.74 (0.58-0.94) in studies reporting on vegetable fiber
Chen et al. (2018) (83)
Esophageal cancer 9 case-control studies   OR: 0.66 (0.44-0.98) for esophageal adenocarcinoma
OR: 0.61 (0.31-1.20) for esophageal squamous cell carcinoma
Coleman et al. (2013) (84)
15 case-control studies OR: 0.52 (0.43-0.64) OR: 0.42 (0.29-0.61) for Barrett’s esophagus (precancerous lesions)
OR: 0.56 (0.37-0.67) for esophageal adenocarcinoma
OR: 0.53 (0.31-0.90) for esophageal squamous cell carcinoma
OR: 0.73 (0.48-1.12) in studies reporting on fruit fiber
OR: 0.61 (0.45-0.83) in studies reporting on vegetable fiber
OR : 0.81 (0.61-1.07) in studies reporting on cereal fiber
OR: 0.85 (0.65-1.11) in studies reporting on grain fiber
OR: 0.40 (0.20-0.78) in studies reporting on soluble fiber
OR: 0.37 (0.18-0.75) in studies reporting on insoluble fiber
Sun et al. (2017) (85)
Gastric cancer 2 prospective cohort and 19 case-control studies OR: 0.58 (0.49-0.67) OR: 0.67 (0.46-0.99) in studies reporting on fruit fiber
OR: 0.72 (0.57-0.90) in studies reporting on vegetable fiber
OR: 0.58 (0.41-0.82) in studies reporting on cereal fiber
OR: 0.41 (0.32-0.52) in studies reporting on soluble fiber
OR: 0.42 (0.34-0.52) in studies reporting on insoluble fiber
Zhang et al. (2013) (86)
Ovarian cancer 5 prospective cohort and 14 case-control studies RR: 0.70 (0.57-0.87) RR: 0.97 (0.85-1.12) in cohort studies
RR: 0.62 (0.47-0.82) in case-control studies
Xu et al. (2018) (87)
4 prospective cohort and 13 case-control studies RR: 0.76 (0.70-0.82) RR: 0.76 (0.63-0.92) in cohort studies
RR: 0.75 (0.68-0.83) in case-control studies
Huang et al. (2018) (88)
Pancreatic cancer 1 prospective cohort and 13 case-control studies OR: 0.52 (0.43-0.63) OR: 1.01 (0.59-1.73)  in the cohort study
OR: 0.54 (0.44-0.67) in case-control studies
Wang et al. (2015)** (89)
1 prospective cohort and 13 case-control studies OR: 0.52 (0.44-0.61) OR: 0.66 (0.51-0.80) in studies reporting on soluble fiber
OR: 0.65 (0.44-0.87) in studies reporting on insoluble fiber
Mao et al. (2017)** (90)
Prostate cancer 5 prospective cohort and 12 case-control studies OR: 0.89 (0.77-1.01) OR: 0.94 (0.77-1.11) in cohort studies
OR: 0.82 (0.68-0.96) in case-control studies
OR: 0.92 (0.81-1.03) in studies reporting on fruit fiber
OR: 0.87 (0.53-1.21) in studies reporting on vegetable fiber
OR: 1.05 (0.94-1.16) in studies reporting on cereal fiber
OR: 0.87 (0.52-1.22) in studies reporting on soluble fiber
OR: 0.80 (0.46-1.13) in studies reporting on insoluble fiber
Sheng et al. (2015) (91)
5 prospective cohort and 11 case-control studies RR: 0.94 (0.85-1.05) OR: 0.99 (0.87-1.14) in cohort studies
OR: 0.89 (0.75-1.06) in case-control studies
Wang et al. (2015) (92)
Renal cell carcinoma 2 prospective cohort and 4 case-control studies RR: 0.84 (0.74-0.96) OR: 0.88 (0.69-1.12) in cohort studies
OR: 0.82 (0.68-1.00) in case-control studies
OR: 0.92 (0.80-1.05) in studies reporting on fruit fiber
OR: 0.70 (0.49-1.00) in studies reporting on vegetable fiber
OR: 1.04 (0.91-1.18) in studies reporting on cereal fiber
OR: 0.83 (0.70-0.97) in studies reporting on soluble fiber
OR: 0.81 (0.69-0.94) in studies reporting on insoluble fiber
Huang et al. (2015) (93)
*for the highest versus lowest level of fiber intake (unless otherwise specified)
**both meta-analyses identified the same 14 observational studies
Colorectal cancer

Three most recent meta-analyses of observational studies have reported evidence of an inverse association between fiber intake and risk of colorectal cancer (see Table 2) (58-60). A recent review that included earlier meta-analyses reached a similar conclusion (61).

