Phytochemicals
Dietary fiber
Reviewed August 2026 · How we write these articles
Dietary fiber is the component in food not broken down by native enzymes and secretions of the gastrointestinal tract but which may be metabolized by the bacteria in the lower gut. Dietary fibers are long-chain carbohydrates (polysaccharides) that are indigestible by the human digestive tract. Dietary fiber comes from the thick cell wall of plants. It is an indigestible complex carbohydrate. Dietary fiber consists of both soluble and insoluble fiber. Soluble fiber dissolves in water while insoluble fiber does not dissolve in water. Both types are important to health in different ways. Soluble fiber includes gums, mucilages, pectin and some hemicelluloses. Cellulose, lignin, and the rest of the hemicelluloses, are all insoluble fibers.
Health benefits of high fiber diet
Dietary fibre supports bowel regularity, slows the absorption of glucose, and contributes to satiety. The main value of dietary fiber is that it provides bulk to the bolus moving through the digestive tract. There are two great advantages to this: by bulking up the bolus, eventually increasing the weight of the stool, it's easier for the digestive system to move it through, and the bulkier stool also tends to retain normal amounts of moisture to make it easier to eliminate with less straining and abrasion.
Soluble fiber has been shown to be able to bind bile salts which may reduce blood cholesterol levels. Low blood cholesterol levels have been associated with a reduced risk of coronary heart disease. Soluble fiber attracts water and forms a gel in the digestive tract. This slows digestion and lowers the rate of nutrient absorption from the stomach and intestine. Soluble fibers can also lower blood sugar levels in people with diabetes. Intake of soluble fiber may also improve glucose tolerance in people with diabetes. Fibers that lower blood cholesterol levels include foods such as apples, barley, beans and other legumes, fruits and vegetables, oatmeal, oat bran and rice hulls.
Insoluble fiber acts as a natural laxative that speeds the passage of foods through the stomach. It also gives stool its bulk and helps it move quickly through the gastrointestinal tract. For this reason, insoluble fiber is partially effective as a treatment for constipation. Because the bowel regulation is mostly due to bulking and not to increased water in the stool, it is very unlikely to cause diarrhea unless if taken in massive amounts.
Dietary fiber provides a feeling of fullness and adds bulk in the diet. Fiber fills the stomach, reducing appetite. In theory, fiber should therefore reduce eating, leading to weight loss. This also assists digestion and elimination. A high-fiber diet may help prevent type 2 diabetes, lower insulin and blood sugar levels, and improve cholesterol and triglyceride levels in people with diabetes.
Fiber intake has also been linked with the metabolic syndrome, a constellation of factors that increases the chances of developing heart disease and diabetes. Fiber also appears to interact with intestinal bacteria and bile acids to process certain food substances. Dietary fiber also has been effectively used for the treatment of diverticulosis. Diverticula are outpouchings that develop on weak areas in the bowel wall.
Sources of dietary fiber
Soluble fiber is found in some fruits (particularly oranges, also apples and bananas), oats, legumes, (peas, soybeans, and other beans), other vegetables, such as broccoli and carrots, and psyllium seed. Legumes also typically contain shorter-chain carbohydrates that are indigestible by the human digestive tract but which are digested by bacteria in the small intestine, which is a cause of flatulence. Whole grains, wheat and corn fiber, and many vegetables like cauliflower, green beans, and potatoes are good sources of insoluble fiber.
The skins of fruits and vegetables are also good sources of insoluble fiber. Because of high water content, fruits and vegetables provide less dietary fiber than the drier grains and cereals. Fruits are generally high in pectins while vegetables contain a high percentage of cellulose. Grains, especially wheat bran, are high in hemicellulose.
Over-the-counter, natural fiber supplements can be used, when inadequate fiber is taken in from the diet. There are many types of soluble fiber supplements: psyllium, methylcellulose, pectins, vegetable gums and polycarbophil. Psyllium is digested and does contain calories, may reduce LDL, absorbs water, and may cause gas. Methylcellulose is created from the cell wall of plants. Methylcellulose is undigestible, unfermentable and doesn't cause gas or have calories that humans can use. Both of these can be used for IBS, constipation, diverticulitis. Polycarbophil is also plant based and is similar to methylcellulose. It usually causes even less bloating.
