Dietary fiber
Encyclopedia
Dietary fiber, dietary fibre, or sometimes roughage is the indigestible portion of plant foods having two main components:
It acts by changing the nature of the contents of the gastrointestinal tract
and by changing how other nutrients and chemicals are absorbed. Soluble fiber absorbs water to become a gelatinous, viscous substance and is fermented
by bacteria in the digestive tract. Insoluble fiber has bulking action and is not fermented. Lignin
, a major dietary insoluble fiber source, may alter the fate and metabolism of soluble fibers.
Chemically, dietary fiber consists of non-starch
polysaccharide
s such as arabinoxylan
s, cellulose
, and many other plant components such as resistant dextrin
s, inulin
, lignin
, wax
es, chitin
s, pectin
s, beta-glucans, and oligosaccharide
s. A novel position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet. The term "fiber" is something of a misnomer
, since many types of so-called dietary fiber are not actually fibrous.
Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees, according to the plant's characteristics.
Advantages of consuming fiber are the production of healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract.
Disadvantages of a diet high in fiber is the potential for significant intestinal gas production and bloating. Constipation can occur if insufficient fluid is consumed with a high-fiber diet.
and polysaccharides. The definition was later changed to also include resistant starches, along with inulin
and other oligosaccharides.
Fiber-rich plants can be eaten directly. Or, alternatively, they can be used to make supplements and fiber-rich processed foods.
The American Dietetic Association
(ADA) recommends consuming a variety of fiber-rich foods.
Some plants contain significant amounts of soluble and insoluble fiber. For example plum
s (or prunes) have a thick skin covering a juicy pulp. The plum's skin is an example of an insoluble fiber source, whereas soluble fiber sources are inside the pulp.
Soluble fiber is found in varying quantities in all plant foods, including:
Sources of insoluble fiber include:
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
and/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 Clostridium 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.
s occurring naturally in most plants, inulin
s have nutritional value as carbohydrates, or more specifically as fructan
s, a polymer
of the natural plant sugar, fructose
. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory
roots or Jerusalem artichoke
s 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 significant potential health value as a prebiotic fermentable fiber.
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 (discussed above) which increase absorption of calcium
, magnesium
, and iron
, resulting from upregulation of mineral-transporting gene
s 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.
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 acacia Senegal gum
.
, and to change how other nutrients and chemicals are absorbed. Soluble fiber binds to bile acids in the small intestine, making them less likely to enter the body; this in turn lowers cholesterol
levels in the blood. Soluble fiber also attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown.
Not yet formally proposed as an essential macronutrient, dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.
Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium. Some plant foods can reduce the absorption of minerals and vitamins like calcium
, zinc
, vitamin C
, and magnesium
, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.
, Institute of Medicine
, suggest that adults should consume 20–35 grams of dietary fiber per day, but the average American's daily intake of dietary fiber is only 12–18 grams.
The ADA recommends a minimum of 20–35 g/day for a healthy adult depending on calorie intake (e.g., a 2000 Cal/8400 kJ diet should include 25g of fiber per day). The ADA's recommendation for children is that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day). No guidelines have yet been established for the elderly or very ill. Patients with current constipation
, vomiting
, and abdominal pain
should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioid
s because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.
The British Nutrition Foundation
has recommended a minimum fiber intake of 18 g/day for healthy adults.
levels seen in many developed countries.
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.
In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels, an important factor for general cardiovascular health, and to lower risk of onset for some types of cancer.
Soluble (fermentable) fiber sources gaining FDA approval are:
Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include inulin
, resistant dextrin
s, fructans, xanthan gum
, cellulose
, guar gum
, fructooligosaccharides (FOS), and oligo- or polysaccharides.
Consistent intake of fermentable fiber through foods like berries and other fresh fruit
, vegetable
s, whole grain
s, seeds, and nuts is now known to reduce risk of some of the world’s most prevalent diseases—obesity
, diabetes, high blood cholesterol
, cardiovascular disease
, and numerous gastrointestinal disorders. In this last category are constipation
, inflammatory bowel disease
, ulcerative colitis
, hemorrhoids, Crohn’s disease, diverticulitis
, and colon cancer—all disorders of the intestinal tract where fermentable fiber can provide healthful benefits.
Insufficient fiber in the diet can complicate defecation
. Low-fiber feces are dehydrated and hardened, making them difficult to evacuate—defining constipation
and possibly leading to development of hemorrhoid
s or anal fissure
s.
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.
issued a statement to define dietary fiber more concisely and list the potential health benefits established to date:
s and starch
es because it cannot be fully absorbed by the body. Sugars and starches provide 4 Calories per gram, and the human body has specific enzymes to break them down into glucose
, fructose
, and galactose
, which can then be absorbed by the body. The human body lacks enzymes to break down fiber. Insoluble fiber does not change inside the body, so the body cannot absorb it and nutritionists say that it contributes 0 Calories per gram. Soluble fiber is partially fermented, with the degree of fermentability varying with the type of fiber, and contributes some energy when broken down and absorbed by the body. Dietitians have not reached a consensus on how much energy is actually absorbed, but some approximate around 2 Calories (8.5 kilojoules) per gram of soluble fiber. Regardless of the type of fiber, the body absorbs fewer than 4 Calories (16.7 kilojoules) per gram of fiber, which can create inconsistencies for actual product nutrition labels. In some countries, fiber is not listed on nutrition labels, and is considered 0 Calories/gram when the food's total Calories are computed. In other countries all fiber must be listed, and is considered 4 Calories per gram when the food's total Calories are computed (because chemically fiber is a type of carbohydrate and other carbohydrates contribute 4 Calories per gram). In the US, soluble fiber must be counted as 4 Calories per gram, but insoluble fiber may be (and usually is) treated as 0 Calories per gram and not mentioned on the label.