Several mechanisms have been proposed to explain why consuming fiber can have a protective effect against colorectal cancer. First, the presence of insoluble, coarse fiber can increase stool bulk thereby promoting the fecal excretion of carcinogens like nitrosamines (62). Fiber can also reduce exposure of the gut mucosa to carcinogens by shortening transit time (62). Secondly, fiber consumption influences the composition of the gut microbiota. In vitro studies have shown inulin to selectively stimulate the proliferation of beneficial bacteria while limiting the growth of potentially pathogenic bacteria (reviewed in 14). Gut bacterial imbalance (dysbiosis) has been associated with the incidence of several conditions, including colorectal cancer (63). The major health benefits conferred by the consumption of fiber are thus likely mediated by the bacteria the fiber contributes to feed. Depending on the physicochemical characteristics of fiber, some fiber, like inulin, can be fermented by colonic bacteria and lead to the formation of short-chain fatty acids, namely acetate, propionate, and butyrate. These short-chain fatty acids have been found to protect against gastrointestinal bacterial pathogens (64) and to display anti-inflammatory and anti-carcinogenic actions (65, 66).

A few controlled clinical trials have examined the effect of fiber consumption on the recurrence of colorectal adenomas (precancerous polyps), but none have been conducted in the last two decades. These trials examined the effect of wheat bran fiber (67-71), psyllium (72), and a high-fiber diet (73) on the risk of adenomas in participants with a history of adenomas. A 2017 meta-analysis of these trials found no difference between intervention and control groups in the number of participants with at least one new adenomatous polyp during the follow-up period (2-8 years), regardless of the type of fiber intervention (74).

Breast cancer
A review of four meta-analyses of observational studies reported substantial evidence of an inverse association between dietary fiber intake and breast cancer risk (61). The two most recent meta-analyses of prospective cohort studies found dietary fiber intake to be associated with a 7%-9% lower risk of breast cancer (Table 2) (75, 76). Prolonged exposure to estrogens has been associated with an increased risk of breast cancer (77). As mentioned in the previous section, several mechanisms might support the potential protective effect of dietary fiber against cancer, including breast cancer. The results of small, short-term intervention trials in premenopausal and postmenopausal women suggested that a low-fat (10-25% of total energy), high-fiber (25-40 g/day) diet could decrease circulating estrogen concentrations by increasing the excretion of estrogens and by promoting the metabolism of estrogens to less estrogenic forms (78, 79). Estrogens are conjugated in the liver and excreted within the bile into the gastrointestinal lumen; they are then de-conjugated by bacterial β-glucuronidase, re-absorbed as free estrogens through the enterohepatic circulation, and delivered to different organs and tissues like the breast.

Dietary fiber might interfere with estrogen reabsorption by reducing β-glucoronidase activity (80). Alterations in gut bacteria composition have also been reported in women with breast cancer and might contribute to increased estrogen metabolism and absorption, resulting in higher circulating estrogen concentrations (81). However, it is not known whether fiber-associated effects on endogenous estrogen concentrations have a clinically significant impact on breast cancer risk (4). Finally, a healthy microbiota might also promote the degradation of plant-derived molecules other than fiber, such as lignans, which are precursors of metabolites with anti-estrogenic activities (82).

