What is Dietary fiber

Dietary fiber is the type of compounds including lignin and complex carbohydrate (made up of many sugar molecules linked together) you can eat that is found in fruits, vegetables, legumes and whole grains ¹, ², ³, ⁴, ⁵, ⁶. The European Food Safety Authority (EFSA) defines dietary fiber as non-digestible carbohydrate plus lignin ⁷, ⁸, ⁹. The Codex Alimentarius in 2009 defined dietary fiber as “carbohydrate polymers with ten or more monomeric units which are not hydrolyzed by the endogenous enzymes in the small intestine of humans” ¹⁰, ¹¹. But unlike other carbohydrates, dietary fiber is bound together in such a way that it cannot be easily digested in the small intestine. All dietary fibers are resistant to digestion in the small intestine, meaning they arrive intact in the large intestine ¹². Although most fibers are carbohydrates, one important factor that determines their susceptibility to digestion by human enzymes is the chemical bonds between sugar molecules (glycosidic bonds) ¹³. Humans lack digestive enzymes capable of breaking apart (hydrolyzing) most beta-glycosidic bonds (the chemical bonds between sugar molecules), which explains why amylose, a glucose polymer (containing different types of sugar monomers) with alpha-1,4 glycosidic bonds, is digestible by human enzymes, while cellulose, a glucose polymer (glucose polysaccharide) with beta-1,4 glycosidic bonds, is indigestible ¹³. Dietary fiber adds bulk to your diet. Dietary fiber is an important part of a healthy diet and getting enough fiber is important for your overall health, including heart and digestive health ¹⁴, ¹⁵, ¹⁶, ¹⁷. Because high-fiber foods make you feel full faster, they can help with weight control. Fiber also aids in your digestion and helps prevent constipation. Dietary fiber is sometimes used for the treatment of diverticulosis, diabetes, and heart disease ¹⁸. Fiber-rich foods offer health benefits when eaten raw or cooked. Taking the peels off fruits and vegetables reduces the amount of fiber you get from the food.

The classification of dietary fibre also stems from water solubility. Broadly, there are 2 main types of dietary fibre: soluble and insoluble fiber. Both soluble and insoluble dietary fiber can make you feel full, which may lower your calorie (food) intake by helping you eat less and yet stay satisfied longer ¹⁹ , ²⁰.

1. Soluble dietary fiber attracts water and forms a thick gel-like substance in your stomach. This slows digestion. Soluble dietary fiber is found in oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables. Soluble dietary fiber is broken down by bacteria in your large intestine and provides some calories. Soluble dietary fiber can interfere with the absorption of dietary fat and cholesterol. This, in turn, can help lower low-density lipoprotein (LDL or “bad”) cholesterol levels in your blood. Research has shown that soluble dietary fiber lowers cholesterol, which can help prevent heart disease. Soluble fiber can also slow digestion and the rate at which carbohydrates and other nutrients are absorbed into your bloodstream. This can help control your blood sugar (blood glucose) level by preventing rapid rises in blood glucose (sugar) following a meal.
2. Insoluble dietary fiber does not dissolve in water and may pass through your gastrointestinal tract relatively intact and, therefore, is not a source of calories. Insoluble fiber appears to speed the passage of foods through your stomach and intestines and adds bulk to the stool. Insoluble dietary fiber is found in foods such as wheat bran, vegetables, and whole grains.

Most Americans (>90 percent of women and 97 percent of men) do not get the recommended amount of dietary fiber (25 to 32 g/day for adult women, 34 to 38 g/day for adult men or 14 g of dietary fiber per 1000 kcal consumed) because of under consumption of fruits, vegetables, and whole grains ²¹, ²², ²⁰, ⁶. 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 ²³. Dietary fiber intake for adults living in European countries was 18 to 24 g per day for men and 16 to 20 g per day for women, with grain products (including bread) providing the largest source of dietary fibre ⁶. Comparison with data from the National Health and Nutrition Examination Survey (NHANES) showed that on average, dietary fiber intake in European countries was higher than in North America. Based on these data, it appears that within Europe and the US, dietary fiber intake is around one third below the recommended level.

Dietary fiber is considered a “dietary component of public health concern” in the Dietary Guidelines for Americans because low intakes are associated with potential health risks ²⁴. The Dietary Guidelines for Americans recommendation for older children, adolescents, and adults is to eat 21 to 38 grams of fiber each day (or 14 g of dietary fiber per 1000 kcal consumed). The Dietary Guidelines for Americans recommend consuming a variety of foods that are good sources of dietary fiber, especially vegetables, fruits, beans, and grains ²¹. The guidelines also recommend consuming at least half of grains as whole grains and limiting the intake of refined grains and products made with refined grains ²¹.

Dietary fiber can reach the colon and influence intestinal physiology, and dietary fiber plays a role in regulating intestinal microbiota ²⁵. Dietary fiber also possesses physiological functions including reduction of inflammatory response ²⁶ and antioxidant ²⁷. Accumulating evidence indicates that dietary fiber intake is beneficial in alleviating several chronic diseases such as high blood pressure ²⁸, diabetes ²⁹, ³⁰, ³¹, hyperlipidemia ³² and cancer ³³. Furthermore, dietary fiber has been demonstrated to have significant antiaging benefits, with people who consume high levels of dietary fiber living longer than their peers and having a reduced risk of disease or premature death ³⁴.

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 ³⁵. These higher fiber foods are currently underconsumed, contributing to only about 6% of total dietary fiber intake ²³. 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 ³⁵. 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 ³⁶.

All plant-based foods contain a mixture of soluble and insoluble fiber ³⁵. 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 4 below. Some strategies for increasing dietary fiber intake include increasing fruit and nonstarchy vegetable intake, increasing intake of legumes, eating whole-grain cereal or oatmeal for breakfast, substituting whole grains for refined grains, and substituting nuts or popcorn for less healthy snacks.

To ensure that you get enough fiber, eat a variety of foods, including:

• Cereals
• Dried beans and peas
• Fruits
• Vegetables
• Whole grains

For breakfast, choose steel cut oats with nuts and berries instead of a low-fiber, refined cereal. At lunch, have a sandwich or wrap on a whole-grain tortilla or whole-grain bread and add veggies, such as lettuce and tomato, or serve with veggie soup. For a snack, have fresh veggies or whole-grain crackers with hummus. With dinner, try brown rice or whole-grain noodles instead of white rice or pasta made with white flour.

When increasing fiber in your diet, be sure to do it gradually over a period of a few weeks to avoid stomach distress and with plenty of fluids. Water helps fiber pass through your digestive system. As dietary fiber travels through the digestive tract, it is similar to a new sponge; it needs water to plump up and pass smoothly. If you consume more than your usual intake of fiber but not enough fluid, you may experience nausea or constipation. All fiber supplements should be taken with sufficient fluids. Most doctors 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 ³⁷, ³⁸.

Before you reach for the fiber supplements, consider this: Fiber is found naturally in nutritious foods. Studies have found the same benefits, such as a feeling of fullness, may not result from fiber supplements or from fiber-enriched foods. If you’re missing out on your daily amount of fiber, you may be lacking in other essential nutrients as well. Your fiber intake is a good gauge for overall diet quality. Try to reach your fiber goal with unprocessed foods so you get all the other benefits they provide as well.

Types of fibers

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. In the 1970s, dietary fiber was defined as leftovers of plant cells that are resistant to digestion by human enzymes ¹⁸. 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) ³⁹. These compounds share many of the characteristics of fiber present in plant foods. In 2001 a panel of experts convened by the Institute of Medicine (National Academy of Medicine) developed definitions of fiber that made a distinction between dietary fiber as fiber that occurs naturally in plant foods and functional fiber which are man-made (synthetic) or isolated fibers that may be added to foods or used as dietary supplements ². However, these distinctions are controversial, and there are other classification systems for dietary fiber.

Dietary fiber

Dietary fiber is an umbrella term for complex carbohydrate polymers with 10 or more monomeric units which are neither digested by the endogenous enzymes in the small intestine of humans nor absorbed; and have beneficial physiological effect; this term includes non-starch polysaccharides, oligosaccharides, lignin, resistant starch, and other plant components (i.e. gums, mucilages) ¹¹. The European Food Safety Authority (EFSA) defines dietary fiber as “non-digestible carbohydrate plus lignin” ⁷, ⁸. The European Food Safety Authority (EFSA) provide a long list of substances that constitute dietary fiber, including non-starch polysaccharides, cellulose, pectins, hydrocolloids, fructo-oligosaccharides and ‘resistant starch’ ⁷, ⁸, ⁹.

Naturally occurring fiber often referred to as “intrinsic fiber” occurs in foods such as vegetables, whole grains, fruits, cereal bran, flaked cereal and flours. The fibers are also considered to be “intact fiber” because they have not been removed from the food. Foods containing these fibers have been shown to be beneficial to human health.

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 ⁴⁰.

Cellulose

Cellulose is a glucose polymer with beta-1,4 glycosidic bonds found in all plant cell walls ³⁹.

Beta-glucans

Beta-glucans are glucose polymers (glucose polysaccharides) with a mixture of beta-1,4 glycosidic bonds and beta-1,3 glycosidic bonds. Beta-glucans are soluble fibers found in bacteria, yeast, fungi, and grains such as oat bran and barley are particularly rich in beta-glucans ⁴¹, ⁴⁰.

Hemicelluloses

Hemicelluloses are a diverse group of sugar polymers (polysaccharides) containing different types of sugar monomers, including glucose, xylose, arabinose, mannose, galactose, rhamnose, or pentose. Glucomannan is a soluble fiber commonly derived from the Amorphophallus konjac root. Glucomannan is a hemicellulose containing about 60% of mannose and 40% of glucose bonded together by beta-1,4 glycosidic linkages. Like cellulose, hemicelluloses are found in plant cell walls.

Pectins

Pectins are linear sugar polymers (polysaccharides) made of 300 to 1,000 monosaccharides, primarily galacturonic acid residues linked together by alpha-1,4 glycosidic bonds ⁴². Pectins are soluble viscous fibers that are particularly abundant in berries and other fruit ².