“the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine.” In this definition:
edible parts of plants
carbohydrates
resistant to digestion and absorption in the human small intestine
complete or partial fermentation in the large intestine
As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence
).
According to a 2002 journal article,
fibers compounds with partial or low fermentability include:
fiber compounds with high fermentability include:
s (SCFA) are produced. SCFAs are involved in numerous physiological processes promoting health, including:
SCFAs that are absorbed by the colonic mucosa pass through the colonic wall into the portal circulation (supplying the liver
), and the liver transports them into the general circulatory system
.
Overall, SCFAs affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.
The major SCFAs in humans are butyrate
, propionate
, and acetate
, where butyrate is the major energy source for colonocyte
s, propionate is destined for uptake by the liver, and acetate enters the peripheral circulation to be metabolized by peripheral tissues.
or barley
soluble fiber as beta-glucans to claim that reduced risk of heart disease
can result from their regular consumption.
The FDA statement template for making this claim is: Soluble fiber from foods such as [name of soluble fiber source, and, if desired, name of food product], as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food product] supplies __ grams of the [necessary daily dietary intake for the benefit] soluble fiber from [name of soluble fiber source] necessary per day to have this effect.
Eligible sources of soluble fiber providing beta-glucan include:
The allowed label may state that diets low in saturated fat and cholesterol and that include soluble fiber from certain of the above foods “may” or “might” reduce the risk of heart disease.
As discussed in FDA regulation 21 CFR 101.81, the daily dietary intake levels of soluble fiber from sources listed above associated with reduced risk of coronary heart disease
are:
Soluble fiber from consuming grains is included in other allowed health claims for lowering risk of some types of cancer and heart disease by consuming fruit and vegetables (21 CFR 101.76, 101.77, and 101.78).
-related death.
- soluble (prebiotic, viscous) fiber that is readily fermented in the colonColon (anatomy)The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
into gases and physiologically active byproducts, and - insoluble fiber that is metabolically inert, absorbing water as it moves through the digestive system, easing defecationDefecationDefecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...
.
It acts by changing the nature of the contents of the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
and by changing how other nutrients and chemicals are absorbed. Soluble fiber absorbs water to become a gelatinous, viscous substance and is fermented
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
by bacteria in the digestive tract. Insoluble fiber has bulking action and is not fermented. Lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
, a major dietary insoluble fiber source, may alter the fate and metabolism of soluble fibers.
Chemically, dietary fiber consists of non-starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
s such as arabinoxylan
Arabinoxylan
Arabinoxylan is a hemicellulose found in both the primary and secondary cell walls of plants, including woods and cereal grains, consisting of copolymers of two pentose sugars – arabinose and xylose.-Structure:...
s, cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, and many other plant components such as resistant dextrin
Dextrin
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α- or α- glycosidic bonds....
s, inulin
Inulin
Inulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes...
, lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
, wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
es, chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
s, pectin
Pectin
Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
s, beta-glucans, and oligosaccharide
Oligosaccharide
An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...
s. A novel position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet. The term "fiber" is something of a misnomer
Misnomer
A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...
, since many types of so-called dietary fiber are not actually fibrous.
Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees, according to the plant's characteristics.
Advantages of consuming fiber are the production of healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract.
Disadvantages of a diet high in fiber is the potential for significant intestinal gas production and bloating. Constipation can occur if insufficient fluid is consumed with a high-fiber diet.
History of definition
Originally, fiber was defined to be the components of plants that resist human digestive enzymes, a definition that includes ligninLignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
and polysaccharides. The definition was later changed to also include resistant starches, along with inulin
Inulin
Inulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes...
and other oligosaccharides.
Sources of fiber
Dietary fiber is found in plants. While all plants contain some fiber, plants with high fiber concentrations are generally the most practical source.Fiber-rich plants can be eaten directly. Or, alternatively, they can be used to make supplements and fiber-rich processed foods.
The American Dietetic Association
American Dietetic Association
The American Dietetic Association is the United States' largest organization of food and nutrition professionals, with nearly 72,000 members. The American Dietetic Association is officially changing its name to the Academy of Nutrition and Dietetics. The announcement was made Saturday, September...
(ADA) recommends consuming a variety of fiber-rich foods.
Plant sources of fiber
Plum
A plum or gage is a stone fruit tree in the genus Prunus, subgenus Prunus. The subgenus is distinguished from other subgenera in the shoots having a terminal bud and solitary side buds , the flowers in groups of one to five together on short stems, and the fruit having a groove running down one...
s (or prunes) have a thick skin covering a juicy pulp. The plum's skin is an example of an insoluble fiber source, whereas soluble fiber sources are inside the pulp.