Other cancer sites

A review of the most recent meta-analyses of observational studies suggests that dietary fiber consumption is inversely associated with the risk of cancer of the esophagus (84, 85), stomach (86), pancreas (89, 90), and ovaries (see Table 2) (87, 88). The evidence linking fiber intake and esophageal cancer was exclusively based on observations from case-control studies, and for the other cancer sites, the evidence is primarily derived from case-control studies (Table 2). It is important to note that the evidence of an inverse association between fiber intake and risk of ovarian cancer (2 meta-analyses) was observed in case-control but not in prospective cohort studies (Table 2). Additionally, there was no evidence of an association between fiber intake and risk of endometrial cancer (1 meta-analysis; 83), prostate cancer (2 meta-analyses; 91, 92), or renal cell carcinoma (1 meta-analysis; 93) (Table 2). At present, evidence from randomized controlled trials of a causal relationship between fiber intake and risk of any cancer is lacking.

Of note, a recent study in a mouse model presenting a dysbiotic microbiota characterized by an increase in fiber-fermenting bacteria showed that the consumption of an obesogenic, high-fat diet enriched with soluble fiber could cause icteric hepatocarcinoma (94). Such findings suggest that a prolonged consumption of fermentable fiber may have detrimental consequences in contexts of dysbiosis. No such observations were found when insoluble fiber were substituted for soluble fiber (94).

Diarrhea associated with enteral nutrition

Gastrointestinal disorders associated with enteral nutrition prolong the time to recovery. A 2015 meta-analysis of 14 intervention studies found that fiber-enriched enteral formulas and/or fiber supplements reduced the overall incidence of diarrhea in patients requiring enteral nutrition (95). There was no reduction in incidental diarrhea when the analysis was restricted to studies that used prebiotic fiber (i.e., fermentable fiber influencing microbiota composition). Subgroup analyses also showed a benefit of fiber in non-critically ill patients but not in critically ill patients (95).

Fecal incontinence

A prospective cohort study that followed nearly 60,000 older women for four years found that the highest versus lowest level of fiber intake (mean, 25 g/day versus 13.5 g/day) was associated with a 17% lower risk of developing fecal incontinence (defined here as an incontinence episode of liquid or solid stool at least once a month) (96). A limited number of studies have also examined whether fiber supplementation might help treat established fecal incontinence. In one placebo-controlled trial, 206 subjects suffering from fecal incontinence were randomized for 32 days to receive fibers with different degrees of fermentability: gum arabic (on average, 16.6 g/day; highly fermentable), sodium carboxymethylcellulose (16.2 g/day; partially fermentable), and psyllium (14.6 g/day; poorly fermentable) (97). The frequency of fecal incontinence increased with sodium carboxymethylcellulose but decreased with gum arabic and psyllium compared to placebo. Stool consistency and amount did not differ among groups (97). A randomized, cross-over trial in 80 community-dwelling participants with at least one fecal incontinence episode per week found that the reduction in fecal incontinence frequency and severity with psyllium supplementation was equivalent to that observed with antidiarrheal drug loperamide (Imodium) (98).

Weight control

It has been suggested that higher fiber intakes could help maintain weight or promote weight loss by increasing satiation (causing meal termination) and/or extending the feeling of fullness after a meal (satiety). Several mechanisms have been proposed to explain the potential satiating effect of fibers and the subsequent reduction in food intake. The presence of fiber in food may indirectly stimulate the production of hormones involved in the regulation of appetite in particular through (i) increasing the processing time in the mouth due to increased efforts required to masticate large particles containing fibers, and/or (ii) increasing the duration of stomach distention due to a lower rate of gastric emptying, and/or (iii) increasing the colonic production of short-chain fatty acids that can bind to receptors present on gut endocrine cells (reviewed in 99). Adverse effects like gas production and bloating observed with the supplementation of isolated fibers (e.g., wheat dextrin) might also reduce hunger and increase satiety.

A 2013 review of 44 randomized controlled trials found that β-glucans from oat or barley, lupin kernel fiber, whole grain rye, rye bran, and a mixed high-fiber diet could enhance satiety, whereas psyllium, whole grain barley, whole grain buckwheat, resistant starch, and wheat bran had no benefit or even reduced measures of satiety (100). However, for many types of fiber examined, results from interventions were mixed, some showing a positive effect on satiety, and others showing no effect. Additionally, there seemed to be no relationship between the physicochemical properties of fiber (i.e., solubility, viscosity, fermentability) and evidence of efficacy. For example, among soluble viscous fibers, β-glucans appeared to enhance satiety, but pectin or psyllium did not. Similarly, among fermentable fibers, β-glucans enhanced satiety, but guar gum, inulin, fructooligosaccharides, and resistant starch did not. Finally, a positive effect of fiber on satiety was not consistently associated with a reduction in food intake (100).