Dosage, intake, recommended daily allowance (RDA)
The American Dietetic Association (ADA) recommends 20-35g/day for a healthy adult depending on calorie intake (eg. 2000 calorie a day diet should include 25g of fiber/day). For children over age 2, the recommended intake is the child's age + 5 grams. Both types of fiber are crucial for regular large bowel function as it increases stool weight, and improves transit time, as well as decreases absorption of other carbohydrates and fats.
Side effects, overdosage symptoms, interactions
A large increase in fiber over a short period of time may result in bloating, diarrhea, gas and general discomfort. An overdose of soluble fiber can cause diarrhea and worsen irritable bowel syndrome. Negative effects of dietary fiber include a reduced absorption of vitamins, minerals, proteins, and calories from the gut. Some insoluble fibers can bind to certain minerals: calcium, magnesium, phosphorus, and iron. Bran, an insoluble fiber, reduces the absorption of calcium enough to cause urinary calcium to fall. Taking fiber supplements without adequate liquids may cause it to swell and, in extreme cases, cause choking. People with esophageal stricture (narrowing of the esophagus) or any other narrowing or obstruction of the gastrointestinal tract should not take fiber supplements.
Inulin is a polysaccharide that belongs to a group of naturally-occurring carbohydrates containing non-digestible fructooligosaccharides (FOS). Inulin is a group of polymers made from a single glucose molecule and several fructose molecules. Inulin is formed of 30 fructose units joined together. Inulins are mainly comprised of fructose units and typically have a terminal glucose. The fructose units in inulins are joined by a beta-(2-1) glycosidic link.
Plant inulins generally contain between 2 to 140 fructose units. Inulins refer to a group of naturally occurring fructose-containing oligosaccharides. They belong to a class of carbohydrates known as fructans. Inulin is not chemically related to insulin; the similarities in name do not relate to any similarity in form or function. Inulin can be transformed into various important food ingredients by hydrolysis and other types of processing.
Inulin is indigestible by human enzymes ptyalin and amylase, which are designed to digest starch. As a result, inulin passes through much of the digestive system intact. Inulin is a highly effective prebiotic, stimulating the growth of beneficial probiotic bacteria in the gut. Inulin is used in low fat products because of its ability to give a creamy smooth texture to products. Inulin is a dietary fibre and is believed to activate beneficial good bacteria in the digestive tract. Inulin has a mildly sweet taste, but does not affect blood sugar levels and is recommended for diabetics.
Inulin has been clinically proven to increase calcium absorption. The inherent calcium in dairy foods is now an even better source of this bone-building mineral when inulin is added because inulin improves the body's uptake. Inulin is also used for diagnosis of kidney functions. People have used plants containing inulin to help relieve diabetes mellitus, a condition characterised by hyperglycemia and/or hyperinsulinemia. Inulin is injected into the bloodstream and, after an appropriate time delay, its concentration is checked for in the urine and bloodstream.
Inulin is found naturally in more than 36,000 types of plants worldwide, including dahlias, asparagus, bananas, Jerusalem artichokes, salsify, wheat, chicory, onions, and garlic. It is estimated that approximately one-third of the earth's vegetation contains this substance. Inulin is a naturally occurring fructo-oligosaccharide composed of a mixture of oligomers of varying degrees of polymerization ("DP") or molecular weights that occurs naturally plants such as onion, garlic, Jerusalem artichoke, dahlia and chicory for plant energy storage. The inulin produced from different plants, at different stages in the growing cycle of a plant, or under different climatic conditions, will normally have different average degrees of polymerization.
Pectin is a heterosaccharide derived from the cell wall of plants. Pectins are variable in their chain lengths, complexity, and the order of each of the monosaccharide units. Pectin is the methylated ester of polygalacturonic acid. It is commercially extracted from citrus peels, apple pomace and sugar beet pulp. A typical pectin molecule comprises 200 to 1000 galacturonic acid units connected in a linear chain. Pectins are heterogeneous materials, with a polysaccharide backbone that is uniform as.alpha.-1,4-linked polygalacturonic acid. Various neutral sugars have been identified in pectins such as xylose, galactose, rhamnose, arabinose.