Soluble fiber is found in varying quantities in all plant foods, including:
- legumes (peaPeaA pea is most commonly the small spherical seed or the seed-pod of the pod fruit Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a flower. However, peas are considered to be a vegetable in cooking...
s, soybeanSoybeanThe soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...
s, lupinLupinLupinus, commonly known as Lupins or lupines , is a genus in the legume family . The genus comprises about 280 species , with major centers of diversity in South and western North America , and the Andes and secondary centers in the Mediterranean region and Africa Lupinus, commonly known as Lupins...
s and other beanBeanBean is a common name for large plant seeds of several genera of the family Fabaceae used for human food or animal feed....
s) - oatOatThe common oat is a species of cereal grain grown for its seed, which is known by the same name . While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed...
s, ryeRyeRye is a grass grown extensively as a grain and as a forage crop. It is a member of the wheat tribe and is closely related to barley and wheat. Rye grain is used for flour, rye bread, rye beer, some whiskeys, some vodkas, and animal fodder...
, chia, and barleyBarleyBarley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods... - some fruitFruitIn broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
s and fruit juiceJuiceJuice is the liquid that is naturally contained in fruit or vegetable tissue.Juice is prepared by mechanically squeezing or macerating fruit or vegetable flesh without the application of heat or solvents. For example, orange juice is the liquid extract of the fruit of the orange tree...
s (including prunePruneA prune is any of various plum cultivars, mostly Prunus domestica or European Plum, sold as fresh or dried fruit. The dried fruit is also referred to as a dried plum...
juice, plumPlumA plum or gage is a stone fruit tree in the genus Prunus, subgenus Prunus. The subgenus is distinguished from other subgenera in the shoots having a terminal bud and solitary side buds , the flowers in groups of one to five together on short stems, and the fruit having a groove running down one...
s, berriesBerryThe botanical definition of a berry is a fleshy fruit produced from a single ovary. Grapes are an example. The berry is the most common type of fleshy fruit in which the entire ovary wall ripens into an edible pericarp. They may have one or more carpels with a thin covering and fleshy interiors....
, bananas, and the insides of appleAppleThe apple is the pomaceous fruit of the apple tree, species Malus domestica in the rose family . It is one of the most widely cultivated tree fruits, and the most widely known of the many members of genus Malus that are used by humans. Apple grow on small, deciduous trees that blossom in the spring...
s and pearPearThe pear is any of several tree species of genus Pyrus and also the name of the pomaceous fruit of these trees. Several species of pear are valued by humans for their edible fruit, but the fruit of other species is small, hard, and astringent....
s) - certain vegetableVegetableThe noun vegetable usually means an edible plant or part of a plant other than a sweet fruit or seed. This typically means the leaf, stem, or root of a plant....
s such as broccoliBroccoliBroccoli is a plant in the cabbage family, whose large flower head is used as a vegetable.-General:The word broccoli, from the Italian plural of , refers to "the flowering top of a cabbage"....
, carrotCarrotThe carrot is a root vegetable, usually orange in colour, though purple, red, white, and yellow varieties exist. It has a crisp texture when fresh...
s, and Jerusalem artichokeJerusalem artichokeThe Jerusalem artichoke , also called the sunroot, sunchoke, earth apple or topinambour, is a species of sunflower native to eastern North America, and found from Eastern Canada and Maine west to North Dakota, and south to northern Florida and Texas...
s - root tubers and root vegetableRoot vegetableRoot vegetables are plant roots used as vegetables. Here "root" means any underground part of a plant.Root vegetables are generally storage organs, enlarged to store energy in the form of carbohydrates. They differ in the concentration and the balance between sugars, starches, and other types of...
s such as sweet potatoSweet potatoThe sweet potato is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are an important root vegetable. The young leaves and shoots are sometimes eaten as greens. Of the approximately 50 genera and more than 1,000 species of...
es and onionOnionThe onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...
s (skins of these are sources of insoluble fiber) - psylliumPsylliumPsyllium , or Ispaghula , is the common name used for several members of the plant genus Plantago whose seeds are used commercially for the production of mucilage.-History:...
seed husk (a mucilageMucilageMucilage is a thick, gluey substance produced by most plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide.It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as...
soluble fiber).
Sources of insoluble fiber include:
- whole grainWhole grainWhole grains are cereal grains that contain cereal germ, endosperm, and bran, in contrast to refined grains, which retain only the endosperm. Whole grains can generally be sprouted while refined grains generally will not sprout. Whole-meal products are made by grinding whole grains in order to make...
foods - wheatWheatWheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
and cornMaizeMaize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
branBranBran is the hard outer layer of grain and consists of combined aleurone and pericarp. Along with germ, it is an integral part of whole grains, and is often produced as a by-product of milling in the production of refined grains. When bran is removed from grains, the grains lose a portion of their... - nutNut (fruit)A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...
s and seedSeedA seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s - potatoPotatoThe potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...
skins - flaxFlaxFlax is a member of the genus Linum in the family Linaceae. It is native to the region extending from the eastern Mediterranean to India and was probably first domesticated in the Fertile Crescent...
and hempHempHemp is mostly used as a name for low tetrahydrocannabinol strains of the plant Cannabis sativa, of fiber and/or oilseed varieties. In modern times, hemp has been used for industrial purposes including paper, textiles, biodegradable plastics, construction, health food and fuel with modest...
seed - lignans
- vegetables such as green beanGreen beanGreen beans , also known as French beans , are the unripe fruit of any kind of bean, including the yardlong bean, the hyacinth bean, the winged bean, and especially the common bean , whose pods are also usually called string beans in the northeastern and western United States, but can also be...
s, cauliflowerCauliflowerCauliflower is one of several vegetables in the species Brassica oleracea, in the family Brassicaceae. It is an annual plant that reproduces by seed...