A 2011 systematic review of 61 randomized controlled studies examined the effect of different fiber types on body weight (101). This analysis found that dextrins and marine polysaccharides reduced body weight in all 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, however, the average weight reduction was only 0.3 kg (0.7 lb) per 4 weeks for a 79-kg person (101). A few randomized controlled trials in overweight or obese subjects suggested that psyllium supplementation may influence body composition and/or promote weight loss (reviewed in 102). In one randomized, placebo-controlled trial in 159 Australian with body mass indices (BMI) ≥25 kg/m2, psyllium (5 g/day) reduced waist circumference, waist-to-hip ratio, and body fat percentage, and increased the percentage of lean mass after 3, 6, and 12 months (103). Psyllium appeared to transiently reduce body weight at 3 and 6 months, yet there was no difference in body weight between psyllium and placebo at the end of the intervention (12 months) (103).

More research needs to be conducted in order to clarify which types of fiber might play a role in appetite regulation and weight management (99).

Mortality

Several prospective cohort studies have examined dietary fiber intake in relation to all-cause and cause-specific mortality. A 2015 report from the NIH-AARP Diet and Health Study, which followed 364,442 older adults for an average of 14 years, found that men and women in the highest versus lowest quintile of dietary fiber intake (mean, 10.2 g/day versus 2.0 g/day) had lower risks of all-cause mortality (-19%) and mortality from cardiovascular disease (-20%), cancer (-15%), diabetes mellitus (-34%), and respiratory diseases (-21%) (104). Another prospective study that made use of data collected from 15,740 participants in the US National Health and Nutrition Examination Survey (NHANES) 1988-1994 found a 13% lower risk of all-cause mortality in subjects with total fiber intake between 14.5 g/day and 22.1 g/day — but not in those with higher intake (>22.1 g/day) — compared to those with fiber intake less than 9.3 g/day, over a mean follow-up period of 13.7 years (105). No associations were found between intakes of either insoluble or soluble fiber and all-cause mortality (105).

A meta-analysis of prospective cohort studies published before 2013, which included a total of 1,752,848 participants followed for a mean 12.4 years, found higher versus lower total fiber intake to be associated with lower risks of all-cause mortality (-23%; 9 studies), cancer-related mortality (-17%; 5 studies), and cardiovascular disease-related mortality (-23%; 16 studies) (106). Another meta-analysis identified 14 prospective cohort studies that examined cereal fiber intake in relation to mortality (107). Participants in the highest versus lowest quartile of cereal fiber intake had lower risks of all-cause mortality (-19%; 3 studies), cardiovascular disease-related mortality (-18%; 10 studies), and cancer-related mortality (-25%; 2 studies) (107).

Disease Treatment

Diabetes mellitus

A 2004 meta-analysis that combined the results of 23 clinical trials in patients with type 1 or type 2 diabetes mellitus found that high-fiber diets (≥20 g/1,000 kcal) lowered postprandial blood glucose concentrations by 13%-21%, serum LDL cholesterol concentrations by 8%-16%, and serum triglyceride concentrations by 8%-13% when compared with low-fiber diets (<10 g/1,000 kcal) (108). 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 slightly higher than recommendations for the general public (14 g/1,000 kcal) (4). However, recommendations from the American Diabetes Association and the Academy of Nutrition and Dietetics to people with diabetes are similar to those prescribed for the population as a whole (50, 109).

Adhering to one of the USDA’s healthy dietary patterns, like the Mediterranean-style diet (which is rich in fruit, vegetables, and whole grains), would contribute to meeting the daily intake recommendation for total fiber (see Intake Recommendations) (110). A 2015 meta-analysis of nine randomized controlled trials in a total of 1,178 participants with type 2 diabetes showed evidence of body weight loss and improvements in glycemic control and blood lipid profile with the consumption of a Mediterranean-style diet compared to a control diet (111).

