Dietary fiber

Dietary fibers are found in fruits, vegetables and whole grains. The amount of fiber contained in common foods are in the following table:[25]

Dietary fiber is found in plants, typically eaten whole, raw or cooked, although fiber can be added to make dietary supplements and fiber-rich processed foods. Grain bran products have the highest fiber contents, such as crude corn bran (79 g per 100 g) and crude wheat bran (43 g per 100 g), which are ingredients for manufactured foods.[25] Medical authorities, such as the Mayo Clinic, recommend adding fiber-rich products to the Standard American Diet (SAD) which is rich in processed and artificially sweetened foods, with minimal intake of vegetables and legumes.[28][29]

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.[30]

Soluble fiber is 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

Sources of insoluble fiber 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[31]

These are a few example forms of fiber that have been sold as supplements or food additives. These may be marketed to consumers for nutritional purposes, treatment of various gastrointestinal disorders, and for such possible health benefits as lowering cholesterol levels, reducing risk of colon cancer, and losing weight.

Soluble fiber supplements may be beneficial for alleviating symptoms of irritable bowel syndrome, such as diarrhea or constipation and abdominal discomfort.[32] Prebiotic soluble fiber products, like those containing inulin or oligosaccharides, may contribute to relief from inflammatory bowel disease,[33] as in Crohn's disease,[34] ulcerative colitis,[35][36] and Clostridium difficile,[37] due in part to the short-chain fatty acids produced with subsequent anti-inflammatory actions upon the bowel.[38][39] Fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.[40]

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[41][42][43] as well as promoting regularity[44] and possibly relief of diarrhea.[45][46][47] One preliminary finding indicates that resistant corn starch may reduce symptoms of ulcerative colitis.[48]

Chemically defined as oligosaccharides occurring naturally in most plants, inulins have nutritional value as carbohydrates, or more specifically as fructans, a polymer of the natural plant sugar, fructose. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory roots or Jerusalem artichokes for use in prepared foods.[49] Subtly sweet, it can be used to replace sugar, fat, and flour, is often used to improve the flow and mixing qualities of powdered nutritional supplements, and has significant potential health value as a prebiotic fermentable fiber.[50]

Inulin is advantageous because it contains 25–30% the food energy of sugar or other carbohydrates and 10–15% the food energy of fat. As a prebiotic fermentable fiber, its metabolism by gut flora yields short-chain fatty acids (see below) which increase absorption of calcium,[51] magnesium,[52] and iron,[53] resulting from upregulation of mineral-transporting genes and their membrane transport proteins within the colon wall. Among other potential beneficial effects noted above, inulin promotes an increase in the mass and health of intestinal Lactobacillus and Bifidobacterium populations.

Inulin's primary disadvantage is its tolerance. As a soluble fermentable fiber, it is quickly and easily fermented within the intestinal tract, which may cause gas and digestive distress at doses higher than 15 grams/day in most people.[54] Individuals with digestive diseases have benefited from removing fructose and inulin from their diet.[55] While clinical studies have shown changes in the microbiota at lower levels of inulin intake, some of the health effects require higher than 15 grams per day to achieve the benefits.[56]

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.[57] Examples of vegetable gum fibers are guar gum and gum arabic.

Dietary fiber has distinct physicochemical properties. Most semi-solid foods, fiber and fat are a combination of gel matrices which are hydrated or collapsed with microstructural elements, globules, solutions or encapsulating walls. Fresh fruit and vegetables are cellular materials.[69][70][71]

• The cells of cooked potatoes and legumes are gels filled with gelatinized starch granules. The cellular structures of fruits and vegetables are foams with a closed cell geometry filled with a gel, surrounded by cell walls which are composites with an amorphous matrix strengthened by complex carbohydrate fibers.
• Particle size and interfacial interactions with adjacent matrices affect the mechanical properties of food composites.
• Food polymers may be soluble in and/or plasticized by water.
• The variables include chemical structure, polymer concentration, molecular weight, degree of chain branching, the extent of ionization (for electrolytes), solution pH, ionic strength and temperature.
• Cross-linking of different polymers, protein and polysaccharides, either through chemical covalent bonds or cross-links through molecular entanglement or hydrogen or ionic bond cross-linking.
• Cooking and chewing food alters these physicochemical properties and hence absorption and movement through the stomach and along the intestine[72]

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

• food compounds
• complex lipids/micellar/aqueous/hydrocolloid and hydrophobic phases
• hydrophilic phases
• solid, liquid, colloidal and gas bubble phases.[73]

Micelles are colloid-sized clusters of molecules which form in conditions as those above, similar to the critical micelle concentration of detergents.[74] In the upper gastrointestinal tract, these compounds consist of bile acids and di- and monoacyl glycerols which solubilize triacylglycerols and cholesterol.[74]

Two mechanisms bring nutrients into contact with the epithelium:

1. intestinal contractions create turbulence; and
2. convection currents direct contents from the lumen to the epithelial surface.[75]

