Inulins are a group of naturally occurring
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
s produced by many types of
plants,
industrially most often extracted from
chicory. The inulins belong to a class of
dietary fiber
Dietary fiber (in British English fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition, and can be grouped generally by t ...
s known as
fructan
A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides. Fructans can be found in over 12% of the angiosperms including both monocots and dicots such as agave, artichokes, asparagus, l ...
s. Inulin is used by some plants as a means of storing energy and is typically found in
root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s or
rhizomes. Most plants that synthesize and store inulin do not store other forms of carbohydrate such as
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
. In the United States in 2018, the
Food and Drug Administration
The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
approved inulin as a dietary fiber ingredient used to improve the
nutrition
Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It provides organisms with nutrients, which can be metabolized to create energy and chemical structures. Failure to obtain sufficient ...
al value of manufactured food products.
Using inulin to measure
kidney function is the
"gold standard" for comparison with other means of estimating
glomerular filtration rate.
Origin and history
Inulin is a natural storage carbohydrate present in more than 36,000 species of plants, including
agave,
wheat
Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...
,
onion
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the on ...
,
bananas,
garlic
Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...
,
asparagus,
Jerusalem artichoke, and chicory. For these plants, inulin is used as an energy reserve and for regulating cold resistance.
Because it is soluble in water, it is osmotically active. Certain plants can change the
osmotic potential of their cells by changing the degree of polymerization of inulin molecules by hydrolysis. By changing osmotic potential without changing the total amount of carbohydrate, plants can withstand cold and drought during winter periods.
Inulin was discovered in 1804 by German scientist Valentin Rose. He found “a peculiar substance” from ''
Inula helenium'' roots by boiling-water extraction.
In the 1920s, Irvine used chemical methods such as methylation to study the molecular structure of inulin, and he designed the isolation method for this new anhydrofructose.
During studies of renal tubules in the 1930s, researchers searched for a substance that could serve as a biomarker that is not reabsorbed or secreted after introduction into tubules.
Richards introduced inulin because of its high molecular weight and its resistance to enzymes.
Inulin is used to determine
glomerular filtration rate of the
kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...
s.
Chemical structure and properties
Inulin is a heterogeneous collection of
fructose
Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...
polymers. It consists of chain-terminating
glucosyl
A glycosyl group is a univalent free radical or substituent structure obtained by removing the hemiacetal hydroxyl group from the cyclic form of a monosaccharide and, by extension, of a lower oligosaccharide.
Glycosyl also reacts with inorga ...
moieties and a repetitive fructosyl moiety, which are linked by β(2,1) bonds. The degree of polymerization (DP) of standard inulin ranges from 2 to 60. After removing the fractions with DP lower than 10 during manufacturing process, the remaining product is high-performance inulin.
Some articles considered the fractions with DP lower than 10 as short-chained fructo-oligosaccharides, and only called the longer-chained molecules inulin.
Because of the β(2,1) linkages, inulin is not digested by enzymes in the human alimentary system, contributing to its functional properties: reduced calorie value, dietary fiber, and
prebiotic effects. Without color and odor, it has little impact on sensory characteristics of food products. Oligofructose has 35% of the sweetness of sucrose, and its sweetening profile is similar to sugar. Standard inulin is slightly sweet, while high-performance inulin is not. Its solubility is higher than the classical fibers. When thoroughly mixed with liquid, inulin forms a gel and a white creamy structure, which is similar to fat. Its three-dimensional gel network, consisting of insoluble submicron crystalline inulin particles, immobilizes a large amount of water, assuring its physical stability.
It can also improve the stability of foams and emulsions.
Uses
Harvesting and extraction
Chicory root is the main source of
extraction Extraction may refer to:
Science and technology
Biology and medicine
* Comedo extraction, a method of acne treatment
* Dental extraction, the surgical removal of a tooth from the mouth
Computing and information science
* Data extraction, the pr ...
for commercial production of inulin. The extraction process for inulin is similar to obtaining
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or do ...
from
sugar beet
A sugar beet is a plant whose root contains a high concentration of sucrose and which is grown commercially for sugar production. In plant breeding, it is known as the Altissima cultivar group of the common beet ('' Beta vulgaris''). Together ...
s.
After harvest, the chicory roots are sliced and washed, then soaked in a
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
(hot water or ethanol); the inulin is then isolated, purified, and spray dried. Inulin may also be
synthesized from
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refine ...
