Dietary fiber (British spelling fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes.^[1] It has two main components:^[2]

Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, pectins, beta-glucans, and oligosaccharides.^[1][2]

Dietary fibers can act by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed.^[5] Some types of soluble fiber absorb water to become a gelatinous, viscous substance which may or may not be fermented by bacteria in the digestive tract. Some types of insoluble fiber have bulking action and are not fermented.^[6] Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers.^[2] Other types of insoluble fiber, notably resistant starch, are fermented to produce short-chain fatty acids, which are physiologically active and confer health benefits.^[1][3][4] Health benefit from dietary fiber and whole grains may include a decreased risk of death and lower rates of coronary heart disease, colon cancer, and type 2 diabetes.^[7]

Food sources of dietary fiber have traditionally been divided according to whether they provide soluble or insoluble fiber.

Plant foods contain both types of fiber in varying amounts, according to the plant's characteristics of viscosity and fermentability.^[1][8] Advantages of consuming fiber depend upon which type of fiber is consumed and which benefits may result in the gastrointestinal system.^[9] Bulking fibers – such as cellulose, hemicellulose and psyllium – absorb and hold water, promoting regularity.^[8] Viscous fibers – such as beta-glucan and psyllium – thicken the fecal mass.^[8] Fermentable fibers – such as resistant starch and inulin – feed the bacteria and microbiota of the large intestine, and are metabolized to yield short-chain fatty acids, which have diverse roles in gastrointestinal health.^[11][12]

Dietary fiber is defined to be plant components that are not broken down by human digestive enzymes.^[1] In the late 20th century, only lignin and some polysaccharides were known to satisfy this definition, but in the early 21st century, resistant starch and oligosaccharides were included as dietary fiber components.^[1][6]

Official definition of dietary fiber varies among different institutions:

Dietary fibers are found in fruits, vegetables and whole grains. The amount of fiber contained in common foods are in the following table:^[17]

Dietary fiber is found in plants, typically eaten whole, raw or cooked, although fiber can be added to make dietary supplements and fiber-rich processed foods. Grain bran products have the highest fiber contents, such as crude corn bran (79 g per 100 g) and crude wheat bran (43 g per 100 g), which are ingredients for manufactured foods.^[17] Medical authorities, such as the Mayo Clinic, recommend adding fiber-rich products to the Standard American Diet (SAD) which is rich in processed and artificially sweetened foods, with minimal intake of vegetables and legumes.^[20][21]

Some plants contain significant amounts of soluble and insoluble fiber.

For example, plums and prunes have a thick skin covering a juicy pulp. The skin is a source of insoluble fiber, whereas soluble fiber is in the pulp. Grapes also contain a fair amount of fiber.^[22]

Soluble fiber is found in varying quantities in all plant foods, including:

Sources of insoluble fiber include:

These are a few example forms of fiber that have been sold as supplements or food additives.

These may be marketed to consumers for nutritional purposes, treatment of various gastrointestinal disorders, and for such possible health benefits as lowering cholesterol levels, reducing risk of colon cancer, and losing weight.

Soluble fiber supplements may be beneficial for alleviating symptoms of irritable bowel syndrome, such as diarrhea or constipation and abdominal discomfort.^[24] Prebiotic soluble fiber products, like those containing inulin or oligosaccharides, may contribute to relief from inflammatory bowel disease,^[25] as in Crohn's disease,^[26] ulcerative colitis,^[27][28] and Clostridium difficile,^[29] due in part to the short-chain fatty acids produced with subsequent anti-inflammatory actions upon the bowel.^[30][31] Fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.^[32]

One insoluble fiber, resistant starch from high-amylose corn, has been used as a supplement and may contribute to improving insulin sensitivity and glycemic management^[33][34][35] as well as promoting regularity^[36] and possibly relief of diarrhea.^[37][38][39] One preliminary finding indicates that resistant corn starch may reduce symptoms of ulcerative colitis.^[40]