Gums

Gums are viscous sugar polymers (polysaccharides) often found in seeds ². Guar gum extracted from guar beans is a galactomannan polysaccharide composed of mannose and galactose residues. Guar gum is used in the food industry for its thickening and stabilizing properties. The viscosity of guar gum is lost when guar gum is partially broken down (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 ⁴³. 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

Resistant starch is a form of starch that is resistant for human enzymatic digestion in the small intestine and can function as a prebiotic or dietary fiber in the large intestine. Resistant starch includes four forms: RS 1 “physically inaccessible”; RS 2 “ungelatinized”; RS 3 “retrograded”; RS 4 “chemically modified”; RS 5 “amylose-lipid complex”. Naturally occurring resistant starch is secluded in plant cell walls and is therefore inaccessible to human digestive enzymes ². Bananas and legumes are sources of naturally occurring resistant starch. Resistant starch is fermented by bacteria in the large intestine (colon). 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” ². 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. Man-made (synthetic) or isolated non-digestible fibers include: gum acacia (gum arabic), alginate, arabinoxylan, beta-glucan soluble fiber, cellulose, cross linked phosphorylated RS4, galactooligosaccharide, glucomannan, guar gum, high amylose starch (resistant starch 2), hydroxypropylmethylcellulose, inulin and inulin-type fructans, locust bean gum, mixed plant cell wall fibers (a broad category that includes fibers like sugar cane fiber and apple fiber, among many others), pectin, polydextrose, psyllium husk, and resistant maltodextrin/dextrin.

The U.S. Food and Drug Administration (FDA) has identified the following isolated or synthetic non-digestible carbohydrates as meeting the dietary fiber definition ³⁸:

• Acacia (gum arabic)
• Alginate
• Arabinoxylan
• Beta-glucan soluble fiber
• Cellulose
• Cross linked phosphorylated RS4
• Galactooligosaccharide
• Glucomannan
• Guar gum
• High amylose starch (resistant starch 2)
• Hydroxypropylmethylcellulose
• Inulin and inulin-type fructans
• Locust bean gum
• Mixed plant cell wall fibers (a broad category that includes fibers like sugar cane fiber and apple fiber, among many others)
• Pectin
• Polydextrose
• Psyllium husk
• Resistant maltodextrin/dextrin

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 ².
• Chitin and chitosan: Chitin is a polysaccharide polymer extracted from the exoskeletons of crustaceans, such as crab and lobster ⁴⁴. Chitin is a polymer of more than 5,000 acetylated glucosamine units linked by beta-1,4 glycosidic bonds. Deacetylated chitin also known as chitosan consists in unbranched chains of glucosamine ⁴².
• Fructooligosaccharides: Fructooligosaccharides are short, synthetic fructose chains terminating with a glucose unit. Fructooligosaccharides are used as food additives ⁴³.
• Galactooligosaccharides: Galactooligosaccharides are produced through the enzymatic conversion of lactose and are classified as prebiotics (a non-digestible food ingredient that promotes the growth of beneficial microorganisms in the intestines) ⁴⁵. Fibers that are fermentable and can stimulate the growth and/or activity of beneficial gut bacteria are called prebiotic fibers ⁴⁶.
• 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 ².
• Resistant dextrins: Resistant dextrins also called resistant maltodextrins are synthetic indigestible polysaccharides formed when starch is heated and treated with enzymes. Maltodextrins are used as food additives ².

Total fiber

Total fiber is defined by the Institute of Medicine as “the sum of dietary fiber and functional fiber” ².

Dietary fiber Other classification systems

Fibers can also be classified into 4 clinically meaningful categories according to their physiochemical properties, i.e., their solubility, viscosity, and fermentability ¹:

Soluble, viscous/gel forming, readily fermented fibers

Beta-glucans (oats and barley) and 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 broken down (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 ¹². Short-chain fatty acids can be absorbed and broken down (metabolized) to produce energy. Interestingly, the preferred energy source for epithelial cells that line the large intestine (colonocytes) is butyrate. Fermentation of fiber is estimated to contribute up to 10% of daily energy intake ³⁶. Fibers that are fermentable and can stimulate the growth and/or activity of beneficial gut bacteria are called prebiotic fibers ⁴⁶. Fibers that are soluble, viscous, and fermentable have been shown to improve blood sugar (glycemic) control and to lower blood cholesterol concentration. However, their water-holding capacity is lost when they are fermented in the large intestine (colon) such that they have no laxative effect.

Soluble, viscous/gel forming, nonfermented fibers

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 ². Psyllium can improve blood sugar (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, psyllium can exert a stool-normalizing effect, preventing constipation or softening hard stool as well as firming loose/liquid stool in diarrhea and fecal incontinence.

Soluble, nonviscous, readily fermented fibers

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. Test tube studies have shown inulin to selectively stimulate the proliferation of beneficial bacteria and limit the growth of potentially pathogenic bacteria ⁴⁷. However, no health benefit is currently associated with this fiber-driven prebiotic effect.

Insoluble, poorly fermented fibers

Wheat bran, cellulose, lignin

These fibers do not dissolve in water, do not trap water, and are poorly fermented. Large or 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.

How dietary fiber is digested

It is now well-established that dietary fibre reaches the large intestine and is fermented by the colonic microflora with the production of short chain fatty acids, hydrogen, carbon dioxide and biomass. This fermentative process dominates human large bowel function and provides a means whereby energy is obtained from carbohydrates not digested in the small bowel, through absorption of short chain fatty acids ⁴⁸.

Fermentation of fiber in the colon (large intestine)

Polysaccharides (a carbohydrate (e.g. starch, cellulose) whose molecules consist of a number of sugar molecules bonded together) cannot penetrate in the bacterial cells. They are first hydrolysed (broken down by chemical reaction with water) in monosaccharides (simple sugar molecule e.g. glucose), by membranous or extra-cellular enzymes secreted by bacteria. Metabolism of these monomeric sugars continue in the bacterial cells using the Embden-Meyerhoff pathway which leads to pyruvate. Pyruvate does not appear in the large bowel because it is immediately converted in end-products. These are short chain fatty acids, mainly acetate, propionate and butyrate, and gases: carbon dioxide (CO2), hydrogen (H2), and methane (CH4) ⁴⁸.

Colonic fermentation is an efficient digestive process since starch is almost totally degraded, as well as lactose, alcohol-sugars and fructans if the intake of these sugars is not too high. More than half of the usually consumed fibres are degraded in the large intestine, the rest being excreted in the stool. A number of factors are likely to affect the utilization of fermentable carbohydrates in the colon. Among these is solubility. The more soluble substrates, being more accessible to hydrolytic enzymes, are likely to be degraded more rapidly. Nevertheless, some soluble fibres such as alginates or carragheenans are poorly fermented. Other factors involving digestive motility and individual differences in microflora could also modulate fermentation. Furthermore, certain metabolic pathways can be modified by the repeated occurrence of some sugars (lactose, lactulose, fructans) in the colon. The mechanisms and the physiological consequences of this adaptation are not completely identified.

Table 1. Colonic fermentability of dietary fibrer in humans

Dietary fiber	Fermentability (%)
Cellulose	20-80
Hemicelluloses	60-90
Pectins	100
Guar gum	100
Ispaghula	55
Wheat bran	50
Resistant starch	100
Inulin, oligosaccharides	100 (if they are not in excess)

[Source ⁴⁸ ]

Absorption and metabolism of dietary fiber end-products

Reducing the rate of digestion of carbohydrate spreads the absorption of carbohydrate along a longer portion of the small intestine and tends to increase the amount of carbohydrate which escapes digestion in the small intestine). For example, the amount of carbohydrate from lentils entering the colon is 2.5 times as great as carbohydrate from bread. Increasing the delivery of starch to the colon has many implications which include those on the health of the colon itself and on systemic metabolism. It is believed that starch entering the colon is completely and rapidly fermented, mostly in the cecum. The fermentation of starch produces relatively more butyrate than the fermentation of dietary fibre and resistant starch produces somewhat different fermentation products than readily digested starch.

A part of the products of fermentation are utilized by bacteria yielding energy and carbon necessary for synthesis and growth of the flora. Another part is eliminated in the stool or rectal gases, but the major part is absorbed by the colonic mucosa. Absorption of short chain fatty acids is rapid and leads to accumulation of bicarbonates and increase of pH in the lumen. Butyrate is considered to be the primary nutrient for the epithelial cells lining the colon and short chain fatty acids stimulate proliferation of colonic epithelial cells and growth of the colon in general. Butyrate is the preferred substrate of colonocytes. Short chain fatty acids which are not metabolized in the mucosa are oxidized in the liver, a part of acetate being also metabolised in the peripheric tissues.

Only a fraction of gases produced during fermentation is available for absorption. Hydrogen and methane are excreted in the breath gases. A large part of gases are consumed in the colonic lumen by ‘bacteria. Acetogenic bacteria produce acetate from CO2 and H2. Methanogenic bacteria produce CH4 by reduction of CO2 with H2. Finally, sulfate reducing bacteria utilise H2 to reduce sulfates and produce sulfites or hydrogen sulfide. Unused gases are excreted through the anus.

Effects of dietary fibre on gut microflora

The composition of microflora appears to be influenced to some degree by diet, age and geographic considerations, but these factors are not thought to be particularly significant, at least as far as the commonly studied bacterial groups are concerned. Recent studies have shown, however, that the ingestion of certain oligosaccharides, such as fructo-oligosaccharides, could modify bacterial composition of the dominant flora by increasing bifidobacteria. Some studies suggest that these bifidobacteria, which are saccharolytic bacteria naturally occurring in the normal colonic flora, might be beneficial to host health. At the present time, however, this has not been conclusively established.

Ingestion of fructo-oligosaccharides have increased faecal counts of endogenous bifidobacteria by a factor of 10, without changing the total anaerobes concentration. The similarity of effects of chemically different substrates is likely due to the capacity of bifidobacteria to hydrolyze all these substrates and to metabolize the produced monomeric sugars (glucose, galactose, fructose). The exact mechanisms whereby only some substrates could stimulate preferentially the growth of bifidobactria are not known. A recent in vitro study suggested that the polymerization degree could be more determinant than the chemical nature of oligosaccharides. The metabolic consequences of the changes in faecal flora composition are unknown. Ingestion of oligosaccharides had no effect on stool weight and pH.