Numerous controlled clinical trials have shown that supplementation with soluble viscous fibers improves markers of glycemic control in people who have type 2 diabetes mellitus. A meta-analysis of 28 trials in 1,394 adults with type 2 diabetes found reductions in HbA1c concentration (20 trials), fasting glucose concentration (28 trials), and insulin resistance (11 trials) with soluble viscous fiber supplementation (median doses of 10.5-15 g/day for 6-8 weeks) (112). Another meta-analysis of 35 trials showed that the effect of psyllium varied with baseline fasting glucose concentration: psyllium supplementation had no effect on markers of glycemic control in euglycemic participants but showed a modest benefit in subjects with impaired glucose tolerance, and a greater effect in those with overt type 2 diabetes (see also Biological Activities) (32).

A small randomized uncontrolled trial in 20 healthy participants suggested that supplemental wheat dextrin, which is partially absorbed as sugar in the small intestine, could increase fasting glucose concentration into the prediabetes range after one month of supplementation (113). Since there is little evidence from clinical trials that increasing nonviscous fiber alone is beneficial (114), individuals with diabetes should preferably increase fiber intake from sources of soluble viscous fibers, such as oats and barley (β-glucans), vegetables, beans, and legumes (108).

Gastrointestinal disorders

Chronic idiopathic constipation

Only insoluble fibers and soluble viscous fibers that resist bacterial fermentation in the colon have a potential laxative effect (see Improving regularity in stool elimination) (15). The prevalence of chronic constipation is higher among people with diabetes mellitus, in women during pregnancy and after delivery, or in older people. The management of constipation in these patients is usually similar to the management in the rest of the population, although the etiology might be different. Bulk-forming laxatives, including psyllium, bran, and methylcellulose, are commonly recommended to improve stool regularity in patients with diabetes mellitus (115). However, there is no evidence that methylcellulose and bran are efficacious in patients with constipation (116). In these patients, psyllium is also recognized to improve glycemic control (see Diabetes mellitus). There is a need for good quality, randomized, double-blind, controlled trials to examine the effect of fiber supplementation in the treatment of constipation in older adults in long-term care (117) or in pregnant women and new mothers (118, 119).

The American College of Gastroenterology recognizes the efficacy of soluble fiber in the treatment of chronic idiopathic constipation. It also recognizes that the evidence from observational studies is mixed, as constipation is associated with low-fiber diets in some, but not all, studies (120). It recommends a gradual increase in fiber intake, in particular to limit the potential adverse effects associated with the intake of insoluble fiber, i.e., bloating, distension, flatulence, and cramping (121).

Irritable bowel syndrome

Irritable bowel syndrome (IBS) is a functional disorder of the intestines, characterized by episodes of abdominal pain or discomfort associated with altered gut mobility and changes in bowel habits (i.e., with constipation, diarrhea, or both) (122). Although the pathophysiology of IBS remains unclear, certain food components have been recognized as a cause for symptoms of IBS. Dietary restriction of highly fermentable, soluble, short-chain carbohydrates, identified as FODMAP (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) and including some dietary fibers (e.g., fructans, galactooligosaccharides), has been found to relieve IBS symptoms, including abdominal pain/discomfort, abdominal bloating/distension, and flatulence (123). On the other hand, soluble, poorly fermentable, long-chain carbohydrate fiber types may improve the symptoms related to excessive gas production. Two meta-analyses of randomized controlled trials and cross-over studies found a beneficial effect of fiber that was limited to soluble fiber, primarily psyllium (124, 125). Accordingly, the American College of Gastroenterology recognizes that soluble fiber like psyllium can provide overall symptom relief in IBS, while insoluble fiber (e.g., wheat bran) can cause bloating and abdominal discomfort (121). More research is needed to document the effect of specific soluble fibers, considering physicochemical properties (viscosity and fermentability), doses, and duration of supplementation, and to provide stronger recommendations to individuals diagnosed with IBS (125). Future trials should also consider subjects with all of the IBS types, i.e., constipation-predominant, diarrhea-predominant, and mixed-diarrhea-and-constipation IBS.