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

1. Nutrients diffuse through the thin, relatively unstirred layer of fluid adjacent to the epithelium.
2. Immobilizing of nutrients and other chemicals within complex polysaccharide molecules affects their release and subsequent absorption from the small intestine, an effect influential on the glycemic index.[75]
3. Molecules begin to interact as their concentration increases. During absorption, water must be absorbed at a rate commensurate with the absorption of solutes. The transport of actively and passively absorbed nutrients across epithelium is affected by the unstirred water layer covering the microvillus membrane.[75]
4. The presence of mucus or fiber, e.g., pectin or guar, in the unstirred layer may alter the viscosity and solute diffusion coefficient.[73]

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

Dietary fiber interacts with pancreatic and enteric enzymes and their substrates. Human pancreatic enzyme activity is reduced when incubated with most fiber sources. Fiber may affect amylase activity and hence the rate of hydrolysis of starch. The more viscous polysaccharides extend the mouth-to-cecum transit time; guar, tragacanth and pectin being slower than wheat bran.[76]

The colon may be regarded as two organs,

1. the right side (cecum and ascending colon), a fermenter.[77] The right side of the colon is involved in nutrient salvage so that dietary fiber, resistant starch, fat and protein are utilized by bacteria and the end-products absorbed for use by the body
2. the left side (transverse, descending, and sigmoid colon), affecting continence.

The presence of bacteria in the colon produces an 'organ' of intense, mainly reductive, metabolic activity, whereas the liver is oxidative. The substrates utilized by the cecum have either passed along the entire intestine or are biliary excretion products. The effects of dietary fiber in the colon are on

1. bacterial fermentation of some dietary fibers
2. thereby an increase in bacterial mass
3. an increase in bacterial enzyme activity
4. changes in the water-holding capacity of the fiber residue after fermentation

Enlargement of the cecum is a common finding when some dietary fibers are fed and this is now believed to be normal physiological adjustment. Such an increase may be due to a number of factors, prolonged cecal residence of the fiber, increased bacterial mass, or increased bacterial end-products. Some non-absorbed carbohydrates, e.g. pectin, gum arabic, oligosaccharides and resistant starch, are fermented to short-chain fatty acids (chiefly acetic, propionic and n-butyric), and carbon dioxide, hydrogen and methane. Almost all of these short-chain fatty acids will be absorbed from the colon. This means that fecal short-chain fatty acid estimations do not reflect cecal and colonic fermentation, only the efficiency of absorption, the ability of the fiber residue to sequestrate short-chain fatty acids, and the continued fermentation of fiber around the colon, which presumably will continue until the substrate is exhausted. The production of short-chain fatty acids has several possible actions on the gut mucosa. All of the short-chain fatty acids are readily absorbed by the colonic mucosa, but only acetic acid reaches the systemic circulation in appreciable amounts. Butyric acid appears to be used as a fuel by the colonic mucosa as the preferred energy source for colonic cells.

Dietary fiber may act on each phase of ingestion, digestion, absorption and excretion to affect cholesterol metabolism,[78] such as the following:

1. Caloric energy of foods through a bulking effect
2. Slowing of gastric emptying time
3. A glycemic index type of action on absorption
4. A slowing of bile acid absorption in the ileum so bile acids escape through to the cecum
5. Altered or increased bile acid metabolism in the cecum
6. Indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fiber fermentation affecting the cholesterol metabolism in the liver.
7. Binding of bile acids to fiber or bacteria in the cecum with increased fecal loss from the entero-hepatic circulation.

One action of some fibers is to reduce the reabsorption of bile acids in the ileum and hence the amount and type of bile acid and fats reaching the colon. A reduction in the reabsorption of bile acid from the ileum has several direct effects.

1. Bile acids may be trapped within the lumen of the ileum either because of a high luminal viscosity or because of binding to a dietary fiber.[79]
2. Lignin in fiber adsorbs bile acids, but the unconjugated form of the bile acids are adsorbed more than the conjugated form. In the ileum where bile acids are primarily absorbed the bile acids are predominantly conjugated.
3. The enterohepatic circulation of bile acids may be altered and there is an increased flow of bile acids to the cecum, where they are deconjugated and 7alpha-dehydroxylated.
4. These water-soluble form, bile acids e.g., deoxycholic and lithocholic are adsorbed to dietary fiber and an increased fecal loss of sterols, dependent in part on the amount and type of fiber.
5. A further factor is an increase in the bacterial mass and activity of the ileum as some fibers e.g., pectin are digested by bacteria. The bacterial mass increases and cecal bacterial activity increases.
6. The enteric loss of bile acids results in increased synthesis of bile acids from cholesterol which in turn reduces body cholesterol.

The fibers that are most effective in influencing sterol metabolism (e.g. pectin) are fermented in the colon. It is therefore unlikely that the reduction in body cholesterol is due to adsorption to this fermented fiber in the colon.