Pectin occurs as a coarse or fine powder, yellowish-white in color, practically odorless, and with a mucilaginous taste. It is almost completely soluble in 20 parts water, forming a viscous solution containing negatively charged, very much hydrated particles. It is acid to litmus and insoluble in alcohol or in diluted alcohol, and in other organic solvents. It dissolves more readily in water, if first moistened with alcohol, glycerol or sugar syrup, or if first mixed with 3 or more parts of sucrose.
It is stable under mildly acidic conditions; more strongly acidic or basic conditions cause depolymerization. A critical property of pectins, which is known to affect their gelation properties, is the extent to which the galacturonic acid units are esterified. The degree of esterification (DE) of pectins found naturally can vary considerably (from 60 to 90%).
Pectin is divided into two main categories: high methoxylated pectin HM pectin), which are characterized by a degree of methoxylation above 50% and low methoxylated pectin (LM pectin) having a degree of methoxylation below 50%. Degree of methoxylation (also referred to as DE or degree of esterification) is intended to mean the extent to which free carboxylic acid groups contained in the polygalacturonic acid chain have been esterified (e.g. by methylation). The LM pectins are further subdivided into two groups: low methoxylated amidated, and low methoxylated conventional.
Low DE pectins are usually prepared by the de-esterification of extracted pectins, normally on a bench scale, by way of an enzymatic process, or, on an industrial scale, by the treatment with acid or ammonia in an alcoholic heterogeneous medium. For pectins with a low degree of methoxylation the gelation properties are known to depend on the DM and the molecular weight of the pectin.
Pectin is an important ingredient in making jams and jellies. Pectin is a general intestinal regulator that is used in many medicinal preparations, especially as an antidiarrhea agent. Modified citrus pectin (MCP) is citrus pectin that has been broken down to less complex molecules by modifying the pH. MCP is dissolves easily in water and absorbed much easily in the body.
Pectin is resistant to human digestion, but is almost completely degraded into short-chain fatty acids in the colon by bacteria. Pectin has a reduced tendency to have a laxative effect and stimulates bacterial growth in the colon.
Pectin can be extracted from many different plant sources. Fruits rich in pectin are the peach, apple, currant, and plum. Pectins are also found in root crops such as carrots and beetroot, as well as in tubers, such as potatoes.
Beta-glucan is a cell wall polysaccharide comprising d-glucan units and is the main structural material in the cell walls of barley and oat grain. In their natural states, yeast and mushrooms contain a mixture of beta-1,3-glucan and beta-1,6-glucan. Oats and barley contain a mixture of beta-1,3-glucan and beta-1,4-glucan.
Beta-glucan is soluble fiber. Soluble dietary fiber is believed to have a beneficial effect in the reduction of high serum cholesterol levels and reducing the risk associated with such elevated levels. Foods high in water insoluble fiber are known to improve regularity and bulk formation. Soluble fibre forms a gel in the gut, which slows glucose absorption and binds bile acids. In addition, soluble dietary fiber can have the additional beneficial effect of reduced constipation and improved stool regularity.
Beta-glucan is a fiber-type complex sugar (polysaccharide) derived from the cell wall of baker's yeast, oat and barley fiber, and many medicinal mushrooms, such as maitake. Beta glucan is a naturally occurring polysaccharide component of cell walls of cereal grains. Beta glucan from cereal grains comprises a family of linear polysaccharides comprising D-glucopyranosyl units. The highest concentrations of beta glucan are found in barley and oats, but beta glucan can also be isolated from wheat and corn. It is also a component of yeasts and fungi.
The two primary uses of beta-glucan are to enhance the immune system and to lower blood cholesterol levels. Beta-1,3-glucan is very effective at activating white blood cells known as macrophages and neutrophils. These cells provide one of the immune system's first lines of defense against foreign invaders. Yeast beta-glucan can bind to various cells of the non-specific immune system, such as macrophages and neutrophils.