, zucchiniZucchiniThe zucchini is a summer squash which often grows to nearly a meter in length, but which is usually harvested at half that size or less. It is a hybrid of the cucumber. Along with certain other squashes, it belongs to the species Cucurbita pepo. Zucchini can be dark or light green...
(courgette), celeryCeleryApium graveolens is a plant species in the family Apiaceae commonly known as celery or celeriac , depending on whether the petioles or roots are eaten: celery refers to the former and celeriac to the latter. Apium graveolens grows to 1 m tall...
, and nopalNopalNopales are a vegetable made from the young cladode segments of prickly pear, carefully peeled to remove the spines. These fleshy pads are flat and about hand-sized. They can be purple or green... - some fruits including avocadoAvocadoThe avocado is a tree native to Central Mexico, classified in the flowering plant family Lauraceae along with cinnamon, camphor and bay laurel...
, and bananas - the skins of some fruits, including kiwifruitKiwifruitThe kiwifruit, often shortened to kiwi in many parts of the world, is the edible berry of a cultivar group of the woody vine Actinidia deliciosa and hybrids between this and other species in the genus Actinidia....
and tomatoTomatoThe word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...
es
Fiber 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 cholesterolCholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
levels, reducing risk of colon cancer, and losing weight.
Soluble fiber supplements may be beneficial for alleviating symptoms of irritable bowel syndrome
Irritable bowel syndrome
Irritable bowel syndrome is a diagnosis of exclusion. It is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any detectable organic cause. In some cases, the symptoms are relieved by bowel movements...
, such as diarrhea
Diarrhea
Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
and/or constipation
Constipation
Constipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
and abdominal discomfort. Prebiotic
Prebiotic (nutrition)
Prebiotics are non-digestible food ingredients that stimulate the growth and/or activity of bacteria in the digestive system in ways claimed to be beneficial to health. They were first identified and named by Marcel Roberfroid in 1995. As a functional food component, prebiotics, like probiotics,...
soluble fiber products, like those containing inulin
Inulin
Inulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes...
or oligosaccharides, may contribute to relief from inflammatory bowel disease
Inflammatory bowel disease
In medicine, inflammatory bowel disease is a group of inflammatory conditions of the colon and small intestine. The major types of IBD are Crohn's disease and ulcerative colitis.-Classification:...
, as in Crohn's disease
Crohn's disease
Crohn's disease, also known as regional enteritis, is a type of inflammatory bowel disease that may affect any part of the gastrointestinal tract from mouth to anus, causing a wide variety of symptoms...
, ulcerative colitis
Ulcerative colitis
Ulcerative colitis is a form of inflammatory bowel disease . Ulcerative colitis is a form of colitis, a disease of the colon , that includes characteristic ulcers, or open sores. The main symptom of active disease is usually constant diarrhea mixed with blood, of gradual onset...
, and Clostridium difficile
Clostridium difficile
Clostridium difficile , also known as "CDF/cdf", or "C...
, due in part to the short-chain fatty acids produced with subsequent anti-inflammatory
Anti-inflammatory
Anti-inflammatory refers to the property of a substance or treatment that reduces inflammation. Anti-inflammatory drugs make up about half of analgesics, remedying pain by reducing inflammation as opposed to opioids, which affect the central nervous system....
actions upon the bowel. fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome
Irritable bowel syndrome
Irritable bowel syndrome is a diagnosis of exclusion. It is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any detectable organic cause. In some cases, the symptoms are relieved by bowel movements...
by modification of food choices.
Inulins
Chemically defined as oligosaccharideOligosaccharide
An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...
s occurring naturally in most plants, inulin
Inulin
Inulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes...
s have nutritional value as carbohydrates, or more specifically as fructan
Fructan
A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides, whereas longer chain fructans are termed inulins...
s, a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
of the natural plant sugar, fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory
Chicory
Common chicory, Cichorium intybus, is a somewhat woody, perennial herbaceous plant usually with bright blue flowers, rarely white or pink. Various varieties are cultivated for salad leaves, chicons , or for roots , which are baked, ground, and used as a coffee substitute and additive. It is also...
roots or Jerusalem artichoke
Jerusalem artichoke
The Jerusalem artichoke , also called the sunroot, sunchoke, earth apple or topinambour, is a species of sunflower native to eastern North America, and found from Eastern Canada and Maine west to North Dakota, and south to northern Florida and Texas...
s 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 significant potential health value as a prebiotic fermentable fiber.
Inulin is advantageous because it contains 25–30% the food energy
Food energy
Food energy is the amount of energy obtained from food that is available through cellular respiration.Food energy is expressed in food calories or kilojoules...
of sugar or other carbohydrates and 10–15% the food energy of fat. As a prebiotic fermentable fiber, its metabolism by gut flora
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....
yields short-chain fatty acids (discussed above) which increase absorption of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, resulting from upregulation of mineral-transporting gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s 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
Lactobacillus
Lactobacillus is a genus of Gram-positive facultative anaerobic or microaerophilic rod-shaped bacteria. They are a major part of the lactic acid bacteria group, named as such because most of its members convert lactose and other sugars to lactic acid. They are common and usually benign...
and Bifidobacterium
Bifidobacterium
Bifidobacterium is a genus of Gram-positive, non-motile, often branched anaerobic bacteria. They are ubiquitous, endosymbiotic inhabitants of the gastrointestinal tract, vagina and mouth of mammals and other animals. Bifidobacteria are one of the major genera of bacteria that make up the colon...
populations.