Chemically defined as oligosaccharides occurring naturally in most plants, inulins have nutritional value as carbohydrates, or more specifically as fructans, a polymer of the natural plant sugar, fructose. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory roots or Jerusalem artichokes for use in prepared foods.^[41] Subtly sweet, it can be used to replace sugar, fat, and flour, is often used to improve the flow and mixing qualities of powdered nutritional supplements, and has significant potential health value as a prebiotic fermentable fiber.^[42]

Inulin is advantageous because it contains 25–30% the food energy of sugar or other carbohydrates and 10–15% the food energy of fat. As a prebiotic fermentable fiber, its metabolism by gut flora yields short-chain fatty acids (see below) which increase absorption of calcium,^[43] magnesium,^[44] and iron,^[45] resulting from upregulation of mineral-transporting genes and their membrane transport proteins within the colon wall. Among other potential beneficial effects noted above, inulin promotes an increase in the mass and health of intestinal Lactobacillus and Bifidobacterium

Inulin's primary disadvantage is its tolerance.

As a soluble fermentable fiber, it is quickly and easily fermented within the intestinal tract, which may cause gas and digestive distress at doses higher than 15 grams/day in most people.^[46] Individuals with digestive diseases have benefited from removing fructose and inulin from their diet.^[47] While clinical studies have shown changes in the microbiota at lower levels of inulin intake, some of the health effects require higher than 15 grams per day to achieve the benefits.^[48]

Vegetable gum fiber supplements are relatively new to the market. Often sold as a powder, vegetable gum fibers dissolve easily with no aftertaste. In preliminary clinical trials, they have proven effective for the treatment of irritable bowel syndrome.^[49] Examples of vegetable gum fibers are guar gum and gum arabic.

Many molecules that are considered to be "dietary fiber" are so because humans lack the necessary enzymes to split the glycosidic bond and they reach the large intestine. Many foods contain varying types of dietary fibers, all of which contribute to health in different ways.

Dietary fibers make three primary contributions: bulking, viscosity and fermentation.^[50]

Bulking fibers can be soluble (e.g. psyllium) or insoluble (e.g. cellulose and hemicellulose).

They absorb water and can significantly increase stool weight and regularity.

Most bulking fibers are not fermented or are minimally fermented throughout the intestinal tract.^[50]

Viscous fibers thicken the contents of the intestinal tract and may attenuate the absorption of sugar, reduce sugar response after eating, and reduce lipid absorption (notably shown with cholesterol absorption).

Their use in food formulations is often limited to low levels, due to their viscosity and thickening effects.

Some viscous fibers may also be partially or fully fermented within the intestinal tract (guar gum, beta-glucan, glucomannan and pectins), but some viscous fibers are minimally or not fermented (modified cellulose such as methylcellulose and psyllium).^[50]

Fermentable fibers are consumed by the microbiota within the large intestines, mildly increasing fecal bulk and producing short-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Resistant starch, inulin, fructooligosaccharide and galactooligosaccharide are dietary fibers which are fully fermented. These include insoluble as well as soluble fibers. This fermentation influences the expression of many genes within the large intestine,^[51] which affect digestive function and lipid and glucose metabolism, as well as the immune system, inflammation and more.^[52]

Dietary fibers can change the nature of the contents of the gastrointestinal tract and can change how other nutrients and chemicals are absorbed through bulking and viscosity.^[2][5] Some types of soluble fibers bind to bile acids in the small intestine, making them less likely to re-enter the body; this in turn lowers cholesterol levels in the blood from the actions of cytochrome P450-mediated oxidation of cholesterol.^[6]

Insoluble fiber is associated with reduced risk of diabetes, but the mechanism by which this is achieved is unknown.^[53] One type of insoluble dietary fiber, resistant starch, may increase insulin sensitivity in healthy people,^[54][55] in type 2 diabetics,^[56] and in individuals with insulin resistance, possibly contributing to reduced risk of type 2 diabetes.^[35][34][33]

Not yet formally proposed as an essential macronutrient, dietary fiber has importance in the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.^{[2][5][60][[61]](https://portal.issn.org/resource/ISSN/the original)}

Dietary fiber has distinct physicochemical properties. Most semi-solid foods, fiber and fat are a combination of gel matrices which are hydrated or collapsed with microstructural elements, globules, solutions or encapsulating walls. Fresh fruit and vegetables are cellular materials.^[62][63][64]