.
Processed foods
Inulin received no-objection status as
generally recognized as safe (GRAS) from the
US Food and Drug Administration (FDA), including long-chain inulin as GRAS. In the early 21st century, the use of inulin in processed foods was due in part to its adaptable characteristics for manufacturing.
It is approved by the FDA as an ingredient to enhance the dietary fiber value of manufactured foods.
[ Its flavor ranges from bland to subtly sweet (about 10% of the sweetness of sugar/sucrose). It can be used to replace sugar, fat, and flour. This is advantageous because inulin contains 25–35% of the food energy of carbohydrates (starch, sugar). In addition to being a versatile ingredient, inulin provides nutritional advantages by increasing ]calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
absorption and possibly magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
absorption, while promoting the growth of intestinal bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
.[ Chicory inulin is reported to increase absorption of calcium in young women with lower calcium absorption and in young men.] In terms of nutrition, it is considered a form of soluble fiber
Dietary fiber (in British English fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition, and can be grouped generally by th ...
and is sometimes categorized as a prebiotic.[ Conversely, it is also considered a ]FODMAP
FODMAPs or fermentable oligosaccharides, disaccharides, monosaccharides, and polyols are short chain carbohydrates that are poorly absorbed in the small intestine and are prone to absorb water and ferment in the colon. They include short chain oli ...
, a class of carbohydrates which are rapidly fermented in the colon producing gas.[ Although FODMAPs can cause certain digestive discomfort in some people, they produce potentially favorable alterations in the intestinal flora that contribute to maintaining health of the colon.][ ][ ]
Due to the body's limited ability to process fructans, inulin has minimal increasing impact on blood sugar
Glycaemia, also known as blood sugar level, blood sugar concentration, or blood glucose level is the measure of glucose concentrated in the blood of humans or other animals. Approximately 4 grams of glucose, a simple sugar, is present in the b ...
, and may potentially have use in managing blood sugar-related illnesses, such as metabolic syndrome
Metabolic syndrome is a clustering of at least three of the following five medical conditions: abdominal obesity, high blood pressure, high blood sugar, high serum triglycerides, and low serum high-density lipoprotein (HDL).
Metabolic syndro ...
.
Medical
Inulin and its analog sinistrin
Sinistrin is a naturally occurring sugar polymer or polysaccharide, also known as polyfructosane. It belongs to the fructan group, like inulin. As it is the case with similar substances, such as fructans or inulin, sinistrin acts as an energy stor ...
are used to help measure kidney function by determining the glomerular filtration rate (GFR), which is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule
Bowman's capsule (or the Bowman capsule, capsula glomeruli, or glomerular capsule) is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form u ...
per unit time. While inulin is the gold standard for measuring the GFR, it is rarely used in practice due to the expense and difficulty in conducting the test; it requires intravenous (IV) access for the infusion of inulin as well as up to twelve blood samples taken from the patient over the course of four hours. In the United States, creatinine clearance is more widely used to estimate GFR.
Inulin enhances the growth and activities of bacteria or inhibits growth or activities of certain pathogenic bacteria.
Research has linked it to several health benefits, such as helping control diabetes, aiding weight loss and improving digestive health.
A 2017 systematic review
A systematic review is a scholarly synthesis of the evidence on a clearly presented topic using critical methods to identify, define and assess research on the topic. A systematic review extracts and interprets data from published studies on t ...
of clinical trial
Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...
results showed that dietary supplementation with inulin reduced biomarkers of metabolic syndrome.[
]
Diet and side effects
The side effects of inulin dietary fiber diet, which may occur, usually in sensitive persons, are:
* Intestinal discomfort, including flatulence, bloating, stomach noises, belching, and cramping
* Diarrhea
Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin w ...
* Inflammation
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
- inulin can cause an allergy-related type of inflammation in the gut and lungs
* Anaphylactic allergic reaction (rare) – inulin is used for GFR testing, and in some isolated cases has resulted in an allergic reaction, possibly linked to a food allergy response.
Industrial use
Nonhydrolyzed inulin can also be directly converted to ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
in a simultaneous saccharification and fermentation process, which may have potential for converting crops high in inulin into ethanol for fuel.