Vegetable gums
Vegetable gumNatural gum
Natural gums are polysaccharides of natural origin, capable of causing a large viscosity increase in solution, even at small concentrations. In the food industry they are used as thickening agents, gelling agents, emulsifying agents, and stabilizers...
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
Irritable bowel syndrome
Irritable bowel syndrome is a diagnosis of exclusion. It is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any detectable organic cause. In some cases, the symptoms are relieved by bowel movements...
. Examples of vegetable gum fibers are guar gum
Guar gum
Guar gum, also called guaran, is a galactomannan. It is primarily the ground endosperm of guar beans. The guar seeds are dehusked, milled and screened to obtain the guar gum. It is typically produced as a free-flowing, pale, off-white-colored, coarse to fine ground powder.-Production:Guar gum is an...
and acacia Senegal gum
Gum arabic
220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal...
.
Mechanism
The main action of dietary fiber is to change the nature of the contents of the gastrointestinal tractGastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
, and to change how other nutrients and chemicals are absorbed. Soluble fiber binds to bile acids in the small intestine, making them less likely to enter the body; this in turn lowers cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
levels in the blood. Soluble fiber also attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown.
Not yet formally proposed as an essential macronutrient, dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.
Effects of fiber intake
Research has shown that fiber may benefit health in several different ways. Lignin and probably related materials that are resistant to enzymatic degradation, diminish the nutritional value of foods.Table legend
Color coding of table entries:- Both Applies to both soluble and insoluble fiber
- Soluble Applies to soluble fiber only
- Insoluble Applies to insoluble fiber only
Dietary fiber functions and benefits
| Functions | Benefits |
|---|---|
| Increases food volume without increasing caloric content, providing satiety | May reduce appetite |
| Attracts water and forms a viscous Viscosity Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity... gel during digestion, slowing the emptying of the stomach and intestinal transit, shielding carbohydrates from enzymes, and delaying absorption of glucose |
Lowers variance in blood sugar levels |
| Lowers total and LDL cholesterol | Reduces risk of cardiovascular disease |
| Regulates blood sugar | May reduce glucose and insulin levels in diabetic patients and may lower risk of diabetes |
| Speeds the passage of foods through the digestive system | Facilitates regular defecation |
| Adds bulk to the stool | Alleviates constipation |
| Balances intestinal pH and stimulates intestinal fermentation production of short-chain fatty acids |
May reduce risk of colorectal cancer |
Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium. Some plant foods can reduce the absorption of minerals and vitamins like calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...
, and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.
Guidelines on fiber intake
Current recommendations from the United States National Academy of SciencesUnited States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
, Institute of Medicine
Institute of Medicine
The Institute of Medicine is a not-for-profit, non-governmental American organization founded in 1970, under the congressional charter of the National Academy of Sciences...
, suggest that adults should consume 20–35 grams of dietary fiber per day, but the average American's daily intake of dietary fiber is only 12–18 grams.
The ADA recommends a minimum of 20–35 g/day for a healthy adult depending on calorie intake (e.g., a 2000 Cal/8400 kJ diet should include 25g of fiber per day). The ADA's recommendation for children is that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day). No guidelines have yet been established for the elderly or very ill. Patients with current constipation
Constipation
Constipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
, vomiting
Vomiting
Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...
, and abdominal pain
Abdominal pain
Abdominal pain can be one of the symptoms associated with transient disorders or serious disease. Making a definitive diagnosis of the cause of abdominal pain can be difficult, because many diseases can result in this symptom. Abdominal pain is a common problem...
should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioid
Opioid
An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract...
s because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.
The British Nutrition Foundation
British Nutrition Foundation
The British Nutrition Foundation is a British registered charity.-Aims:According to its entry in the Charity Commission's register, the aims of the British Nutrition Foundation are:...
has recommended a minimum fiber intake of 18 g/day for healthy adults.
Fiber recommendations in North America
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 obesityObesity
Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...
levels seen in many developed countries.
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.
In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels, an important factor for general cardiovascular health, and to lower risk of onset for some types of cancer.
Soluble (fermentable) fiber sources gaining FDA approval are:
- PsylliumPsylliumPsyllium , or Ispaghula , is the common name used for several members of the plant genus Plantago whose seeds are used commercially for the production of mucilage.-History:...
seed husk (7 grams per day) - Beta-glucanBeta-glucanβ-Glucans are polysaccharides of D-glucose monomers linked by β-glycosidic bonds. β-glucans are a diverse group of molecules that can vary with respect to molecular mass, solubility, viscosity, and three-dimensional configuration...
from oat bran, whole oatOatThe common oat is a species of cereal grain grown for its seed, which is known by the same name . While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed...
s, oatrim, or rolled oatsRolled oatsRolled oats are traditionally oat groats that have been rolled into flat flakes under heavy rollers and then steamed and lightly toasted. The oat, like the other cereals, has a hard, inedible outer husk that must be removed before the grain can be eaten. After the outer husk has been removed from...