Diverticular disease

Diverticular disease or diverticulosis is a rather common gastrointestinal condition in Western countries characterized by the formation of small pouches (diverticula) in the colon (126). Acute inflammation or infection of diverticula — known as diverticulitis — occurs in about 10%-25% of all symptomatic cases of diverticulosis and is caused by the irritation of the mucosa by fecalith obstructing diverticula. Complications of diverticulitis include abscesses, fistulas, obstruction, and perforation (126). The etiology of diverticulosis is thought to be multifactorial, involving both genetic and environmental risk factors. Despite little supporting evidence, it has been proposed that low fiber intakes that characterize Western diets might contribute to increasing the risk of diverticulosis (127, 128). This low-fiber hypothesis is disputed. In particular, a 2012 cross-sectional study of 2,104 adults found higher odds of diverticula (assessed by colonoscopy) among participants in the highest versus lowest quartile of fiber intake, measured by food frequency questionnaires (129).

A 2017 review identified interventions — published over four decades — that examined the effect of dietary or supplemental fiber on the reduction of abdominal pain in patients suffering from symptomatic uncomplicated diverticular disease (SUDD), as well as on the risk of acute diverticulitis (130). However, a meta-analysis could not be conducted nor any conclusion provided regarding the efficacy of fiber in the treatment of SUDD due to the very poor quality of the studies and their substantial heterogeneity in terms of study design and quantity and quality of fiber types used (130). Another recent review of the literature focused on the effect of fiber-restricted diets in the management of acute uncomplicated diverticulitis (131). Based on the review of three randomized controlled trials and two observational studies, the authors found a reduced length of hospital stay with non-restricted diets compared to restricted diets, but no difference regarding the incidence of treatment failure (i.e., the risk of no clinical improvement with therapy and the development of complications) and the risk of post-discharge reoccurrence of diverticulitis. While there appears to be no clinical benefit in restricting fiber intake in subjects with uncomplicated diverticulitis, the quality of the studies was once again deemed to be very low (131).

Despite the lack of high-quality evidence regarding the potential benefit of fiber in the management of diverticular disease, many national guidelines recommend the use of high-fiber diets in patients with SUDD and for the prevention of diverticulitis (132, 133).

Hemorrhoids

A limited number of interventions have examined the effect of fiber supplementation in subjects with symptomatic hemorrhoids. A randomized controlled trial in 67 participants found that supplementation with psyllium (7 g/day for 6 weeks) improved stool consistency and regularity, reduced the use of laxatives, and increased the quality of life compared to a placebo (134). Another randomized controlled trial in 50 patients with hemorrhoidal prolapse (grades II-IV) and rectal bleeding showed that psyllium supplementation (11.6 g/day for 40 days) reduced the number of bleeding episodes and the number of congested hemorrhoidal cushions but had no effect on the degree of prolapse (135). Finally, a more recent uncontrolled intervention in 102 individuals with advanced hemorrhoids (grades II-IV) examined the effect of counseling patients to follow a therapy meant to improve defecatory habits and involving an increase of psyllium intake to 20 g/day-25 g/day. A follow-up for a median 40 months suggested that psyllium supplementation might help halt the progression of hemorrhoidal prolapse and reduce the number of bleeding episodes (136).

Sources

Food sources

An analysis of the 2009-2010 US National Health and Nutrition Examination Survey (NHANES) data reported average dietary fiber intakes of 13.6 g/day in children and adolescents and 17 g/day in adults—well below recommended intake levels (see Intake Recommendations) (137). Fiber is identified as a shortfall nutrient of public health concern in the 2010-2015 Dietary Guidelines for Americans (51).

Good sources of dietary fiber include legumes, nuts, whole grains, bran products, fruit, and nonstarchy vegetables. Legumes (e.g., dry beans and peas), nuts, seeds, and whole grains are generally more concentrated sources of fiber than fruit and vegetables (138). These higher fiber foods are currently underconsumed, contributing to only about 6% of total dietary fiber intake (137). Although refined grains are often perceived as being poor sources of fiber, they can provide as much fiber as either fruit or vegetables when comparable serving sizes are consumed (138). In addition, not all whole grains are good sources of fiber, yet they provide key micronutrients and phytochemicals that contribute to the health benefit associated with whole grain consumption (see the article on Whole Grains) (12).