Biochemistry
Inulins are polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s composed mainly of fructose units ( fructans), and typically have a terminal glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
. The fructose units in inulins are joined by a β(2→1) glycosidic bond. The molecule is almost exclusively linear, with only a few percent branching. In general, plant inulins contain between 2 and 70 fructose units or sometimes as high as 200, but molecules with less than 10 units are called fructo-oligosaccharides, the simplest being 1-kestose
Kestose is a class of sugars that belongs to a group of fructooligosaccharides.
Chemical properties
Kestoses are typical fructooligosaccharides, and in its structure, one fructose molecule is combined with sucrose to form a trisaccharide. In ...
, which has two fructose units and one glucose unit. Bacterial inulin is more highly branched (more than 15% branching) and contains on the order of tens or hundreds of subunits.
Inulins are named in the following manner, where n is the number of fructose residues and py is the abbreviation for pyranosyl:
* Inulins ''with'' a terminal glucose are known as ''alpha''-D-glucopyranosyl- eta-D-fructofuranosyln-1)-D-fructofuranosides, abbreviated as GpyFn.
* Inulins ''without'' glucose are ''beta''-D-fructopyranosyl- -fructofuranosyln-1)-D-fructofuranosides, abbreviated as FpyFn.
Hydrolysis of inulins may yield fructo-oligosaccharides, which are oligomers with a degree of polymerization
The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule.
For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by ...
(DP) of 10 or less.
Calculation of glomerular filtration rate
Inulin is uniquely treated by nephrons
The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structur ...
in that it is completely filtered at the glomerulus but neither secreted nor reabsorbed by the tubules. This property of inulin allows the clearance of inulin to be used clinically as a highly accurate measure of glomerular filtration rate (GFR) — the rate of plasma from the afferent arteriole that is filtered into Bowman's capsule measured in ml/min.
It is informative to contrast the properties of inulin with those of para-aminohippuric acid (PAH). PAH is partially filtered from plasma at the glomerulus and not reabsorbed by the tubules, in a manner identical to inulin. PAH is different from inulin in that the fraction of PAH that bypasses the glomerulus and enters the nephron's tubular cells (via the peritubular capillaries) is completely secreted. Renal clearance of PAH is thus useful in calculation of renal plasma flow (RPF), which empirically is (1- hematocrit) times renal blood flow. Of note, the clearance of PAH is reflective only of RPF to portions of the kidney that deal with urine formation, and, thus, underestimates the actual RPF by about 10%.
The measurement of GFR by inulin or sinistrin is still considered the gold standard
A gold standard is a monetary system in which the standard economic unit of account is based on a fixed quantity of gold. The gold standard was the basis for the international monetary system from the 1870s to the early 1920s, and from th ...
. However, it has now been largely replaced by other, simpler measures that are approximations of GFR. These measures, which involve clearance of such substrates as EDTA, iohexol, cystatin C
Cystatin C or cystatin 3 (formerly gamma trace, post-gamma-globulin, or neuroendocrine basic polypeptide), a protein encoded by the CST3 gene, is mainly used as a biomarker of kidney function. Recently, it has been studied for its role in predicti ...
, 125I- iothalamate (sodium radioiothalamate), the chromium radioisotope 51Cr (chelated with EDTA), and creatinine, have had their utility confirmed in large cohorts of patients with chronic kidney disease.
For both inulin and creatinine, the calculations involve concentrations in the urine and in the serum. However, unlike creatinine, inulin is not naturally present in the body. This is an advantage of inulin (because the amount infused will be known) and a disadvantage (because an infusion is necessary).
Metabolism ''in vivo''
Inulin is indigestible by the human enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s ptyalin
α-Amylase is an enzyme (EC 3.2.1.1; systematic name 4-α-D-glucan glucanohydrolase) that hydrolyses α bonds of large, α-linked polysaccharides, such as starch and glycogen, yielding shorter chains thereof, dextrins, and maltose:
:Endohydr ...
and amylase, which are adapted to digest starch. As a result, it passes through much of the digestive system
The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
intact. Only in the colon do bacteria metabolise inulin, with the release of significant quantities of carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
, hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
, and/or methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
. Inulin-containing foods can be rather gassy, in particular for those unaccustomed to inulin, and these foods should be consumed in moderation at first.
Inulin is a soluble fiber, one of three types of dietary fiber including soluble, insoluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
, and resistant starch. Soluble fiber dissolves in water to form a gelatinous material. Some soluble fibers may help lower blood cholesterol and glucose levels.
Because normal digestion does not break inulin down into monosaccharide
Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built.
