Dietary fiber (also spelled fibre), or roughage, is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition and can be grouped generally by their solubility, viscosity and fermentability, which affect how fibers are processed in the body. Dietary fiber has two main subtypes: soluble and insoluble. They are components of many foods from plants, such as legumes, whole grains, cereals, vegetables, fruits, nuts, and seeds. Regularly consuming substantial dietary fiber is generally associated with supporting health and lowering the risks of several diseases. Dietary fiber consists of non-starch polysaccharides and other plant components, such as cellulose, resistant starch, resistant dextrins, inulins, lignins, chitins, pectins, beta-glucans, and oligosaccharides.
Food sources of dietary fiber have traditionally been divided according to proportion of soluble vs. insoluble fiber. The two types are defined according to their characteristics of viscosity and fermentability, with different advantages between the types. Plant foods contain both types in varying amounts. Bulking fibers, such as cellulose and hemicellulose (including psyllium), absorb and hold water, promoting bowel movement regularity. Viscous fibers, such as beta-glucan and psyllium, thicken the fecal mass. Fermentable fibers, such as resistant starch, xanthan gum, and inulin, feed the bacteria and microbiota of the large intestine and are metabolized to yield short-chain fatty acids, which have diverse roles in gastrointestinal health.
Soluble fiber (fermentable fiber, or prebiotic fiber)which dissolves in wateris generally fermented in the colon into gases and physiologically active by-products, such as short-chain fatty acids, produced in the colon by gut bacteria. Examples are beta glucans (in oats, barley, and mushrooms) and raw guar gum. Psylliumsoluble, viscous, and non-fermented fiberis a bulking fiber, retaining water as it moves through the digestive system, easing defecation. Soluble fiber is generally viscous and delays gastric emptying, which in humans can result in an extended feeling of fullness. Inulin (in chicory root), wheat dextrin, oligosaccharides, and resistant starches (in legumes and bananas) are soluble, non-viscous fibers. Regular intake of soluble fibers, such as beta-glucans, from oats or barley, have been shown to lower blood levels of LDL cholesterol. Soluble fiber supplements also significantly lower LDL cholesterol.
Insoluble fiber does not dissolve in water and is inert, unaffected by digestive enzymes in the upper gastrointestinal tract. Examples include wheat bran, cellulose, and lignin. Coarsely ground insoluble fiber triggers the secretion of mucus in the large intestine, providing bulking. However, finely ground insoluble fiber does not have this effect and can instead cause constipation. Some forms of insoluble fiber, such as resistant starches, can be fermented in the colon.
Definition Dietary fiber is defined to be plant components that are not broken down by human digestive enzymes. In the late 20th century, only lignin and some polysaccharides were known to satisfy this definition, but in the early 21st century, resistant starch and oligosaccharides were included as dietary fiber components. The most accepted definition of dietary fiber is "all polysaccharides and lignin, which are not digested by the endogenous secretion of the human digestive tract". Currently, most animal nutritionists are using either a physiological definition, "the dietary components resistant to degradation by mammalian enzymes", or a chemical definition, "the sum of non-starch polysaccharides (NSP) and lignin".
Types and sources
+ Water-insoluble dietary fibers Nutrient E number Source, comments β-glucans (a few of which are water-soluble) Cellulose E 460 Cereals, fruit, vegetables (in all plants in general) Chitin In fungi, exoskeleton of insects and crustaceansHemicellulose Cereals, bran, timber, legumes Hexoses Wheat, barley Pentose Rye, oatLignin Stones of fruits, vegetables (filaments of the garden bean), cerealsXanthan gum E 415 Production with Xanthomonas-bacteria from sugar substratesResistant starch Can be starch protected by seed or shell (type RS1), granular starch (type RS2) or retrograded starch (type RS3) Resistant starch High-amylose corn, barley, high-amylose wheat, legumes, raw bananas, cooked and cooled pasta and potatoes
+ water-soluble dietary fibers Nutrient E number Source/CommentsArabinoxylan (a hemicellulose) PsylliumFructans Replace or complement in some plant taxa the starch as storage carbohydrate Inulin In diverse plants, e.g. topinambour, chicory, etc.Polyuronide Pectin E 440 In fruit skins (mainly of apples, quinces), vegetables Alginic acids (Alginates) E 400E 407 In algae Sodium alginate E 401 Potassium alginate E 402 Ammonium alginate E 403 Calcium alginate E 404 Propylene glycol alginate (PGA) E 405 agar E 406 carrageenan E 407 Red algaeRaffinose LegumesPolydextrose E 1200 Synthetic polymer, c.