Effects of dietary fibre on gut function

In the gastrointestinal tract, some fibres form a matrix with fibrous characteristics. That is, some fibres, because of their ability to swell within the aqueous medium, can trap water and nutrients, especially water-soluble ones such as sugars. The physical characteristics of the gastric and small intestinal contents are altered by fibre sources. The bulk or amount of material in the gastrointestinal tract is greater because fibre is not digestible and hence remains during the transit of digesta through the small intestine. The volume increase is due to the water-holding capacity of certain fibres. The viscosity of the intestinal contents increases due to the presence of fibre sources containing viscous polysaccharides.

The changes in the physical characteristics of the intestinal contents may influence gastric emptying, dilute enzymes and absorbable compounds in the gut, prevent starch from hydrolyzing, and slow the diffusion or mobility of enzymes, substrates and nutrients to the absorptive surface. These effects result in the slower appearance of nutrients such as glucose and some lipid molecules in the plasma following a meal.

The effects of purified dietary fibres on bowel function may or may not be similar to those of intact fibres in whole foods. This is presumably due, at least in part, to interactions between fibre and starch, and the presence of fibre associated substances such as phytate and lectins which are present in the whole food. This makes it very difficult to make valid generalizations about the physiologic effects of fibre based simply on fibre analysis. For example, when considering the effect of fibre on postprandial blood glucose responses, purified viscous fibres have been found to produce a significant reduction in glycemic response in 33 of 50 studies (66%) reviewed in 1992, compared to only 3 of 14 (21%) studies with insoluble fibre (166). The effects of purified fibres appear to be directly related to their viscosity. This would suggest that the blood glucose responses of foods should be more closely related to their soluble than insoluble fibre content, however the opposite is the case. For 52 foods, the food glycemic index (as the indicator of rise in blood glucose) was weakly related to the amount of total fibre per 50g carbohydrate, and insoluble fibre explained a larger proportion of the variance in glycemic index, 17%, than soluble fibre, 9%.

Effects on carbohydrate digestion and absorption

• Gastric emptying

Dietary fibres may affect gastric emptying in several ways. First, they may slow gastric filling, due to their bulking and energetic dilution capacity, which might in turn slow gastric emptying. Secondly, when certain soluble fibres are mixed in liquid meals or in liquid/solid meals, they delay emptying of gastric liquids by increasing viscosity of gastric contents. Such an increase in the viscosity of chyma could also slow the gastric emptying of solid components of the meal. On this issue, results are very controversial. Moreover, by acting as an emulsifier, viscous fibre can stabilize the gastric chyma and prevent separation of the solid from the liquid phase, impairing selective retention of the largest particles, and thereby increasing their rate of passage into the small intestine. Besides the effects of soluble fibres, insoluble fractions may also alter gastric emptying by mechanisms depending on their water retention capacity or size of particles.

• Enzyme-substrate interaction

Available evidence suggests that fibre has little, if any, direct acute effect on the secretory function of the exocrine pancreas suggesting that the primary effect of fibre on carbohydrate digestion is exerted in the intestinal lumen. In the lumen, enzymes and substrates may be diluted with the addition of non-digestible material. Evidence from in vitro studies and from duodenal aspirates suggest that most of the tested fibres can alter the activity of pancreatic amylase (88). The inhibitory effects of fibre on pancreatic enzyme activities have been attributed to various factors including pH changes, ion-exchange properties, enzyme inhibitors and adsorption. Rather than a chemical enzyme-fibre interaction, the presence of fibre, through its particulate or viscous nature, probably impedes enzyme-substrate interaction.

The presence of fibre in a form that restricts starch gelatinization or access of the hydrolytic enzymes to starch can slow the rate of digestion of the starch. For instance, the slow rate of digestion of legumes may be related to the entrapment of starch in fibrous thick-walled cells, which prevents its complete swelling during cooking. In addition, resistance of starch to pancreatic hydrolysis may result from the presence of intact cell walls, which survive processing and cooking and insulate starch in such a manner that portions of it cannot be digested or absorbed.

• Small intestinal motility

There is evidence that viscous fibres can influence accessibility of available carbohydrates to the mucosal surface and slow their absorption. One of the major mechanisms of this action is related to the effects of dietary fibre on small intestinal motility. Small intestinal contractions create turbulences and convective currents which cause fluid circulation and mixing of luminal contents. These movements allow glucose to be brought from the centre of the lumen close to the epithelium. When it reaches proximity to the epithelium, glucose must then diffuse across the unstirred water layer. This layer is created by a gradient of progressively poorer stirring as the mucosa is approached and forms an aqueous diffusion barrier separating mixed bulk luminal contents from the brush border. Thickness of the unstirred water layer depends on small intestinal contractions and is inversely related to the magnitude of the stir rate. When there is no contraction, fluid moves through the small intestine with laminar flow comparable to that occurring in a pipe. In this flow, there is no movement in the radial direction (from the centre of the lumen toward the epithelium), and consequently the stirring is very poor and the unstirred water layer very thick. On the contrary, normal motility generates both longitudinal and radial convection currents, hence creating turbulences and stirring of luminal fluid. Beside the effects of mixing contractions on glucose movement, small intestinal motility may alter absorption by influencing transit rate which determines area and time of contact between glucose and the epithelium.

Dietary fibres which alter small intestinal motility could thus influence glucose absorption by this mechanism. Viscous fibres, such as guar gum, stimulate motility but decrease transit rate, because they resist propulsive contractions. However, though guar gum slows transit it does not affect the distribution of glucose in the human upper small intestine. It is thus unlikely that guar gum delays glucose absorption by reducing contact area. As they resist propulsion, viscous fibres should similarly resist mixing contractions, hence inhibiting the effects of motility on fluid stirring. This is probably the mechanism by which they increase thickness of the unstirred water layer, and diminish passage of glucose across the epithelium.

• Effects of dietary fibre on large bowel function

The major effects of dietary fibre occur in the colon. Here each type of dietary fibre interacts with the microflora, and the colonic mucosa and muscle to produce several possible effects. The actions of an individual fibre source depends to a large extent on its fermentability. The range of fermentability of different fibre is great and difficult to predict. Dietary fibre, however, can be roughly divided into those which are rapidly fermented, such as oligosaccharides, those which are more slowly fermented, such as gums, and those which are hardly fermented at all, such as wheat bran. The least fermentable fibres are the most likely to increase stool output. Dietary fibre which is highly fermentable is unlikely to have much effect on stool output but will affect bacterial fermentation products in the proximal colon and hence colonic and systemic physiology. Fibres which are slowly fermented may have a major influence in the distal colon even if they do not increase stool output significantly. Furthermore, the effect of each type of fibre is determined by dose.

• Stool output

The dietary fibres which have the greatest effects on stool output are in general the least fermentable. These fibres probably act by virtue of their water holding capacity. The relationship between water holding capacity and stool output is not simple. Dietary fibres with high water holding capacity are those which are the most fermentable and are lost before they reach the rectum. There are exceptions such as ispaghula which has high water holding capacity but resists fermentation. Moreover, one of the most reliable stool bulkers is wheat bran which has a water holding capacity that is as low as the rest of faecal contents on a normal low fibre diet. It appears that the most important factor for a large effect on stool output is simply for the fibre to appear in stool. The effect is then dependent on the amount of fibre present as well as its residual water holding capacity. The contribution of bacterial cells to faecal mass should not be forgotten, as the water content of bacteria is high. The effects of fibre are not restricted to increasing output. Dietary fibre has also a role in changing the consistency of the stool by increasing the water content and the plasticity, and increasing stool frequency.

• Colonic motility and transit time

Certain fibres are known to have a laxative effect, in that their presence in the colon affects its motility and modifies colonic transit time. Two major mechanisms to explain this effect depend on the physicochemical properties and fermentative fate of fibre. These mechanisms refer to stimulation by the bulking effect of fibre as well as changes in the contractile activity and secretion of the colon.

Increasing the volume of colonic contents distends the colon wall and stimulates propulsion of digesta through the activation of intramuscular mechanoreceptors. Dietary fibre can increase the faecal bulk by several mechanisms. First, the volume occupied by undegraded fibres adds to the volume of the rest of contents. This explains why the least fermentable fibres, such as wheat and corn bran, ispaghula or some algal polysaccharides, are particularly efficient laxatives. Also, these residues can trap water within their matrix, thus leading to a greater bulk. A third possible mechanism to increase intraluminal volume and stretch colonic muscle is the production of gases occurring during the fermentation of fibre.

In addition to their bulking effects, dietary fibre can reduce transit time by modulating contractile activity and water movements in the colon. Here again, they can act in several ways. First, the edges of solid particles can stimulate mechanoreceptors located in the submucosa and by that, modify the contractile pattern of the colon in favour of a greater propulsion of digesta, as has been shown with plastic particles. Fibre could also release compounds trapped in the small intestine (such as biliary salts or fatty acids) into the colon during fermentation. Such compounds have been shown to stimulate secretion and rectosigmoid motility.

Finally, a large part of fibre is fermented by microflora yielding several metabolites which can themselves influence colonic motility. For instance, short chain fatty acids stimulate contractions in the terminal ileum of humans and may also affect colonic motility as has been demonstrated with rats.

It has recently been appreciated that dietary starch bulks the stool, presumably because undigested starch provides energy for colonic bacterial growth. Thus, some of the faecal bulking effect of dietary fibre, at least in intact foods, could be due to the associated increase in starch delivery to the colon.

Dietary fiber health benefits

Dietary fiber contributes to health and wellness in a number of ways ⁶, ⁴⁹, ¹⁹. First, dietary fiber aids in providing fullness after meals, which helps promote a healthy weight. Second, adequate fiber intake can help to lower cholesterol. Third, it helps prevent constipation and diverticular disease. And fourth, adequate fiber from food helps keep blood sugar levels within a healthy range.

Observational studies and intervention trials have also identified associations between high-fiber intakes and reductions in chronic disease risk. The U.S. Food and Drug Administration (FDA) approved specific health claims related to the cardioprotective effects of two soluble, gel-forming fibers only: beta-glucan and psyllium.