All plant-based foods contain a mixture of soluble and insoluble fiber (138). Bran flaxseed, oat cereal, legumes, nuts, fruit, and vegetables are good sources of soluble viscous and nonviscous fiber. Wheat bran, brown rice, barley, cabbage, celery, and whole grains are rich sources of insoluble fiber. The total fiber content of some fiber-rich foods is presented in Table 3. 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.

Table 3. Some Food Sources of Dietary Fiber
Food Serving Fiber (g)
Legumes
Navy beans, cooked, boiled ½ cup 9.6
Split peas, cooked, boiled ½ cup 8.1
Lentils, cooked, boiled ½ cup 7.8
Refried beans, canned ½ cup 5.7
Kidney beans, canned ½ cup 5.5
Cereal and grains
All-bran (wheat) cereal ½ cup 19.5
Oats ½ cup 4.1
Bulgur, cooked ½ cup 4.1
Cereal, instant oatmeal ½ cup 4.0
Pearled barley, cooked ½ cup 3.0
Oat bran, cooked ½ cup 2.8
Quinoa, cooked ½ cup 2.6
Rice, long-grain, brown, cooked ½ cup 1.6
Vegetables
Winter squash, butternut, cooked, baked 1 cup 6.6
Artichoke hearts, cooked ½ cup 4.8
Spinach, frozen, cooked ½ cup 3.5
Mushrooms, white, cooked from fresh 1 cup 3.4
Brussel sprouts, frozen, cooked ½ cup 3.2
Fruit
Plums, dried (prunes), uncooked ½ cup, pitted 6.2
Guava, fresh ½ cup 4.5
Pear 1 small pear 4.6
Asian pear 1 small pear 4.4
Raspberries, fresh ½ cup 4.0
Blackberries, fresh ½ cup 3.8
Plums, fresh 2 plums 1.8
Nuts and Seeds
Almonds 1 ounce (23 kernels) 3.5
Pistachios 1 ounce (49 kernels) 3.0
Pine nuts 1 ounce 3.0
Hazelnuts 1 ounce (21 kernels) 2.7
Pecans 1 ounce (19 halves) 2.7
Peanuts 1 ounce 2.4

Isolated fibers and supplements

β-Glucans

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

Glucomannan

Glucomannan, sometimes called konjac mannan, is classified as a soluble fiber isolated from konjac flour, which is derived from the plant Amorphophallus konjac. Glucomannan is available as powder and in capsules, which should be taken with plenty of liquids (8). Glucomannan forms gels that are firmer than regular gelatin products (e.g., "jello") and do no melt in the mouth. The FDA has banned gel candies containing glucomannan (e.g., "mini-cup jelly products") because of their potential to cause choking (139).

Pectin

Pectins are readily fermented soluble viscous fibers, most often extracted from citrus peels and apple pulp. Pectins are widely used as gelling agents in food but are also available as dietary supplements without a prescription (8).

Inulins and oligofructose

Inulins and oligofructose, extracted from chicory root or synthesized from sucrose, are used as food additives (9). 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 (140). Encouraging the growth of Bifidobacteria might promote intestinal health by suppressing the growth of pathogenic bacteria known to cause diarrhea or by enhancing the immune response (141). 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 (11, 142).

Guar gum

Raw guar gum is a viscous, fermentable fiber derived from the Indian guar or 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 there is no evidence of their efficacy (143). Unlike guar gum, partially hydrolyzed guar gum is nonviscous and therefore does not exhibit the biological activities of guar gum (i.e., it has no effect on serum cholesterol and glycemic control) (see Biological Activities).

Psyllium

Psyllium, a viscous, soluble, gel-forming fiber isolated from psyllium seed husks, is available without a prescription in laxatives, ready-to-eat cereal, and dietary supplements (8). Psyllium is proven to be efficacious to lower serum cholesterol and improve glycemic control (see Biological Activities). Because it also normalizes stool form, psyllium is the only fiber recommended by the American College of Gastroenterology to treat chronic constipation and irritable bowel syndrome (see Gastrointestinal disorders).