Following a meal, the stomach and upper gastrointestinal contents consist of

Micelles are colloid-sized clusters of molecules which form in conditions as those above, similar to the critical micelle concentration of detergents.^[67] In the upper gastrointestinal tract, these compounds consist of bile acids and di- and monoacyl glycerols which solubilize triacylglycerols and cholesterol.^[67]

Two mechanisms bring nutrients into contact with the epithelium:

The multiple physical phases in the intestinal tract slow the rate of absorption compared to that of the suspension solvent alone.

Adding viscous polysaccharides to carbohydrate meals can reduce post-prandial blood glucose concentrations. Wheat and maize but not oats modify glucose absorption, the rate being dependent upon the particle size. The reduction in absorption rate with guar gum may be due to the increased resistance by viscous solutions to the convective flows created by intestinal contractions.

Dietary fiber interacts with pancreatic and enteric enzymes and their substrates.

Human pancreatic enzyme activity is reduced when incubated with most fiber sources.

Fiber may affect amylase activity and hence the rate of hydrolysis of starch. The more viscous polysaccharides extend the mouth-to-cecum transit time; guar, tragacanth and pectin being slower than wheat bran.^[69]

The colon may be regarded as two organs,

The presence of bacteria in the colon produces an ‘organ’ of intense, mainly reductive, metabolic activity, whereas the liver is oxidative.

The substrates utilized by the cecum have either passed along the entire intestine or are biliary excretion products.

The effects of dietary fiber in the colon are on

Enlargement of the cecum is a common finding when some dietary fibers are fed and this is now believed to be normal physiological adjustment.

Such an increase may be due to a number of factors, prolonged cecal residence of the fiber, increased bacterial mass, or increased bacterial end-products.

Some non-absorbed carbohydrates, e.g. pectin, gum arabic, oligosaccharides and resistant starch, are fermented to short-chain fatty acids (chiefly acetic, propionic and n-butyric), and carbon dioxide, hydrogen and methane.

Almost all of these short-chain fatty acids will be absorbed from the colon.

This means that fecal short-chain fatty acid estimations do not reflect cecal and colonic fermentation, only the efficiency of absorption, the ability of the fiber residue to sequestrate short-chain fatty acids, and the continued fermentation of fiber around the colon, which presumably will continue until the substrate is exhausted.

The production of short-chain fatty acids has several possible actions on the gut mucosa.

All of the short-chain fatty acids are readily absorbed by the colonic mucosa, but only acetic acid reaches the systemic circulation in appreciable amounts.

Butyric acid appears to be used as a fuel by the colonic mucosa as the preferred energy source for colonic cells.

Dietary fiber may act on each phase of ingestion, digestion, absorption and excretion to affect cholesterol metabolism,^[71] such as the following:

An important action of some fibers is to reduce the reabsorption of bile acids in the ileum and hence the amount and type of bile acid and fats reaching the colon.

A reduction in the reabsorption of bile acid from the ileum has several direct effects.

The fibers that are most effective in influencing sterol metabolism (e.g. pectin) are fermented in the colon.

It is therefore unlikely that the reduction in body cholesterol is due to adsorption to this fermented fiber in the colon.

Feces consist of a plasticine-like material, made up of water, bacteria, lipids, sterols, mucus and fiber.

Water is distributed in the colon in three ways:

Fecal weight is dictated by:

Wheat bran is minimally fermented and binds water and when added to the diet increases fecal weight in a predictable linear manner and decreases intestinal transit time.

The particle size of the fiber is all-important, coarse wheat bran being more effective than fine wheat bran.

The greater the water-holding capacity of the bran, the greater the effect on fecal weight.

For most healthy individuals, an increase in wet fecal weight, depending on the particle size of the bran, is generally of the order of 3–5 g/g fiber.

The fermentation of some fibers results in an increase in the bacterial content and possibly fecal weight.

Other fibers, e.g. pectin, are fermented and have no effect on stool weight.

Research has shown that fiber may benefit health in several different ways.