Contents in food
Dietary fiber is found in fruits, vegetables and whole grains. The amounts of fiber contained in common foods are listed in the following table:
+ Food group Serving mean Fiber mass per serving Fruit 120 mL (cup)U.S. Government Printing Office—Electronic Code of Federal Regulations U.S. Food and Drug Administration—Guidelines for Determining Metric Equivalents of Household Measures 1.1 g Dark green vegetables 120mL (cup) Orange vegetables 120 mL (cup) Cooked dry beans (legumes) 120 mL (cup) Starchy vegetables 120 mL (cup) Other vegetables 120 mL (cup) Whole grains Meat
Dietary fiber is found in plants, typically eaten whole, raw or cooked, although fiber can be added to make dietary supplements and fiber-rich processed foods. Grain bran products have the highest fiber contents, such as crude corn bran (79g per 100g) and crude wheat bran (43g per 100g), which are ingredients for manufactured foods. Medical authorities, such as the Mayo Clinic, recommend adding fiber-rich products to the Standard American Diet because fiber intake is lower on such a diet that's rich in processed and artificially sweetened foods with minimal intake of vegetables and legumes.
Plant sources Some plants contain significant amounts of soluble and insoluble fiber. For example, plums and prunes have a thick skin covering a juicy pulp. The skin is a source of insoluble fiber, whereas soluble fiber is in the pulp. Grapes also contain a fair amount of fiber.
Soluble fiber Found in varying quantities in all plant foods, including: Legumes (peas, soybeans, lupins and other beans) Oats, rye, chia, and barley Some fruits (including figs, avocados, plums, prunes, berries, ripe bananas, and the skin of apples, quinces and pears) Certain vegetables such as broccoli, carrots, and Jerusalem artichokes Root tubers and root vegetables such as sweet potatoes and onions (skins of these are sources of insoluble fiber also) Psyllium seed husks (a mucilage soluble fiber) and flax seeds nuts, with almonds being the highest in dietary fiber
Insoluble fiber Sources include:
Whole grain foods Wheat and corn bran Legumes such as beans and peas Nuts and seeds Potato skins Lignans Vegetables such as green beans, cauliflower, zucchini (courgette), celery, and nopal Some fruits including avocado, and unripe bananas The skins of some fruits, including kiwifruit, grapes and tomatoes
Supplements These are a few example forms of fiber that have been sold as supplements or food additives. These may be marketed to consumers for nutritional purposes, treatment of various gastrointestinal disorders, and for such possible health benefits as lowering cholesterol levels, reducing the risk of colon cancer, and losing weight.
Soluble fiber Soluble fiber supplements may be beneficial for alleviating symptoms of irritable bowel syndrome, such as diarrhea or constipation and abdominal discomfort. Prebiotic soluble fiber products, like those containing inulin or oligosaccharides, may contribute to relief from inflammatory bowel disease, as in Crohn's disease, ulcerative colitis, and Clostridioides difficile, due in part to the short-chain fatty acids produced with subsequent anti-inflammatory actions upon the bowel. Fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.
Insoluble fiber One insoluble fiber, resistant starch from high-amylose corn, has been used as a supplement and may contribute to improving insulin sensitivity and glycemic management as well as promoting regularity and possibly relief of diarrhea. One preliminary finding indicates that resistant corn starch may reduce symptoms of ulcerative colitis.
Inulins
Chemically defined as oligosaccharides and occurring naturally in most plants, inulins have nutritional value as carbohydrates, or more specifically as fructans, a polymer of the natural plant sugar, fructose. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory roots or Jerusalem artichokes for use in prepared foods. Subtly sweet, it can be used to replace sugar, fat, and flour, is often used to improve the flow and mixing qualities of powdered nutritional supplements, and has potential health value as a prebiotic fermentable fiber.
As a prebiotic fermentable fiber, inulin is metabolized by gut flora to yield short-chain fatty acids (see below), which increase absorption of calcium, magnesium, and iron.
The primary disadvantage of inulin is its fermentation within the intestinal tract, possibly causing flatulence and digestive distress at doses higher than 15g/day in most people. Individuals with digestive diseases have benefited from removing fructose and inulin from their diet. While clinical studies have shown changes in the microbiota at lower levels of inulin intake, higher intake amounts may be needed to achieve effects on body weight.
Vegetable gums Vegetable gum fiber supplements are relatively new to the market. Often sold as a powder, vegetable gum fibers dissolve easily with no aftertaste. In preliminary clinical trials, they have proven effective for the treatment of irritable bowel syndrome. Examples of vegetable gum fibers are guar gum and gum arabic.
Activity in the gut
Many molecules that are considered to be "dietary fiber" are so because humans lack the necessary enzymes to split the glycosidic bond and they reach the large intestine. Many foods contain varying types of dietary fibers, all of which contribute to health in different ways.
Dietary fibers make three primary contributions: bulking, viscosity and fermentation. Different fibers have different effects, suggesting that a variety of dietary fibers contribute to overall health. Some fibers contribute through one primary mechanism. For instance, cellulose and wheat bran provide excellent bulking effects, but are minimally fermented. Alternatively, many dietary fibers can contribute to health through more than one of these mechanisms. For instance, psyllium provides bulking as well as viscosity.
Bulking fibers can be soluble (e.g. psyllium) or insoluble (e.g. cellulose and hemicellulose). They absorb water and can significantly increase stool weight and regularity. Most bulking fibers are not fermented or are minimally fermented throughout the intestinal tract.