(3 grams per day) - Beta-glucan from whole grain or dry-milled barleyBarleyBarley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...
(3 grams per day)
Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include inulin
Inulin
Inulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes...
, resistant dextrin
Dextrin
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α- or α- glycosidic bonds....
s, fructans, xanthan gum
Xanthan gum
Xanthan gum is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris, used as a food additive and rheology modifier, commonly used as a food thickening agent and a stabilizer...
, cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, guar gum
Guar gum
Guar gum, also called guaran, is a galactomannan. It is primarily the ground endosperm of guar beans. The guar seeds are dehusked, milled and screened to obtain the guar gum. It is typically produced as a free-flowing, pale, off-white-colored, coarse to fine ground powder.-Production:Guar gum is an...
, fructooligosaccharides (FOS), and oligo- or polysaccharides.
Consistent intake of fermentable fiber through foods like berries and other fresh fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
, vegetable
Vegetable
The noun vegetable usually means an edible plant or part of a plant other than a sweet fruit or seed. This typically means the leaf, stem, or root of a plant....
s, whole grain
Whole grain
Whole grains are cereal grains that contain cereal germ, endosperm, and bran, in contrast to refined grains, which retain only the endosperm. Whole grains can generally be sprouted while refined grains generally will not sprout. Whole-meal products are made by grinding whole grains in order to make...
s, seeds, and nuts is now known to reduce risk of some of the world’s most prevalent diseases—obesity
Obesity
Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...
, diabetes, high blood cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
, cardiovascular disease
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...
, and numerous gastrointestinal disorders. In this last category are constipation
Constipation
Constipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
, inflammatory bowel disease
Irritable bowel syndrome
Irritable bowel syndrome is a diagnosis of exclusion. It is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any detectable organic cause. In some cases, the symptoms are relieved by bowel movements...
, ulcerative colitis
Ulcerative colitis
Ulcerative colitis is a form of inflammatory bowel disease . Ulcerative colitis is a form of colitis, a disease of the colon , that includes characteristic ulcers, or open sores. The main symptom of active disease is usually constant diarrhea mixed with blood, of gradual onset...
, hemorrhoids, Crohn’s disease, diverticulitis
Diverticulitis
Diverticulitis is a common digestive disease particularly found in the large intestine. Diverticulitis develops from diverticulosis, which involves the formation of pouches on the outside of the colon...
, and colon cancer—all disorders of the intestinal tract where fermentable fiber can provide healthful benefits.
Insufficient fiber in the diet can complicate defecation
Defecation
Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...
. Low-fiber feces are dehydrated and hardened, making them difficult to evacuate—defining constipation
Constipation
Constipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
and possibly leading to development of hemorrhoid
Hemorrhoid
Hemorrhoids or haemorrhoids , are vascular structures in the anal canal which help with stool control. They become pathological or piles when swollen or inflamed. In their physiological state they act as a cushion composed of arterio-venous channels and connective tissue that aid the passage of...
s or anal fissure
Anal fissure
An anal fissure is a break or tear in the skin of the anal canal. Anal fissures may be noticed by bright red anal bleeding on the toilet paper, sometimes in the toilet. If acute they may cause severe periodic pain after defecation but with chronic fissures pain intensity is often less...
s.
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.
Fiber recommendations in the UK
In June 2007, the British Nutrition FoundationBritish Nutrition Foundation
The British Nutrition Foundation is a British registered charity.-Aims:According to its entry in the Charity Commission's register, the aims of the British Nutrition Foundation are:...
issued a statement to define dietary fiber more concisely and list the potential health benefits established to date:
‘Dietary fibre’ has been used as a collective term for a complex mixture of substances with different chemical and physical properties which exert different types of physiological effects.
The use of certain analytical methods to quantify dietary fiber by nature of its indigestibility results in many other indigestible components being isolated along with the carbohydrateCarbohydrateA carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
components of dietary fiber. These components include resistant starchResistant starchResistant starch is starch and starch degradation products that escape digestion in the small intestine of healthy individuals. Resistant starch is considered the third type of dietary fiber, as it can deliver some of the benefits of insoluble fiber and some of the benefits of soluble fiber.Some...
es and oligosaccharides along with other substances that exist within the plant cell structure and contribute to the material that passes through the digestive tract. Such components are likely to have physiological effects.
Yet, some differentiation has to be made between these indigestible plant components and other partially digested material, such as proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
, that appears in the large bowel. Thus, it is better to classify fiber as a group of compounds with different physiological characteristics, rather than to be constrained by defining it chemically.
Diets naturally high in fiber can be considered to bring about several main physiological consequences:
- helps prevent constipation
- reduces the risk of colon cancer
- improvements in gastrointestinal health
- improvements in glucose tolerance and the insulin
InsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
response- reduction of hyperlipidemia
HyperlipidemiaHyperlipidemia, hyperlipoproteinemia, or hyperlipidaemia is the condition of abnormally elevated levels of any or all lipids and/or lipoproteins in the blood...
, hypertensionHypertensionHypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
, and other coronary heart diseaseCoronary heart diseaseCoronary artery disease is the end result of the accumulation of atheromatous plaques within the walls of the coronary arteries that supply the myocardium with oxygen and nutrients. It is sometimes also called coronary heart disease...
risk factors- reduction in the risk of developing some cancers
- increased satiety and hence some degree of weight management
Therefore, it is not appropriate to state that fiber has a single all encompassing physiological property as these effects are dependent on the type of fiber in the diet. 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.