• Metabolic disease: Improved insulin sensitivity (mainly insoluble fibres); reduced risk of developing type 2 diabetes (mainly insoluble cereal fibres and whole grains); improved glycemic status and lipid profiles (mainly soluble fibres); reduced body weight and abdominal adiposity ²⁰
• Heart attack, stroke, and cardiovascular disease: Fiber may help prevent heart disease by helping reduce cholesterol. Epidemiologic studies show that a diet high in fiber consumption from whole foods is strongly associated with a reduced risk of heart attack, stroke, and cardiovascular disease ⁵⁰, ⁵¹.
• Weight management: Fiber slows the speed at which food passes from the stomach to the rest of the digestive system – this can make us feel full longer. Foods that are higher in dietary fiber often are lower in calories as well. Consumption of dietary fiber has been associated with lower risk of obesity ⁵², ⁵³.
• Diabetes: Because fiber slows down how quickly food is broken down, it may help control blood sugar levels for people with diabetes by reducing blood sugar levels after meals.
• Digestive issues: Fiber increases bulk in the intestinal tract and may help improve the frequency of bowel movements.

Improving regularity in bowel movements

Of all the beneficial effects of dietary fiber and the most widely known is the effect on gut motility and prevention of constipation. 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 ⁵⁴. 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.

In one randomized controlled double-blind trial on the effects of ‘vege-powder’ consisting of chicory, broccoli and whole grains on constipation alleviation in >90 participants, compared with the control group, those who received ‘vege-powder’ had significant improvements in symptoms of constipation including stool hardness, defecation frequency and straining to defecate at 2 and 4 weeks ⁵⁵. Further evidence to support the clinical utility of dietary fiber as an effective treatment of constipation stems from a systematic review reported by Rao et al. ⁵⁶. Evidence was sought for dietary fiber intake and restrictions in fermentable oligosaccharide, disaccharide, monosaccharide and polyol (FODMAP-restricted diet) in the management of chronic constipation and irritable bowel syndrome (IBS) ⁵⁶. For chronic constipation, dietary fiber was beneficial in five of the seven studies examined, and all three of the studies of IBS-associated constipation ⁵⁶. The FODMAP-restricted diet also appeared to improve overall IBS symptoms ⁵⁶. Current evidence would appear to support the beneficial effects of dietary fiber on gut motility and as an effective management strategy for both the prevention and treatment of constipation.

A 2016 review of fiber interventions conducted by McRorie and Chey ⁵⁷ examined the potential laxative effect of fermentable fiber; the main findings of this review are summarized in Table 2 below.

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 2) ⁵⁷. 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 ⁵⁸.

Table 2. Laxative effect of fermentable fiber

Fiber	Type of Fiber	Number of Studies	Doses	Laxative Effects	Adverse Effects
Beta-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 (beta-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

[Source ⁵⁷ ]

Chronic idiopathic constipation

Only insoluble fibers and soluble viscous fibers that resist bacterial fermentation in the colon have a potential laxative effect ⁵⁴. 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 cause might be different. Bulk-forming laxatives, including psyllium, bran, and methylcellulose, are commonly recommended to improve stool regularity in patients with diabetes mellitus ⁵⁹. 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. 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 ⁶⁰ or in pregnant women and new mothers ⁶¹, ⁶².

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 ⁶³. The American College of Gastroenterology 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 ⁶⁴.

Irritable bowel syndrome (IBS)

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) ⁶⁵. 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 ⁶⁶. 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 ⁶⁷, ⁶⁸. 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 ⁶⁴. 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 ⁶⁸. 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 ⁶⁹. 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 ⁷⁰. The cause 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 ⁷⁰, ⁷¹. 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 ⁷².

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 ⁷³. However, a meta-analysis could not be conducted nor any conclusion provided regarding the efficacy of fiber in the treatment of symptomatic uncomplicated diverticular disease (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 ⁷³. Another recent review of the literature focused on the effect of fiber-restricted diets in the management of acute uncomplicated diverticulitis ⁷⁴. 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 ⁷⁴.

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 symptomatic uncomplicated diverticular disease (SUDD) and for the prevention of diverticulitis ⁷⁵, ⁷⁶.

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 ⁷⁷. 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 ⁷⁸. 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 ⁷⁹.

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 ⁸⁰. 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 ⁸⁰.

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) ⁸¹. 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) ⁸². 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 ⁸². 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) ⁸³.

Lowering blood cholesterol

Some, but not all, fibers can improve blood lipid (fat) abnormalities observed in conditions like hyperlipidemia, 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 beta-glucan (found in oat bran), raw guar gum, and psyllium, has been shown to decrease total and low-density-lipoprotein (LDL or “bad”) cholesterol concentrations when compared to appropriate controls (e.g., fiber-free supplement, low-fiber supplement, or supplementation with insoluble fiber) ⁵⁴. A 2009 meta-analysis that combined the results of 21 randomized controlled trials in 1,717 participants with high cholesterol (hypercholesterolemia) found dose- and time-dependent reductions in total and LDL (“bad”) cholesterol concentrations with psyllium supplementation (3.0-20.4 g/day for >2 weeks) ⁸⁴. 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 high cholesterol (hypercholesterolemia) ⁸⁵. 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) ⁸⁶ — non-HDL-cholesterol and apo B appear to better predict cardiovascular events than LDL-cholesterol ⁸⁷. 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) ⁸⁶.

Despite considerable differences across the studies, the cholesterol-lowering efficacy of other highly viscous fibers, including beta-glucans from oat or barley and glucomannan (a hemicellulose), was also reported in recent meta-analyses of trials conducted by one research team ⁸⁸, ⁸⁹. The cholesterol-lowering effect of soluble fibers, such as psyllium and beta-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 ⁹⁰. Accordingly, low-viscosity soluble fibers (e.g., gum arabic/acacia gum, methylcellulose, low molecular weight beta-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 ⁵⁴.

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 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, liver cells (hepatocytes) must increase LDL-cholesterol clearance to synthesize more bile acids as cholesterol is a component of bile ⁵⁴.

These findings led the U.S. Food and Drug Administration (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 beta-glucan from whole oat or barley or ≥1.7 g/serving of psyllium ⁹¹.

Cardiovascular disease prevention

Prospective cohort studies have consistently reported associations between high intakes of fiber-rich foods and low risks of coronary heart disease (coronary artery disease) and total cardiovascular disease. Three large prospective cohort studies ⁹², ⁵¹, ⁹³ 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 coronary artery disease; these results are the basis for the Institute of Medicine’s Adequate Intake (AI) recommendation for fiber ². 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 ⁹⁴. The most recent meta-analysis that included 18 prospective studies, with a total of 672,408 participants, found a 7% lower risk of coronary heart disease and a 17% lower risk of coronary heart disease-related mortality with the highest versus lowest intakes of total dietary fiber ⁹⁵. 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 coronary heart disease ⁹⁵.

The U.S. Food and Drug Administration (FDA) has approved health claims like the following on the labels of foods containing at least 0.75 g/serving of beta-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” ⁹¹. 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” ⁹¹.

While the cholesterol-lowering effect of viscous/gel-forming soluble fibers is recognized as a major contributor to the cardioprotective effects of fiber, 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 coronary heart disease (coronary artery disease) and total cardiovascular disease and intakes of insoluble fiber ⁹⁴, ⁹⁵. 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 (“bad”) cholesterol concentration, but also with blood pressure, body mass index (BMI), and serum insulin concentration ⁹⁶. 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 coronary heart disease (coronary artery disease) risk.

Improving blood sugar control

The efficacy of fiber on blood sugar (glycemic) control is dependent on its 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 after meal and/or fasting blood glucose concentrations ⁵⁴. Supplementation with insoluble fiber also failed to improve blood sugar control in subjects with elevated fasting blood glucose concentrations ⁹⁷. In contrast, the capacity of dietary viscous fiber ⁹⁸, ⁹⁹ and isolated viscous fibers ¹⁰⁰, ¹⁰¹, ¹⁰², ¹⁰³ to improve blood sugar control has been demonstrated in numerous controlled clinical trials conducted over three decades ⁵⁴.

A 2015 review of psyllium showed evidence of reductions in after meal blood glucose concentration following a single meal in people with type 2 diabetes mellitus (6 studies) as well as in nondiabetic or normal glycemic subjects (11 studies) ¹⁰⁴. Supplementation with psyllium also resulted in reductions in after meal 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 ¹⁰⁴.

The role of soluble viscous fibers on blood sugar control is related to their ability to increase chyme (the semifluid mass of partly digested food expelled by the stomach into the duodenum) 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 ⁵⁴. 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 beta-cells in response to glucose absorption and is involved in the regulation of food intake at the central nervous system level ¹⁰⁵.

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 ¹⁰⁶. 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 ¹⁰⁶. 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 ¹⁰⁶. Similar findings were reported in another meta-analysis of 18 prospective cohort studies in 617,968 participants ¹⁰⁷. 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) ¹⁰⁷. 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).

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 ¹⁰⁸. 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 ¹⁰⁹. 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 ⁹⁷. Although observational data suggest a protective association of cereal fiber against type 2 diabetes, the current evidence from intervention trials does not support a role for insoluble fiber in blood sugar 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, beta-glucan, raw guar gum). Soluble viscous fibers in cereal, particularly beta-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 ¹¹⁰. Apart from fiber, other bioactive compounds in cereal, like magnesium, might contribute to improving blood sugar control in people with impaired glucose tolerance ¹¹¹.

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) ¹¹². 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) ². 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 ¹¹³, ¹¹⁴.

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 ¹¹⁵. Adherence to a Mediterranean-style diet (rich in fruit, vegetables, and whole grains), the composition of which intends to meet the adequate intake for fiber, has been associated with a lower risk of developing type 2 diabetes ¹¹⁶, ¹¹⁷. 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 ¹¹⁸.

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) ¹¹⁹. 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 ¹⁰⁴.

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 ¹²⁰. Since there is little evidence from clinical trials that increasing nonviscous fiber alone is beneficial ¹²¹, individuals with diabetes should preferably increase fiber intake from sources of soluble viscous fibers, such as oats and barley (beta-glucans), vegetables, beans, and legumes ¹¹².

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 high blood pressure (hypertension) ¹²². 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 normal blood pressure 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 ¹²³. Further analyses showed that, among all the soluble viscous fiber under examination (beta-glucan [oat], guar gum, konjac glucomannan, pectin, and psyllium), only psyllium could reduce systolic blood pressure (mean reduction of 2.39 mm Hg) ¹²³. 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.