Lignin and probably related materials that are resistant to enzymatic degradation, diminish the nutritional value of foods.^[74]

Color coding of table entries:

Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium.^[79][80][81] Some plant foods can reduce the absorption of minerals and vitamins like calcium, zinc, vitamin C, and magnesium, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.^[82]

A study of 388,000 adults ages 50 to 71 for nine years found that the highest consumers of fiber were 22% less likely to die over this period.^[83] In addition to lower risk of death from heart disease, adequate consumption of fiber-containing foods, especially grains, was also associated with reduced incidence of infectious and respiratory illnesses, and, particularly among males, reduced risk of cancer-related death.

An experiment designed with a large sample and conducted by NIH-AARP Diet and Health Study studied the correlation between fiber intake and colorectal cancer.

The analytic cohort consisted of 291,988 men and 197,623 women aged 50–71 years.

Diet was assessed with a self-administered food-frequency questionnaire at baseline in 1995–1996; 2,974 incident colorectal cancer cases were identified during five years of follow-up.

The result was that total fiber intake was not associated with colorectal cancer.^[84]

Although many researchers believe that dietary fiber intake reduces risk of colon cancer, one study conducted by researchers at the Harvard School of Medicine of over 88,000 women did not show a statistically significant relationship between higher fiber consumption and lower rates of colorectal cancer or adenomas.^[85] Similarly, a 2010 study of 58,279 men found no relationship between dietary fiber and colorectal cancer.^[86]

Dietary fiber has many functions in diet, one of which may be to aid in energy intake control and reduced risk for development of obesity.

The role of dietary fiber in energy intake regulation and obesity development is related to its unique physical and chemical properties that aid in early signals of satiation and enhanced or prolonged signals of satiety. Early signals of satiation may be induced through cephalic- and gastric-phase responses related to the bulking effects of dietary fiber on energy density and palatability, whereas the viscosity-producing effects of certain fibers may enhance satiety through intestinal-phase events related to modified gastrointestinal function and subsequent delay in fat absorption. In general, fiber-rich diets, whether achieved through fiber supplementation or incorporation of high fiber foods into meals, have a reduced energy density compared with high fat diets. This is related to fiber’s ability to add bulk and weight to the diet. There are also indications that women may be more sensitive to dietary manipulation with fiber than men. The relationship of body weight status and fiber effect on energy intake suggests that obese individuals may be more likely to reduce food intake with dietary fiber inclusion.^[87]

Current recommendations from the United States National Academy of Sciences, Institute of Medicine, state that for Adequate Intake, adult men ages 14–50 consume 38 grams of dietary fiber per day, men 51 and older 30 grams, women ages 19–50 to consume 25 grams per day, women 51 and older 21 grams. These are based on an observed intake level of 14 grams per 1,000 Calories among those with lower risk of coronary heart disease.^[2][82]

The AND (Academy of Nutrition and Dietetics, previously ADA) recommends a minimum of 20–35 g/day for a healthy adult depending on calorie intake (e.g., a 2000 Cal/8400 kJ diet should include 25 g of fiber per day). The AND's recommendation for children is that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day). No guidelines have yet been established for the elderly or very ill. Patients with current constipation, vomiting, and abdominal pain should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioids because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.

As of 2018, the British Nutrition Foundation has recommended a minimum fiber intake of 30 grams per day for healthy adults.^[88]

On average, North Americans consume less than 50% of the dietary fiber levels recommended for good health.

In the preferred food choices of today's youth, this value may be as low as 20%, a factor considered by experts as contributing to the obesity levels seen in many developed countries.^[89] Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the Food and Drug Administration (FDA) of the United States have given approvals to food products making health claims for fiber. The FDA classifies which ingredients qualify as being "fiber", and requires for product labeling that a physiological benefit is gained by adding the fiber ingredient.^[90] As of 2008, the FDA approved health claims for qualified fiber products to display labeling that regular consumption may reduce blood cholesterol levels – which can lower the risk of coronary heart disease^[91] – and also reduce the risk of some types of cancer.^[92]

Viscous fiber sources gaining FDA approval are:

Other examples of bulking fiber sources used in functional foods and supplements include cellulose, guar gum and xanthan gum. Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include resistant starch, inulin, fructans, fructooligosaccharides, oligo- or polysaccharides, and resistant dextrins, which may be partially or fully fermented.