Fiber and calories
Fiber, a type of carbohydrate, contributes less energy (measured in Calories or kilojoules) than sugarSugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s and starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
es because it cannot be fully absorbed by the body. Sugars and starches provide 4 Calories per gram, and the human body has specific enzymes to break them down into glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
, and galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
, which can then be absorbed by the body. The human body lacks enzymes to break down fiber. Insoluble fiber does not change inside the body, so the body cannot absorb it and nutritionists say that it contributes 0 Calories per gram. Soluble fiber is partially fermented, with the degree of fermentability varying with the type of fiber, and contributes some energy when broken down and absorbed by the body. Dietitians have not reached a consensus on how much energy is actually absorbed, but some approximate around 2 Calories (8.5 kilojoules) per gram of soluble fiber. Regardless of the type of fiber, the body absorbs fewer than 4 Calories (16.7 kilojoules) per gram of fiber, which can create inconsistencies for actual product nutrition labels. In some countries, fiber is not listed on nutrition labels, and is considered 0 Calories/gram when the food's total Calories are computed. In other countries all fiber must be listed, and is considered 4 Calories per gram when the food's total Calories are computed (because chemically fiber is a type of carbohydrate and other carbohydrates contribute 4 Calories per gram). In the US, soluble fiber must be counted as 4 Calories per gram, but insoluble fiber may be (and usually is) treated as 0 Calories per gram and not mentioned on the label.
Soluble fiber fermentation
The American Association of Cereal Chemists has defined soluble fiber this way:“the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine.” In this definition:
edible parts of plants
- indicates that some parts of a plant we eat—skin, pulp, seeds, stems, leaves, roots—contain fiber. Both insoluble and soluble sources are in those plant components.
carbohydrates
- complex carbohydrates, such as long-chained sugars also called starchStarchStarch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
, oligosaccharides, or polysaccharides, are sources of soluble fermentable fiber.
resistant to digestion and absorption in the human small intestine
- foods providing nutrients are digested by gastric acidGastric acidGastric acid is a digestive fluid, formed in the stomach. It has a pH of 1 to 2 and is composed of hydrochloric acid , and large quantities of potassium chloride and sodium chloride...
and digestive enzymes in the stomach and small intestine where the nutrients are released then absorbed through the intestinal wall for transport via the blood throughout the body. A food resistant to this process is undigested, as insoluble and soluble fibers are. They pass to the large intestine only affected by their absorption of water (insoluble fiber) or dissolution in water (soluble fiber).
complete or partial fermentation in the large intestine
- the large intestine comprises a segment called the colonColon (anatomy)The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
within which additional nutrient absorption occurs through the process of fermentation. Fermentation occurs by the action of colonic bacteria on the food mass, producing gases and short-chain fatty acids. It is these short-chain fatty acids—butyricButyric acidButyric acid , also known under the systematic name butanoic acid, is a carboxylic acid with the structural formula CH3CH2CH2-COOH. Salts and esters of butyric acid are known as butyrates or butanoates...
, aceticAcetic acidAcetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
(ethanoic), propionicPropionic acidPropanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...
, and valericValeric acidValeric acid, or pentanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula C5H10O2. Like other low-molecular-weight carboxylic acids, it has a very unpleasant odor. It is found naturally in the perennial flowering plant valerian , from which it gets its name. Its...
acids—that scientific evidence is revealing to have significant health properties.
As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence
Flatulence
Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply gas...
).
According to a 2002 journal article,
fibers compounds with partial or low fermentability include:
- celluloseCelluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, a polysaccharidePolysaccharidePolysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,... - hemicelluloseHemicelluloseA hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
, a polysaccharide - lignanLignanThe lignans are a group of chemical compounds found in plants. Lignans are one of the major classes of phytoestrogens, which are estrogen-like chemicals and also act as antioxidants. The other classes of phytoestrogens are the isoflavones and coumestans...
s, a group of phytoestrogens - plant waxWaxthumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
es - resistant starchResistant starchResistant starch is starch and starch degradation products that escape digestion in the small intestine of healthy individuals. Resistant starch is considered the third type of dietary fiber, as it can deliver some of the benefits of insoluble fiber and some of the benefits of soluble fiber.Some...
es
fiber compounds with high fermentability include:
- beta-glucans, a group of polysaccharides
- pectinPectinPectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
s, a group of heteropolysaccharides - natural gumNatural gumNatural gums are polysaccharides of natural origin, capable of causing a large viscosity increase in solution, even at small concentrations. In the food industry they are used as thickening agents, gelling agents, emulsifying agents, and stabilizers...
s, a group of polysaccharides - inulinInulinInulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes...
s, a group of polysaccharides - oligosaccharideOligosaccharideAn oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...
s, a group of short-chained or simple sugars - resistant dextrinDextrinDextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α- or α- glycosidic bonds....
s
Short-chain fatty acids
When soluble fiber is fermented, short-chain fatty acidShort chain fatty acid
Short-chain fatty acids are a sub-group of fatty acids with aliphatic tails of less than six carbons. They include:* Acetic acid* Propionic acid* Isobutyric acid * Butyric acid* Isovaleric acid...