Metabolic syndrome

Metabolic syndrome also called insulin resistance syndrome is the name for a group of conditions that together raise your risk of coronary heart disease, diabetes, stroke, and other serious health problems. You can have just one condition, but people often have several of them together. When you have at least three of them, it is called metabolic syndrome. These risk factors or conditions include:

• A large waistline, also called abdominal obesity or “having an apple shape”. Too much fat around your stomach area is a greater risk factor for heart disease than too much fat in other parts of your body.
• High triglyceride level. Triglycerides are a type of fat found in your blood. High levels of triglycerides can raise your levels of LDL cholesterol (bad cholesterol). This raises your risk of heart disease.
• Low HDL cholesterol level. HDL is sometimes called the “good” cholesterol because it helps remove cholesterol from your arteries. Blood cholesterol levels are important for heart health. “Good” HDL cholesterol can help remove “bad” LDL cholesterol from your blood vessels. “Bad” LDL cholesterol can cause plaque buildup in your blood vessels.
• Having high blood pressure. If your blood pressure stays high over time, it can damage your heart and lead to other health problems. High blood pressure can also cause plaque, a waxy substance, to build up in your arteries. Plaque can cause heart and blood vessel diseases such as heart attack or stroke.
• High fasting blood sugar levels. Mildly high blood sugar may be an early sign of diabetes. Mildly high blood sugar can damage your blood vessels and raise your risk of getting blood clots . Blood clots can cause heart and blood vessel diseases.

The more risk factors you have, the higher your risk for heart disease, diabetes, and stroke is.

Metabolic syndrome is common in the United States. Metabolic syndrome is estimated to affect nearly 35% of US adults and half of those older than 60 years ¹²⁴. 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 (observational studies that analyze data from a population at a single point in time) but not in prospective cohort studies (a type of observational study focused on following a group of people [a cohort] over a period of time, collecting data on their exposure to a factor of interest) ¹²⁵, ¹²⁶.

In this study ¹²⁷, 111 overweight adults with features of the metabolic syndrome were randomly assigned to one of four 18-week isoenergetic diets, including control, high cereal fiber, high protein and mixed high cereal fiber and protein (Mix) groups. Amongst the 84 participants who completed the dietary intervention, it was demonstrated that compared with the high protein diet group, insulin sensitivity was significantly (25%) higher in the high cereal fiber group ¹²⁷. Furthermore, high cereal fiber intake prevented the reduction of insulin sensitivity upon increased protein intake ¹²⁷. The effects of high cereal fiber on insulin sensitivity at 18 weeks likely related to diminished adherence to the high protein diet ¹²⁷.

There have also been reports from observational studies to corroborate the metabolic benefits of dietary fiber. In one such study, Morimoto and colleagues ¹²⁸ reported on a study on a Japanese cohort (n = 190) without type 2 diabetes to explore the metabolic effects of dietary fiber intake in the context of a diet and exercise program. Increases in the ratio of dietary fiber to carbohydrate intake during the 5-month period of the study associated significantly with a reduction in HbA1C ¹²⁸. The authors suggested the potential utility of an increased dietary ratio of fiber to carbohydrate in the prevention of type 2 diabetes ¹²⁸. In a separate study from Mexico on 217 adolescents ¹²⁹, it was demonstrated that those with the highest dietary fiber intake had lower odds of Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) >2.97, following adjustments for age, sex, body fat percentage and intake of saturated fatty acids. In a recent systematic review and meta-analysis on the effects of dietary fiber and whole grains in the management of Diabetes Mellitus, there was an association of high-fiber diet with improved insulin sensitivity, including many other aspects of metabolic health, such as HbA1C, lipid profile, body weight and C-reactive protein ¹³⁰. A recent systematic review and meta-analysis of twenty-one randomized controlled trials in patients with type 2 diabetes reported that, compared to controls, dietary fiber at a median daily dose of 10 g/day for a mean intervention duration of 8 weeks significantly reduced glycated hemoglobin A1c (HbA1c), fasting glucose and insulin, and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) ¹³¹. Both soluble fiber products and fiber from natural foods were found to be effective in improving glycemic control and insulin sensitivity in type 2 diabetes patients, with the former yielding better effects ¹³¹. There is, however, a deficiency of published longer-term controlled studies (>12 months) on the metabolic effects of increasing dietary fiber intake ¹³⁰, and this should be a focus for future research in this field.

A high intake of soluble dietary fiber appears to have additional metabolic benefits, including improved glycemic index of carbohydrate-rich foods and lipid profiles ¹³², ¹³³, ¹³⁴. However, it is mainly the consumption of insoluble cereal dietary fiber and whole grains and not soluble fiber that associate consistently with a reduced risk for the development of type 2 diabetes in large prospective cohort studies ¹⁹, ¹³⁵, ¹³⁶. Other effects of dietary fiber that may also have an impact on overall metabolic health include the release of various gut hormones ¹³⁷, ¹³⁸, ¹³⁹, ¹⁴⁰, ¹⁴¹, ¹⁴², adipokines ¹⁴³, bile acids ¹⁴⁴ and the metabolic signatures of amino acids ¹⁴⁵. Based on the current scientific evidence from prospectively designed and controlled studies, dietary fiber does appear to associate with improvements in insulin sensitivity and overall metabolic status. Future studies should focus on the underlying mechanisms implicated, which mediate the metabolic benefits of dietary fiber.

Cancer prevention

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) ¹⁴⁶. This European Prospective Investigation into Cancer and Nutrition (EPIC) 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 ¹⁴⁶. 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 ¹⁴⁶. 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 ¹⁴⁶.

The preventive effect of dietary fiber on the development of cancer is consistent with the results of a meta-analysis of a large sample published by Andrew Reynolds in The Lancet ¹⁴⁷, which showed a 13% reduction of cancer mortality in those who consumed the most dietary fiber compared to those who ate the least. Dietary fiber can increase stool volume and decrease stool transit time, therefore diluting the concentration of carcinogens in the colon and reducing the time of intestinal exposure to carcinogens ¹⁴⁸. Dietary fiber binds bile acids and alters the enterohepatic axis, which can reduce cholesterol levels involved in the cause of colon cancer ¹⁴⁹. In contrast, secondary bile acids produced by bile acid metabolism are thought to be promoters of colorectal cancer ¹⁵⁰, ¹⁵¹, which can cause significant damage to the colonic mucosa, such as oxidative stress and inflammation ¹⁵²:S67–S69. doi: 10.1097/MCG.0000000000000252)). Dietary fiber is broken down by intestinal flora into short-chain fatty acids (SCFA), such as acetate, propionate, and butyrate, which can decrease intestinal luminal pH, which helps reduce the conversion of proto-bile acids to carcinogenic secondary bile acids ¹⁵³. Moreover, butyrate not only provides energy to normal colonic epithelial cells but also induces excessive activation of Wnt signaling in colon cancer cells, an event necessary to achieve high levels of apoptosis in these cells, where Wnt activity is associated with cancer cell proliferation ¹⁵⁴. Furthermore, under acidic conditions, dietary fiber has been shown to remove nitrite from the stomach ¹⁵⁵ and decrease the concentration of nitroso compounds, which increase the risk of gastric cancer ¹⁵⁶. It has been shown that dietary fiber is negatively associated with precancerous lesions in esophageal cancer, and the potential mechanisms include improvement of gastroesophageal reflux and weight control, adsorption of carcinogens contained in food, improvement of cancer-associated esophageal ecological dysregulation, and direct action on cancer cells ¹⁵⁷, ¹⁵⁸, ¹⁵⁹, ¹⁶⁰.

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 3 below.

Table 3. Dietary fiber intake and cancer risk

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. 161
	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. 162
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. 163
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. 164
		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. 165
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. 166
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. 167
	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. 168
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. 169
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. 170
	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. 171
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. ** 172
	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)** 172
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. 173
	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. 174
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. 175

Footnotes:

* for the highest versus lowest level of fiber intake (unless otherwise specified)
** both meta-analyses identified the same 14 observational studies

Risk Ratio (RR) also called relative risk = Risk Ratio or Relative Risk is a ratio of the probability of an event occurring in the exposed group versus the probability of the event occurring in the non-exposed group.

Relative Risk = (Probability of event in exposed group) / (Probability of event in not exposed group) ¹⁷⁶

In cancer research, risk ratios (relative risks) are used in prospective (forward looking) studies, such as cohort studies and clinical trials. A risk ratio (relative risk) of 1 means the risk is comparable in the two groups and there is no difference between two groups in terms of their risk of cancer, based on whether or not they were exposed to a certain substance or factor, or how they responded to two treatments being compared. A risk ratio (relative risk) of greater than one (RR>1) or of less than one (RR<1) usually means that being exposed to a certain substance or factor either increases (risk ratio greater than one) or decreases (risk ratio less than one) the risk of cancer, or that the treatments being compared do not have the same effects ¹⁷⁶.

Odds Ratio (OR) is used to compare the relative odds of the occurrence of the outcome of interest (e.g. cancer), given exposure to the variable of interest (e.g. dietary fiber). The odds ratio can also be used to determine whether a particular exposure is a risk factor for a particular outcome, and to compare the magnitude of various risk factors for that outcome.

• OR=1 Exposure does not affect odds of outcome
• OR>1 Exposure associated with higher odds of outcome
• OR<1 Exposure associated with lower odds of outcome

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 3) ¹⁶³, ¹⁶⁵, ¹⁶⁴. A recent review that included earlier meta-analyses reached a similar conclusion ¹⁷⁷.

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 ¹⁷⁸. Fiber can also reduce exposure of the gut mucosa to carcinogens by shortening transit time ¹⁷⁸. Secondly, fiber consumption influences the composition of the gut microbiota. Test tube studies have shown inulin to selectively stimulate the proliferation of beneficial bacteria while limiting the growth of potentially pathogenic bacteria ⁴⁷. Gut bacterial imbalance (dysbiosis) has been associated with the incidence of several conditions, including colorectal cancer ¹⁷⁹. The major health benefits conferred by the consumption of fiber are 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 ¹⁸⁰ and to display anti-inflammatory and anti-carcinogenic actions ¹⁸¹, ¹⁸².

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 ¹⁸³, ¹⁸⁴, ¹⁸⁵, ¹⁸⁶, ¹⁸⁷, psyllium ¹⁸⁸ and a high-fiber diet ¹⁸⁹ 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 ¹⁹⁰.