Chitosan

Chitosan is an indigestible glucosamine polymer derived from chitin. Chitosan is available as a dietary supplement without a prescription in the US, being marketed to promote weight loss and lower cholesterol. A 2018 meta-analysis of randomized controlled, clinical trials found a lowering of total and LDL-cholesterol concentrations with chitosan supplementation (0.3-6.75 g/day for 4-24 weeks) and no effect on HDL-cholesterol or triglycerides (144). Another recent pooled analysis of trials found chitosan to be more effective than placebo in promoting weight loss (145).

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 (146, 147).

Safety

Adverse effects

Dietary fiber

Some people experience abdominal cramping, bloating, or gas when they abruptly increase their dietary fiber intake (146, 147). 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, primarily in people with impaired intestinal motility or difficulty chewing (148-151). The National Academy of Medicine (formerly, 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). It is recommended to gradually introduce a new fiber supplement, not exceeding 3 to 4 g/day the first week, in order to minimize gastrointestinal symptoms (152). In subjects who are constipated, the initiation of a fiber supplement should start once the hard stool is cleared (152). Use of a guar gum-containing supplement for weight loss has been associated with esophageal and small bowel obstruction (153). 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 (154, 155).

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) (72).

Allergy and anaphylaxis: Since chitin is isolated from the exoskeletons of crustaceans, such as crabs and lobsters, and chitosan is derived from chitin, people with shellfish allergies should avoid taking chitosan supplements (8). Anaphylaxis has been reported after intravenous (IV) administration of inulin (156), as well as ingestion of margarine containing inulin extracted from chicory (157). Anaphylaxis has also been reported after the ingestion of cereal containing psyllium, and asthma has occasionally been reported in people with occupational exposure to psyllium powder (158).

Drug interactions

Gel-forming fibers (e.g., β-glucan, psyllium, raw guar gum, pectin) have the potential to slow the absorption of drugs if taken at the same time. Psyllium may reduce the absorption of lithium, carbamazepine (Tegretol), digoxin (Lanoxin), and warfarin (Coumadin) when taken at the same time (8). 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 (159). Pectin may decrease the absorption of lovastatin (Mevacor) when taken at the same time (160). Concomitant administration of a kaolin-pectin antidiarrheal suspension has been reported to decrease the absorption of clindamycin, tetracycline, and digoxin, but it is not known whether kaolin, pectin, or both were responsible for the interaction (8). In general, medications should be taken at least one hour before or two hours after fiber supplements and gel-forming dietary fibers (e.g., oatmeal).

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 (161). 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 (162, 163). The addition of pectin and guar gum to a meal significantly reduced the absorption of the carotenoids β-carotene, lycopene, and lutein from that meal (164, 165).

Intake Recommendations

The Adequate Intake (AI) for total fiber

The Adequate Intake (AI) recommendations for total fiber intake, set by the Food and Nutrition Board of the Institute of Medicine, 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). The FDA approved specific health claims related to the cardioprotective effects of two soluble, gel-forming fibers only: β-glucan and psyllium (see Disease Prevention). 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 Table 4 (4).

Table 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 fruit and vegetables daily (see the article on Fruit and Vegetables).
  • Substitute whole grains for refined grains (see the article on Whole Grains).
  • Eat oatmeal, whole-grain cereal, or bran cereal for breakfast.
  • Eat beans, split peas, or lentils at least once weekly (see the article on Legumes).
  • Substitute nuts or popcorn for less healthful snacks like potato chips or candy (see the article on Nuts).

Adopting one of the USDA healthy dietary patterns (i.e., healthy US-style, healthy Mediterranean-style, and healthy vegetarian dietary patterns) recommended in the 2015-2020 Dietary Guidelines for Americans will help meet the recommendations for total fiber intake (110). Fruit, vegetables, and whole grains available in the USDA dietary patterns contribute nearly 90% of the recommended dietary fiber intake. Within the vegetable group, beans, peas, and starchy vegetables are the main contributor of total fiber intake (22%). Refined grains provide 9% of total fiber intake (110).


Authors and Reviewers

Originally written in 2004 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

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

Updated in August 2009 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

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

Updated in March 2019 by:
Barbara Delage, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in June 2019 by:
Johnson W. McRorie, Jr., Ph.D., F.A.C.G., A.G.A.F., F.A.C.N.
Procter & Gamble
Mason, OH

Copyright 2004-2019  Linus Pauling Institute


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