Consistent intake of fermentable fiber may reduce the risk of chronic diseases.^[93][94][95] Insufficient fiber in the diet can lead to constipation.^[96]

In 2018, the British Nutrition Foundation issued a statement to define dietary fiber more concisely and list the potential health benefits established to date, while increasing its recommended daily intake to 30 grams for healthy adults.^[1] Statement: 'Dietary fibre' has been used as a collective term for a complex mixture of substances with different chemical and physical properties which exert different types of physiological effects.

The use of certain analytical methods to quantify dietary fiber by nature of its indigestibility results in many other indigestible components being isolated along with the carbohydrate components of dietary fiber. These components include resistant starches and oligosaccharides along with other substances that exist within the plant cell structure and contribute to the material that passes through the digestive tract. Such components are likely to have physiological effects.

Diets naturally high in fiber can be considered to bring about several main physiological consequences:^[1]

Fiber is defined by its physiological impact, with many heterogenous types of fibers.

Some fibers may primarily impact one of these benefits (i.e., cellulose increases fecal bulking and prevents constipation), but many fibers impact more than one of these benefits (i.e., resistant starch increases bulking, increases colonic fermentation, positively modulates colonic microflora and increases satiety and insulin sensitivity).^[3][4] The beneficial effects of high fiber diets are the summation of the effects of the different types of fiber present in the diet and also other components of such diets.

Defining fiber physiologically allows recognition of indigestible carbohydrates with structures and physiological properties similar to those of naturally occurring dietary fibers.^[1]

The American Association of Cereal Chemists has defined soluble fiber this way: "the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine." ^[97] In this definition:

As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence).

According to a 2002 journal article,^[93] fiber compounds with partial or low fermentability include:

fiber compounds with high fermentability include:

When fermentable fiber is fermented, short-chain fatty acids (SCFA) are produced. SCFAs are involved in numerous physiological processes promoting health, including:^[98]

SCFAs that are absorbed by the colonic mucosa pass through the colonic wall into the portal circulation (supplying the liver), and the liver transports them into the general circulatory system.

Overall, SCFAs affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.^[100][101]

The major SCFAs in humans are butyrate, propionate, and acetate, where butyrate is the major energy source for colonocytes, propionate is destined for uptake by the liver, and acetate enters the peripheral circulation to be metabolized by peripheral tissues.

The United States FDA allows manufacturers of foods containing 1.7 g per serving of psyllium husk soluble fiber or 0.75 g of oat or barley soluble fiber as beta-glucans to claim that regular consumption may reduce the risk of heart disease.^[102]

The FDA statement template for making this claim is: Soluble fiber from foods such as [name of soluble fiber source, and, if desired, name of food product], as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food product] supplies __ grams of the [necessary daily dietary intake for the benefit] soluble fiber from [name of soluble fiber source] necessary per day to have this effect. ^[102]

Eligible sources of soluble fiber providing beta-glucan include:

The allowed label may state that diets low in saturated fat and cholesterol and that include soluble fiber from certain of the above foods "may" or "might" reduce the risk of heart disease.

As discussed in FDA regulation 21 CFR 101.81, the daily dietary intake levels of soluble fiber from sources listed above associated with reduced risk of coronary heart disease are:

Soluble fiber from consuming grains is included in other allowed health claims for lowering risk of some types of cancer and heart disease by consuming fruit and vegetables (21 CFR 101.76, 101.77, and 101.78).^[102]

In December 2016, FDA approved a qualified health claim that consuming resistant starch from high-amylose corn may reduce the risk of type 2 diabetes due to its effect of increasing insulin sensitivity. The allowed claim specified: "High-amylose maize resistant starch may reduce the risk of type 2 diabetes. FDA has concluded that there is limited scientific evidence for this claim."

^[104] In 2018, the FDA released further guidance on the labeling of isolated or synthetic dietary fiber.^[105]