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 ¹⁷⁷. 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 3) ¹⁶¹, ¹⁶². Prolonged exposure to estrogens has been associated with an increased risk of breast cancer ¹⁹¹. 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 ¹⁹², ¹⁹³. 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 beta-glucoronidase activity ¹⁹⁴. 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 ¹⁹⁵. However, it is not known whether fiber-associated effects on endogenous estrogen concentrations have a clinically significant impact on breast cancer risk ¹³. 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 ¹⁹⁶.

Other cancers

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 ¹⁶⁷, ¹⁶⁸, stomach ¹⁶⁹, pancreas ¹⁹⁷, ¹⁷², and ovaries (see Table 3) ¹⁷⁰, ¹⁷¹. 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 3). 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 3). Additionally, there was no evidence of an association between fiber intake and risk of endometrial cancer (1 meta-analysis) ¹⁶⁶, prostate cancer (2 meta-analyses) ¹⁷³, ¹⁷⁴, or renal cell carcinoma (1 meta-analysis) ¹⁷⁵. At present, evidence from randomized controlled trials of a causal relationship between fiber intake and risk of any cancer is lacking.

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 ¹⁹⁸. 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 ¹⁹⁸.

Dietary fiber for weight loss

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) ¹⁹⁹. 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 ²⁰⁰. Fiber-enriched meals decrease the hunger hormone ghrelin and increase the satiety hormone peptide YY ²⁰¹. This finding suggests that fiber-rich meals might reduce appetite and increase satiety, although the data are inconsistent. 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.

Solah and colleagues ¹⁹⁹ performed a three-arm, parallel, blind, randomized controlled trial with either 4.5 g of PolyGlycopleX (PGX) as softgel, 5 g PGX as granules or 5 g of rice flour as a control. Compared with the rice flour control, in the PGX granules group, there was a significant reduction in waist circumference (−2.5 cm), body weight (−1.4 kg), Body Mass Index (BMI) (−0.5 kg/m²) and number of eating occasions and intake of grain food ¹⁹⁹. However, in other studies, the effects of fiber consumption on weight loss and changes in anthropometric markers were, at best, minor, or no significant and clinically relevant effects were observed ²⁰², ²⁰³.

A 2013 review of 44 randomized controlled trials found that beta-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 the feeling of fullness after a meal (satiety) ²⁰⁴. 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, beta-glucans appeared to enhance satiety, but pectin or psyllium did not. Similarly, among fermentable fibers, beta-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 ²⁰⁴.

To explore further the association of dietary fiber intake in the form of legume consumption (e.g., beans, lentils, and peas) on body weight and composition, Kim and colleagues reported on a systematic review and meta-analysis of 21 randomised controlled trials including 940 participants ²⁰⁵. Pooled data showed an overall significant reduction in body weight of −0.34 kg for diets containing dietary legumes versus diets without pulse intervention (median duration of 6 weeks). Data from six of the trials also suggested an association of dietary legume consumption with reduced body fat percentage ²⁰⁵. In a separate systematic review and meta-analysis of the literature on the effects of viscous fiber ingestion on body weight and composition from 62 trials and 3877 participants, there was a similar significant association with weight loss of −0.33 kg ²⁰⁶. In the ‘Preventing Overweight Using Novel Dietary Strategies’ (POUNDS Lost) study, in 345 participants who consumed a calorie-restricted diet (−750 kcal per day) for 6 months, the most successful predictor of reduction in body weight from a variety of anthropometric and dietary factors was intake of dietary fiber ²⁰⁷. Furthermore, dietary fiber was also strongly associated with adherence to the macronutrient prescriptions within the calorie-restricted diets ²⁰⁷.

A 2011 systematic review of 61 randomized controlled studies examined the effect of different fiber types on body weight ²⁰⁸. 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 ²⁰⁸.

Based on current published evidence, dietary fiber appears to associate only with small improvements in body weight, and evidence for changes in body composition (including fat mass) is less clear ²⁰. The association of dietary fiber intake with only small improvements in body weight suggests that dietary fiber per se, is not a realistic solution for effective weight reduction. More research needs to be conducted in order to clarify which types of fiber might play a role in appetite regulation and weight management ²⁰⁰. Future studies should also focus on the longer-term benefits of dietary fiber on weight loss and maintenance of body weight.

Psyllium

Psyllium seed is a soluble fiber supplement derived from the husk of Plantago psyllium. Psyllium is purported to promote weight loss by acting as a bulking agent in the gut, which results in delayed gastric emptying and increased satiety. A few randomized controlled trials in overweight or obese subjects suggested that psyllium supplementation may influence body composition and/or promote weight loss ²⁰⁹. A study in overweight and obese individuals noted a dose-response relationship with psyllium supplementation, with a dose greater than 30 g of fiber per day leading to the most robust weight loss effect compared to a placebo ²¹⁰. 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 ²¹¹. 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) ²¹¹.

Many human studies show no improvement in body weight and composition after psyllium consumption. One early study of psyllium supplementation showed improved glucose and lipid control compared to placebo in 125 overweight patients with type 2 diabetes, although no weight loss occurred ²¹². Recent studies have demonstrated similar results with improved metabolic parameters without statistically significant weight reduction. A more recent trial in patients with type 2 diabetes supported these positive findings, demonstrating that 10.5 g of psyllium supplementation daily for eight weeks was associated with a significant reduction in body weight, waist circumference, and hip circumference ²¹³.

Psyllium is less readily fermented and does not cause as much flatulence and abdominal distension as other fiber supplements. Still, the most commonly reported side effects are gastrointestinal, including flatulence, abdominal pain, diarrhea, constipation, and nausea. Generally, psyllium is well tolerated for short-term use without serious adverse effects ²¹⁴.

Beta-glucans

Beta-glucans are glucose polymers (glucose polysaccharides) with a mixture of beta-1,4 glycosidic bonds and beta-1,3 glycosidic bonds. Beta-glucans are soluble fibers found in bacteria, yeast, fungi, and grains such as oat bran and barley are particularly rich in beta-glucans ⁴¹, ⁴⁰. Weight loss effects may be due to increased satiety and decreased food intake ⁴¹. A meta-analysis published in 2019 showed that most trials reported non-significant or no weight loss from beta-glucans administered at a dose of three to ten grams per day for four to twelve weeks ²¹⁵. Data does not currently support the use of beta-glucans for the treatment of obesity. Adverse effects include increased flatulence ²¹⁶.

Glucomannan

Glucomannan is a soluble fiber commonly derived from the Amorphophallus konjac root. Glucomannan is a hemicellulose containing about 60% of mannose and 40% of glucose bonded together by beta-1,4 glycosidic linkages. Because human salivary and pancreatic amylase cannot split beta-1,4 glycosidic linkages, glucomannan passes relatively unchanged into the colon, where the gut microbiota ferment it ²¹⁷. While glucomannan may be an ingredient in weight-loss products, it is more commonly used to treat constipation or elevated glucose and cholesterol. Glucomannan reportedly promotes satiety, slows gastrointestinal transit, and reduces fat and protein absorption through fecal loss. In animal studies, glucomannan suppressed cholesterol synthesis in the liver and increased the fecal excretion of bile acids and cholesterol ²¹⁸. In a 2008 systematic review and meta-analysis, glucomannan significantly lowered total cholesterol, low-density lipoprotein (LDL or “bad”) cholesterol, triglycerides, body weight, and fasting blood glucose. This led to a statistically significant, although unlikely clinically significant weight loss of 0.79 kg ²¹⁹.However, 2 recent reviews showed no statistically significant change in body weight compared to a placebo, contradicting the earlier meta-analysis.

Glucomannan appears to be well tolerated for short-term use. Minor adverse gastrointestinal effects include belching, bloating, frequent loose stools, flatulence, constipation, and abdominal discomfort ²²⁰. There have been reports of esophageal obstruction following the consumption of glucomannan-containing compounds, specifically with the tablet formulation and in patients with esophageal pathology. The capsule form of glucomannan supplement has not been associated with this effect ²²¹.

Guar Gum

Guar Gum is a soluble fiber supplement derived from the Indian bean Cyamopsis tetragonolobus and is generally found in food products as a thickening agent. Guar gum is purported to promote weight loss by acting as a bulking agent in the gut, which results in delayed gastric emptying and increased satiety. Several studies have evaluated the effect of guar gum on weight reduction. A meta-analysis of 11 randomized, double-blind, placebo-controlled clinical trials of guar gum at dosages of nine to thirty grams daily for three weeks to six months found no significant difference in weight loss compared to placebo ²²². More recently, a clinical trial in 44 patients with type 2 diabetes evaluated the effects of ten grams per day of guar gum on metabolic syndrome parameters and found a significant reduction in waist circumference but no effect on weight loss ²²³.

Reported guar gum adverse effects include gastrointestinal complaints, such as abdominal pain, flatulence, diarrhea, nausea, and cramps. More research is needed to support the use of guar gum as a weight-loss supplement.

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 ²²⁴. Another recent pooled analysis of trials found chitosan to be more effective than placebo in promoting weight loss ²²⁵.

Irvingia Gabonensis

Irvingia gabonensis also known as African mango is a fruit native to Africa and commonly consumed in African cuisine ²²⁶. The plant has many advantageous properties, including high fiber content and anti-diabetic and anti-lipidemic effects. Several studies have examined different formulations of Irvingia gabonensis (African mango) on weight and found beneficial results. A 2013 systematic review of the efficacy of Irvingia gabonensis (African mango) for weight management reported significant reductions in body weight, waist circumference, and total cholesterol ²²⁷. An article published in 2018 studied the effect of 150 mg (milligrams) of Irvingia gabonensis (African mango) taken twice a day over 90 days and found improved and statistically significant differences in waist circumference, serum glucose, and triglycerides ²²⁸. While multiple studies have reported beneficial results on metabolic disease markers, small sample sizes and mixed methods make it difficult to extrapolate a true clinical benefit. Overall, Irvingia gabonensis (African mango) appears to be safe and well-tolerated. The most common side effects include headache, flatulence, and difficulty sleeping ²²⁹, ²³⁰. Further studies are needed to determine efficacy, dosing, and safety.