s (SCFA) are produced. SCFAs are involved in numerous physiological processes promoting health, including:
- stabilize blood glucoseGlucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
levels by acting on pancreatic insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
release and liver control of glycogenGlycogenGlycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
breakdown - stimulate gene expressionGene expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
of glucose transporterGlucose transporterGlucose transporters are a wide group of membrane proteins that facilitate the transport of glucose over a plasma membrane. Because glucose is a vital source of energy for all life these transporters are present in all phyla...
s in the intestinal mucosa, regulating glucose absorption - provide nourishment of colonocytes, particularly by the SCFA butyrate
- suppress cholesterolCholesterolCholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
synthesis by the liver and reduce blood levels of LDL cholesterol and triglycerides responsible for atherosclerosisAtherosclerosisAtherosclerosis is a condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol... - lower colonic pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
(i.e., raises the acidity level in the colonColon (anatomy)The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
) which protects the lining from formation of colonic polyps and increases absorption of dietary minerals - stimulate production of T helper cells, antibodies, leukocytes, cytokines, and lymphLymphLymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary...
mechanisms having crucial roles in immune protection - improve barrier properties of the colonic mucosal layer, inhibiting inflammatoryInflammationInflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
and adhesionAdhesionAdhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
irritants, contributing to immune functions
SCFAs that are absorbed by the colonic mucosa pass through the colonic wall into the portal circulation (supplying the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
), and the liver transports them into the general circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
.
Overall, SCFAs affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.
The major SCFAs in humans are butyrate
Butyric acid
Butyric acid , also known under the systematic name butanoic acid, is a carboxylic acid with the structural formula CH3CH2CH2-COOH. Salts and esters of butyric acid are known as butyrates or butanoates...
, propionate
Propionic acid
Propanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...
, and acetate
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
, where butyrate is the major energy source for colonocyte
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
s, propionate is destined for uptake by the liver, and acetate enters the peripheral circulation to be metabolized by peripheral tissues.
FDA-approved health claims
The FDA allows producers of foods containing 1.7g per serving of psyllium husk soluble fiber or 0.75g of oatOat
The common oat is a species of cereal grain grown for its seed, which is known by the same name . While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed...
or barley
Barley
Barley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...
soluble fiber as beta-glucans to claim that reduced risk of heart disease
Heart disease
Heart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...
can result from their regular consumption.
The FDA statement template for making this claim is: Soluble fiber from foods such as [name of soluble fiber source, and, if desired, name of food product], as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food product] supplies __ grams of the [necessary daily dietary intake for the benefit] soluble fiber from [name of soluble fiber source] necessary per day to have this effect.
Eligible sources of soluble fiber providing beta-glucan include:
- Oat bran
- Rolled oats
- Whole oat flour
- Oatrim
- Whole grain barley and dry milled barley
- Soluble fiber from psyllium husk with purity of no less than 95%
The allowed label may state that diets low in saturated fat and cholesterol and that include soluble fiber from certain of the above foods “may” or “might” reduce the risk of heart disease.
As discussed in FDA regulation 21 CFR 101.81, the daily dietary intake levels of soluble fiber from sources listed above associated with reduced risk of coronary heart disease
Coronary heart disease
Coronary artery disease is the end result of the accumulation of atheromatous plaques within the walls of the coronary arteries that supply the myocardium with oxygen and nutrients. It is sometimes also called coronary heart disease...
are:
- 3g or more per day of beta-glucan soluble fiber from either whole oats or barley, or a combination of whole oats and barley
- 7g or more per day of soluble fiber from psyllium seed husk.
Soluble fiber from consuming grains is included in other allowed health claims for lowering risk of some types of cancer and heart disease by consuming fruit and vegetables (21 CFR 101.76, 101.77, and 101.78).
Potential longevity
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. In addition to lowering the risk of death from heart disease, adequate consumption of fiber-containing foods, especially grains, also appeared to reduce the incidence of infectious and respiratory illnesses, and, particularly among males, lowered the risk of cancerCancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
-related death.
See also
- High residue dietHigh residue diet- General guidelines :Women should aim for at least 21 to 25 grams of fiber daily. Men should try for 30 to 38 grams daily.- Foods to include :High fiber foods include whole grain breads and cereals, vegetables and fruits .- External links :* livestrong.com...
- Low residue dietLow residue dietA low residue diet is a diet designed to reduce the frequency and volume of stools while prolonging intestinal transit time. It is similar to a low fiber diet, but typically includes restrictions on foods that increase bowel activity, such as milk, milk products, and prune juice. A low residue diet...
- MethylcelluloseMethylcelluloseMethyl cellulose is a chemical compound derived from cellulose. It is a hydrophilic white powder in pure form and dissolves in cold water, forming a clear viscous solution or gel. It is sold under a variety of trade names and is used as a thickener and emulsifier in various food and cosmetic...
- PrebioticPrebiotic (nutrition)Prebiotics are non-digestible food ingredients that stimulate the growth and/or activity of bacteria in the digestive system in ways claimed to be beneficial to health. They were first identified and named by Marcel Roberfroid in 1995. As a functional food component, prebiotics, like probiotics,...
—indigestible matter which encourages growth of gut floraGut floraGut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora.... - Resistant starchResistant starchResistant starch is starch and starch degradation products that escape digestion in the small intestine of healthy individuals. Resistant starch is considered the third type of dietary fiber, as it can deliver some of the benefits of insoluble fiber and some of the benefits of soluble fiber.Some...