1. There might be alterations in the end-products of bile acid bacterial metabolism or the release of short chain fatty acids which are absorbed from the colon, return to the liver in the portal vein and modulate either the synthesis of cholesterol or its catabolism to bile acids.
2. The prime mechanism whereby fiber influences cholesterol metabolism is through bacteria binding bile acids in the colon after the initial deconjugation and dehydroxylation. The sequestered bile acids are then excreted in feces.[80]
3. Fermentable fibers e.g., pectin will increase the bacterial mass in the colon by virtue of their providing a medium for bacterial growth.
4. Other fibers, e.g., gum arabic, act as stabilizers and cause a significant decrease in serum cholesterol without increasing fecal bile acid excretion.

Kids eating dietary fiber food

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

1. Feces are 75% water; bacteria make a large contribution to the dry weight, the residue being unfermented fiber and excreted compounds.
2. Fecal output may vary over a range of between 20 and 280 g over 24 hours. The amount of feces egested a day varies for any one individual over a period of time.
3. Of dietary constituents, only dietary fiber increases fecal weight.

Water is distributed in the colon in three ways:

1. Free water which can be absorbed from the colon.
2. Water that is incorporated into bacterial mass.
3. Water that is bound by fiber.

Fecal weight is dictated by:

1. the holding of water by the residual dietary fiber after fermentation.
2. the bacterial mass.
3. There may also be an added osmotic effect of products of bacterial fermentation on fecal mass.

A study of 388,000 adults ages 50 to 71 for nine years found that the highest consumers of fiber were 22% less likely to die over this period.[88] In addition to lower risk of death from heart disease, adequate consumption of fiber-containing foods, especially grains, was also associated with reduced incidence of infectious and respiratory illnesses, and, particularly among males, reduced risk of cancer-related death.

An experiment designed with a large sample and conducted by NIH-AARP Diet and Health Study studied the correlation between fiber intake and colorectal cancer. The analytic cohort consisted of 291,988 men and 197,623 women aged 50–71 years. Diet was assessed with a self-administered food-frequency questionnaire at baseline in 1995–1996; 2,974 incident colorectal cancer cases were identified during five years of follow-up. The result was that total fiber intake was not associated with colorectal cancer.[89]

Although many researchers believe that dietary fiber intake reduces risk of colon cancer, one study conducted by researchers at the Harvard School of Medicine of over 88,000 women did not show a statistically significant relationship between higher fiber consumption and lower rates of colorectal cancer or adenomas.[90] Similarly, a 2010 study of 58,279 men found no relationship between dietary fiber and colorectal cancer.[91]

According to the European Food Safety Authority (EFSA) Panel on Nutrition, Novel Foods and Food Allergens (NDA), which deals with the establishment of Dietary Reference Values for carbohydrates and dietary fibre, "based on the available evidence on bowel function, the Panel considers dietary fibre intakes of 25 g per day to be adequate for normal laxation in adults".[95][22]

On average, North Americans consume less than 50% of the dietary fiber levels recommended for good health. In the preferred food choices of today's youth, this value may be as low as 20%, a factor considered by experts as contributing to the obesity levels seen in many developed countries.[96] Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the Food and Drug Administration (FDA) of the United States have given approvals to food products making health claims for fiber. The FDA classifies which ingredients qualify as being "fiber", and requires for product labeling that a physiological benefit is gained by adding the fiber ingredient.[97] As of 2008, the FDA approved health claims for qualified fiber products to display labeling that regular consumption may reduce blood cholesterol levels – which can lower the risk of coronary heart disease[98] – and also reduce the risk of some types of cancer.[99]

Viscous fiber sources gaining FDA approval are:

• Psyllium seed husk (7 grams per day)
• Beta-glucan from oat bran, whole oats, or rolled oats (3 grams per day)
• Beta-glucan from whole grain or dry-milled barley (3 grams per day)

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

Consistent intake of fermentable fiber may reduce the risk of chronic diseases.[100][101][102] Insufficient fiber in the diet can lead to constipation.[103]

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

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

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

• increases fecal bulk and helps prevent constipation by decreasing fecal transit time in the large intestine
• improves gastro-intestinal health
• improves glucose tolerance and the insulin response following a meal
• increases colonic fermentation and short-chain fatty acid production
• positively modulates colonic microflora
• reduces hyperlipidemia, hypertension, and other coronary heart disease risk factors
• increases satiety and hence may contribute to weight management

Fiber is defined by its physiological impact, with many heterogenous types of fibers. Some fibers may primarily impact one of these benefits (i.e., cellulose increases fecal bulking and prevents constipation), but many fibers impact more than one of these benefits (i.e., resistant starch increases bulking, increases colonic fermentation, positively modulates colonic microflora and increases satiety and insulin sensitivity).[5][4] The beneficial effects of high fiber diets are the summation of the effects of the different types of fiber present in the diet and also other components of such diets.

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