Chronic Inflammation

It has been hypothesized that low intake of dietary fiber is a risk factor for both local and systemic chronic inflammation ²³¹, ²³². The current belief suggests that limited dietary fiber intake hampers the establishment and maintenance of a healthy, viable and diverse colonic microbiota that, in turn, limits the local production of short-chain fatty acids (SCFAs), including butyrate. Signalling pathways that implicate nuclear factor kappa-B (NF-ĸB) and inhibition of deacetylase influence inflammatory processes both locally including gut-wall leakiness and colonic inflammation in patients with inflammatory bowel disease and systemically ²³³ and both are likely influenced by levels of butyrate within the colon ²³¹. Furthermore, butyrate may improve oxidative stress within the colon through effects on gene expression implicated in glutathione and uric acid metabolism ²³⁴.

In support of a role for dietary fiber in influencing inflammatory pathways, Miller and colleagues ²³⁵ reported a cross-sectional study on >140 overweight Hispanic and African-American adolescents. This study revealed that the highest consumption of dietary fiber intake compared with the lowest dietary fiber intake had significantly lower plasma markers of inflammatory status, including 36% and 43% lower levels of plasminogen activator inhibitor-1 (PAI-1) and resistin, respectively, with similar data for insoluble fiber ²³⁵. In a much larger study from the UK, Gibson and colleagues ²³⁶ reported on data from the Airwave Health Monitoring Study, a cross-sectional analysis on 6898 participants with 7-day food records. Data from this study revealed a significant inverse linear trend across fifths of total fiber intake and consumption of fiber from fruit with C-reactive protein (CRP, a plasma measure of general inflammatory status) and BMI, percentage body fat and waist circumference ²³⁶. In this study, given the association of dietary fiber intake with body fat percentage, it is possible that the favorable effects of dietary fiber on inflammatory status (indicated by plasma level of CRP) are actually mediated, at least in part, by changes in body composition rather than a direct effect of dietary fiber ²³⁶. In a further study, Kabisch and colleagues ²³⁷ demonstrated an interventional interaction effect of dietary fiber supplementation on inflammation. Interestingly, whilst the effect size for the anti-hyperglycaemic properties of insoluble dietary fiber seemed to depend mainly on the prevailing metabolic state, the anti-inflammatory effect of the particular supplement used in this study related primarily to the presence or absence of obesity ²³⁷.

Given the likely effects of dietary fiber on colonic microbiota diversity and production of short-chain fatty acids (SCFAs) and the known effects of butyrate in the mediation of inflammatory pathways, it is entirely plausible, and that dietary fiber does have at least some influence on inflammatory status both within the colon and systemically ²⁰. The mechanisms implicated should form a focus for future research. Given the multiple possible beneficial effects of dietary fiber and the complexity of the implicated mechanisms including involvement of the colonic microflora, identifying the actual mechanisms that mediate the anti-inflammatory effects of dietary fiber will likely be challenging and necessitate a variety of approaches, including prospectively designed Randomized Controlled Trials (RCTs) and rodent-based mechanistic studies ²⁰.

Depression

Large cohort studies have demonstrated that high diet quality and healthy dietary patterns are associated with reduced symptoms of depression ²³⁸, ²³⁹. More recently, adequate dietary fiber intake (often a hallmark of diet quality) has also emerged as an important factor in supporting mental well-being by lowering odds of developing depression ²⁴⁰. Although the underlying mechanisms remain incompletely understood, it has been hypothesised that inflammation may mediate the link between dietary fiber and depression, and that the association between a high-fiber diet and a reduction in inflammatory compounds may alter the concentrations of certain neurotransmitters that, in turn, could reduce the risk for the development of depression ²⁴¹. Consistent with a role for the gut microbiota in the mediation of fiber-effects on mental health, a meta-analysis of controlled clinical trials showed a small but significant effect of probiotics on depression and anxiety ²⁴². Furthermore, proof of the concept that a healthy diet improves depressive symptoms was provided in the SMILES trial ²⁴³, in which a modified Mediterranean diet with nutrition counselling sessions in adult patients with poor quality diets and major depressive disorders was shown to associate with improvements in depressive symptoms compared with the control group. Given the association of poor diet and obesity with depression and other mental health problems, it is important for future studies to provide insights into the mechanisms linking diet including dietary fiber with mental health ²⁰. Future guidance on the prevention and management of depression and other mental health disorders may also include a high-fiber diet as an important factor to consider ²⁰.

Good sources of dietary fiber

Whole grains and beans tend to be higher in fiber than fruits and vegetables, but these foods are good natural sources of dietary fiber and contribute other important nutrients. Make sure you include a variety of these foods regularly to meet your dietary fiber needs. Eating the skin or peel of fruits and vegetables provides a greater dose of fiber. Fiber is also found in beans and lentils, whole grains, nuts and seeds. Typically, the more refined or processed a food is, the lower its fiber content. For example, one medium apple with the peel contains 4.4 grams of fiber, while ½ cup of applesauce contains 1.4 grams, and 4 ounces of apple juice contains no fiber. By including certain foods, you can increase your fiber intake.

Here are a few foods that are naturally high in fiber:

• Beans, peas, and lentils
• Fruits
• Nuts
• Seeds
• Vegetables
• Wheat bran
• Whole grains (such as whole oats, brown rice, popcorn, and quinoa) and foods made with whole grain ingredients (such as some breads, cereals, crackers, and pasta).

Table 4. High fiber foods

FOOD bc	PORTION d	CALORIES	FIBER (g)
GRAINS
Ready-to-eat cereal, high fiber, unsweetened (e.g., bran)	1/2 cup	62	14
Ready-to-eat cereal, whole grain kernels	1/2 cup	209	7.5
Ready-to-eat cereal, wheat, shredded	1 cup	172	6.2
Popcorn	3 cups	169	5.8
Ready-to-eat cereal, bran flakes	3/4 cup	98	5.5
Bulgur, cooked	1/2 cup	76	4.1
Spelt, cooked	1/2 cup	123	3.8
Teff, cooked	1/2 cup	128	3.6
Barley, pearled, cooked	1/2 cup	97	3
Ready-to-eat cereal, toasted oat	1 cup	111	3
Oat bran	1/2 cup	44	2.9
Crackers, whole wheat	1 ounce	122	2.9
Chapati or roti, whole wheat	1 ounce	85	2.8
Tortillas, whole wheat	1 ounce	88	2.8
VEGETABLES
Lima beans (white), cooked*	1 cup	216	13.2
Artichoke, cooked	1 cup	89	9.6
Navy beans, cooked*	1/2 cup	128	9.6
Small white beans, cooked*	1/2 cup	127	9.3
Yellow beans, cooked*	1/2 cup	128	9.2
Green peas, cooked	1 cup	134	8.8
Adzuki beans, cooked*	1/2 cup	147	8.4
French beans, cooked*	1/2 cup	114	8.3
Split peas, cooked*	1/2 cup	116	8.2
Breadfruit, cooked	1 cup	170	8
Lentils, cooked*	1/2 cup	115	7.8
Lupini beans, cooked*	1/2 cup	115	7.8
Mung beans, cooked*	1/2 cup	106	7.7
Black turtle beans, cooked*	1/2 cup	120	7.7
Pinto beans, cooked*	1/2 cup	123	7.7
Cranberry (roman) beans, cooked*	1/2 cup	121	7.6
Black beans, cooked*	1/2 cup	114	7.5
Fufu, cooked	1 cup	398	7.4
Pumpkin, canned	1 cup	83	7.1
Taro root (dasheen or yautia), cooked	1 cup	187	6.7
Brussels sprouts, cooked	1 cup	65	6.4
Chickpeas (garbanzo beans), cooked*	1/2 cup	135	6.3
Sweet potato, cooked	1 cup	190	6.3
Great northern beans, cooked*	1/2 cup	105	6.2
Parsnips, cooked	1 cup	110	6.2
Nettles, cooked	1 cup	37	6.1
Jicama, raw	1 cup	46	5.9
Winter squash, cooked	1 cup	76	5.7
Pigeon peas, cooked*	1/2 cup	102	5.7
Kidney beans, cooked*	1/2 cup	113	5.7
White beans, cooked*	1/2 cup	125	5.7
Black-eyed peas, dried and cooked*	1/2 cup	99	5.6
Cowpeas, dried and cooked*	1/2 cup	99	5.6
Yam, cooked	1 cup	158	5.3
Broccoli, cooked	1 cup	54	5.2
Tree fern, cooked	1 cup	56	5.2
Luffa gourd, cooked	1 cup	100	5.2
Soybeans, cooked*	1/2 cup	148	5.2
Turnip greens, cooked	1 cup	29	5
Drumstick pods (moringa), cooked	1 cup	42	5
Avocado	1/2 cup	120	5
Cauliflower, cooked	1 cup	34	4.9
Kohlrabi, raw	1 cup	36	4.9
Carrots, cooked	1 cup	54	4.8
Collard greens, cooked	1 cup	63	4.8
Kale, cooked	1 cup	43	4.7
Fava beans, cooked*	1/2 cup	94	4.6
Chayote (mirliton), cooked	1 cup	38	4.5
Snow peas, cooked	1 cup	67	4.5
Pink beans, cooked*	1/2 cup	126	4.5
Spinach, cooked	1 cup	41	4.3
Escarole, cooked	1 cup	22	4.2
Beet greens, cooked	1 cup	39	4.2
Salsify, cooked	1 cup	92	4.2
Cabbage, savoy, cooked	1 cup	35	4.1
Cabbage, red, cooked	1 cup	41	4.1
Wax beans, snap, cooked	1 cup	44	4.1
Edamame, cooked*	1/2 cup	94	4.1
Okra, cooked	1 cup	36	4
Green beans, snap, cooked	1 cup	44	4
Hominy, canned	1 cup	115	4
Corn, cooked	1 cup	134	4
Potato, baked, with skin	1 medium	161	3.9
Lambsquarters, cooked	1 cup	58	3.8
Lotus root, cooked	1 cup	108	3.8
Swiss chard, cooked	1 cup	35	3.7
Mustard spinach, cooked	1 cup	29	3.6
Carrots, raw	1 cup	52	3.6
Hearts of palm, canned	1 cup	41	3.5
Mushrooms, cooked	1 cup	44	3.4
Bamboo shoots, raw	1 cup	41	3.3
Yardlong beans, cooked*	1/2 cup	101	3.3
Turnip, cooked	1 cup	34	3.1
Red bell pepper, raw	1 cup	39	3.1
Rutabaga, cooked	1 cup	51	3.1
Plantains, cooked	1 cup	215	3.1
Nopales, cooked	1 cup	22	3
Dandelion greens, cooked	1 cup	35	3
Cassava (yuca), cooked	1 cup	267	3
Asparagus, cooked	1 cup	32	2.9
Taro leaves, cooked	1 cup	35	2.9
Onions, cooked	1 cup	92	2.9
Cabbage, cooked	1 cup	34	2.8
Mustard greens, cooked	1 cup	36	2.8
Beets, cooked	1 cup	49	2.8
Celeriac, raw	1 cup	66	2.8
FRUITS
Sapote or Sapodilla	1 cup	217	9.5
Guava	1 cup	112	8.9
Nance	1 cup	82	8.4
Raspberries	1 cup	64	8
Loganberries	1 cup	81	7.8
Blackberries	1 cup	62	7.6
Soursop	1 cup	148	7.4
Boysenberries	1 cup	66	7
Gooseberries	1 cup	66	6.5
Pear, Asian	1 medium	75	6.5
Blueberries, wild	1 cup	80	6.2
Passion fruit	1/4 cup	57	6.1
Persimmon	1 fruit	118	6
Pear	1 medium	103	5.5
Kiwifruit	1 cup	110	5.4
Grapefruit	1 fruit	130	5
Apple, with skin	1 medium	104	4.8
Cherimoya	1 cup	120	4.8
Durian	1/2 cup	179	4.6
Starfruit	1 cup	41	3.7
Orange	1 medium	73	3.7
Figs, dried	1/4 cup	93	3.7
Blueberries	1 cup	84	3.6
Pomegranate seeds	1/2 cup	72	3.5
Mandarin orange	1 cup	103	3.5
Tangerine (tangelo)	1 cup	103	3.5
Pears, dried	1/4 cup	118	3.4
Peaches, dried	1/4 cup	96	3.3
Banana	1 medium	112	3.2
Apricots	1 cup	74	3.1
Prunes or dried plum	1/4 cup	105	3.1
Strawberries	1 cup	49	3
Dates	1/4 cup	104	3
Blueberries, dried	1/4 cup	127	3
Cherries	1 cup	87	2.9
PROTEIN FOODS
Wocas, yellow pond lily seeds	1 ounce	102	5.4
Pumpkin seeds, whole	1 ounce	126	5.2
Chia seeds	1 Tbsp	58	4.1
Almonds	1 ounce	164	3.5
Chestnuts	1 ounce	106	3.3
Sunflower seeds	1 ounce	165	3.1
Pine nuts	1 ounce	178	3
Pistachio nuts	1 ounce	162	2.9
Flax seeds	1 Tbsp	55	2.8
Hazelnuts (filberts)	1 ounce	178	2.8
Other Sources
Coconut	1 ounce	187	4.6

Footnotes:

^* Beans, peas, and lentils are listed under Vegetables but can also be counted in the Protein Foods group.

^a All foods listed are assumed to be in nutrient-dense forms; lean or low-fat and prepared with minimal or no added sugars, saturated fat, and sodium.

^b Some fortified foods and beverages are included. Other fortified options may exist on the market, but not all fortified foods are nutrient-dense. For example, some foods with added sugars may be fortified and would not be examples in the lists provided here.

^c Some foods or beverages are not appropriate for all ages, (e.g., nuts, popcorn), particularly young children for whom some foods could be a choking hazard

^d This list includes “Standard” portions, which provide at least 2.8 g of dietary fiber. Portions listed are not necessarily recommended serving sizes.

[Source ²⁴⁴ ]

Fiber Intake Recommendations

The Adequate Intake 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 (coronary artery disease). Total fiber is defined by the Institute of Medicine as “the sum of dietary fiber and functional fiber” ². For adults who are 50 years of age and younger, the Adequate Intake 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 Adequate Intake recommendations for males and females of all ages are presented in Table 5 below.

Table 5. Fiber Intake Recommendations (Adequate Intake 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

Footnote: *ND = Not determined

[Source ² ]

Fiber supplements

Fiber supplements are man-made (synthetic) or isolated fiber and do not provide many of the health benefits associated with fiber-rich whole foods (eg, fruits, vegetables, legumes, whole grains) ¹.

There are 4 main characteristics of fiber supplements that drive their health benefits ¹:

• Solubility. Solubility defines whether a fiber supplement will dissolve in water (soluble) or remain as discreet insoluble particles ¹⁴, ²⁴⁵. A fiber that is 70% soluble will be considered a soluble fiber
• Degree or rate of fermentation. Fermentation refers to the rate and degree to which a dietary fiber, after resisting enzymatic digestion in the small bowel, can be degraded by gut bacteria, producing fermentation byproducts such as short chain fatty acids and gas ¹⁴
• Viscosity. For soluble fibers, viscosity refers to the ability of some fibers to “thicken” when water is added, in a concentration-dependent manner ¹⁴, ²⁴⁶, ²⁴⁷, ²⁴⁵.
• Gel formation. Gel formation refers to the ability of a subset of soluble viscous fibers to form cross-links, resulting in a viscoelastic gel when hydrated ¹⁴, ²⁴⁵.

Using the 4 fiber characteristics described above, fiber supplements can be divided into 4 clinically meaningful categories ¹:

• Insoluble, poorly fermented (e.g, wheat bran): when you think of “insoluble fiber”, think of plastic (clinical studies described later actually used plastic particles to mimic effects of wheat bran): does not dissolve in water (no water-holding capacity); poorly fermented; can exert a laxative effect by mechanical irritation/stimulation of gut mucosa if particles are sufficiently large and coarse (“plastic effect”); small smooth particles (eg, wheat bran flour/bread) have no significant laxative effect; insoluble fiber does not gel or alter viscosity and thus does not provide other (gel-dependent) fiber health benefits.
• Soluble, nonviscous, readily fermented (e.g, inulin, wheat dextrin, oligosaccharides, resistant starches): dissolves in water; no increase in viscosity; rapidly and completely fermented (once fermented, the fiber is no longer present in stool, rapid gas formation, increased flatulence, energy harvest [calorie uptake] from fermentation by-products); may alter the numbers of specific bacteria in the gut (eg, “prebiotic” effect); no laxative effect at physiologic doses; does not gel or alter viscosity and thus does not provide any of the gel-dependent fiber health benefits. Readily fermented fibers are part of an emerging area of science relating to their effects on the gut microbiome, but to date, the marketed fiber supplements have no established clinically meaningful health benefits.
• Soluble viscous or gel forming, readily fermented (e.g, beta-glucan [oats, barley], raw guar gum): dissolves in water, forms a viscous gel (eg, oatmeal), increases chyme viscosity to slow nutrient absorption and improve glycemic control, lowers elevated serum cholesterol, readily fermented (gas formation, energy harvest [calorie uptake] from fermentation by-products), fermentation results in loss of gel and water-holding capacity, and thus, no significant laxative effect and no retained gel to attenuate diarrhea.
• Soluble viscous or gel forming, nonfermented (i.e, psyllium): dissolves in water; forms a viscous gel; increases chyme viscosity to slow nutrient absorption and improve glycemic control, lowers elevated serum cholesterol; not fermented (no gas production, no calorie harvest from fermentation by-products); because it is not fermented, it remains gelled throughout the large bowel, providing a dichotomous “stool-normalizing” effect: softens hard stool in constipation (relieves/prevents constipation) and firms/forms loose/liquid stool in diarrhea (relieves/prevents diarrhea), and normalizes stool form in irritable bowel syndrome (IBS).

Beta-glucans

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

Glucomannan

Glucomannan also 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 ⁴². 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 ²⁴⁸.

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 ⁴².

Inulins and oligofructose

Inulins and oligofructose, extracted from chicory root or synthesized from sucrose, are used as food additives ²⁴⁹. 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 ²⁵⁰. 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 ²⁵¹. 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 ²⁵², ¹.

Guar gum

Raw guar gum is a viscous, fermentable fiber derived from the Indian guar or cluster bean ². Guar gum 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 ²²². 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 blood sugar control). Use of a guar gum-containing supplement for weight loss has been associated with esophageal and small bowel obstruction ²⁵³.

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 ⁴². Psyllium is proven to be efficacious to lower serum cholesterol and improve blood sugar control. 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 (IBS). Several cases of intestinal obstruction by psyllium have been reported when taken with insufficient fluids or by people with impaired swallowing or gastrointestinal motility ²⁵⁴, ²⁵⁵. 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 ²⁵⁶. 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 ¹⁸⁸.

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 ²²⁴. Another recent pooled analysis of trials found chitosan to be more effective than placebo in promoting weight loss ²²⁵.

Dietary fiber side effects

Eating a large amount of fiber in a short period of time can cause intestinal gas (flatulence), bloating, diarrhea and abdominal cramps. This problem often goes away once the natural bacteria in your digestive system get used to the increase in fiber. 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 ²⁵⁷. These symptoms can be minimized or avoided by increasing intake of fiber-rich foods gradually and increasing fluid intake to at least ~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 ²⁵⁸, ²⁵⁹, ²⁶⁰, ²⁶¹. Too much fiber may interfere with the absorption of minerals such as iron, zinc, magnesium, and calcium. In most cases, this is not a cause for too much concern because high-fiber foods tend to be rich in minerals. The National Academy of Medicine (the Institute of Medicine) has not established a tolerable upper intake level (UL) for dietary or functional fiber ².

Gastrointestinal symptoms

The following dietary 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 ². 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 ²⁵⁷. In subjects who are constipated, the initiation of a fiber supplement should start once the hard stool is cleared ²⁵⁷. Use of a guar gum-containing supplement for weight loss has been associated with esophageal and small bowel obstruction ²⁵³. 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 ²⁵⁴, ²⁵⁵.

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 ¹⁸⁸.

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 ⁴². Anaphylaxis has been reported after intravenous (IV) administration of inulin ²⁶², as well as ingestion of margarine containing inulin extracted from chicory ²⁶³. 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 ²⁵⁶.

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 ²⁶⁴. 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 ². Evidence from animal studies and limited research in humans suggests that inulin and oligofructose may enhance calcium absorption ²⁶⁵, ²⁶⁶. 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 ²⁶⁷, ²⁶⁸.

Drug interactions

Gel-forming fibers (e.g., beta-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 ⁴². 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 ²⁶⁹. Pectin may decrease the absorption of lovastatin (Mevacor) when taken at the same time ²⁷⁰. 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 ⁴². 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).

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