They are usually colorless, water- so ...
s, it does not elevate blood sugar levels and may, therefore, be helpful in the management of diabetes. Inulin also stimulates the growth of bacteria in the gut.
Inulin passes through the stomach and duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
undigested and is highly available to the gut bacterial flora. This makes it similar to resistant starches and other fermentable carbohydrates.
Some traditional diets contain over 20 g per day of inulin or fructo-oligosaccharides. The diet of the prehistoric hunter-forager in the Chihuahuan Desert has been estimated to include 135 g per day of inulin-type fructans. Many foods naturally high in inulin or fructo-oligosaccharides, such as chicory, garlic, and leek
The leek is a vegetable, a cultivar of '' Allium ampeloprasum'', the broadleaf wild leek ( syn. ''Allium porrum''). The edible part of the plant is a bundle of leaf sheaths that is sometimes erroneously called a stem or stalk. The genus '' Al ...
, have been seen as "stimulants of good health" for centuries.
Due to its resistance to digestive enzymes, inulin resists absorption during its transit through the upper gastrointestinal tract. After reaching the large intestine, inulin is converted by colonic bacteria to a gel known as a prebiotic, a food ingredient that is highly nourishing to gut microflora. As of 2013, no regulatory authority had permitted health claims in the marketing of prebiotics as a class. Inulin's health effects had been studied in small clinical trials, which showed that it causes gastrointestinal adverse effects such as bloating and flatulence, does not affect triglyceride levels or development of fatty liver
Fatty liver disease (FLD), also known as hepatic steatosis, is a condition where excess fat builds up in the liver. Often there are no or few symptoms. Occasionally there may be tiredness or pain in the upper right side of the abdomen. Complica ...
, may help prevent traveler’s diarrhea, and may help increase calcium absorption in adolescents.[
]
Natural sources
Plants that contain high concentrations of inulin include:
* Agave (''Agave'' spp.)
* Banana
A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", disting ...
and plantain (Musaceae
Musaceae is a family of flowering plants composed of three genera with about 91 known species, placed in the order Zingiberales. The family is native to the tropics of Africa and Asia. The plants have a large herbaceous growth habit with leaves ...
)
* Burdock
''Arctium'' is a genus of biennial plants commonly known as burdock, family Asteraceae. Native to Europe and Asia, several species have been widely introduced worldwide. Burdock's clinging properties, in addition to providing an excellent mech ...
(''Arctium lappa'')
* Camas (''Camassia'' spp.)
* Chicory (''Cichorium intybus'')
* Coneflower ('' Echinacea'' spp.)
* Costus
''Costus'' is a group of herbaceous perennial plants in the family Costaceae, described by Linnaeus as a genus in 1753. It was formerly known as ''Hellenia'' after the Finnish botanist Carl Niclas von Hellens. It is widespread through tropical ...
(''Saussurea lappa'')
* Dandelion (''Taraxacum officinale'')
* Elecampane (''Inula helenium'')
* Garlic
Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chive, Welsh onion and Chinese onion. It is native to South Asia, Central Asia and northeas ...
(''Allium sativum'')
* Globe artichoke (''Cynara scolymus'', ''Cynara cardunculus'' var. ''scolymus'')
* Jerusalem artichoke (''Helianthus tuberosus'')
* Jicama (''Pachyrhizus erosus'')
* Leopard's bane Leopard's bane or leopard's-bane may refer to:
*''Aconitum'', also known as aconite, monkshood, wolf's bane, women's bane, Devil's helmet or blue rocket, a genus of flowering plants belonging to the buttercup family
*''Arnica montana'', also known ...
('' Arnica montana'')
* Mugwort root (''Artemisia vulgaris
''Artemisia vulgaris'', the common mugwort, is a species of flowering plant in the daisy family Asteraceae. It is one of several species in the genus '' Artemisia'' commonly known as mugwort, although ''Artemisia vulgaris'' is the species most ...
'')
* Onion
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the on ...
(''Allium cepa'')
* Wild yam (''Dioscorea
''Dioscorea'' is a genus of over 600 species of flowering plants in the family Dioscoreaceae, native throughout the tropical and warm temperate regions of the world. The vast majority of the species are tropical, with only a few species extendi ...
'' spp.)
* ''Yacón
The yacón (''Smallanthus sonchifolius'') is a species of perennial daisy traditionally grown in the northern and central Andes from Colombia to northern Argentina for its crisp, sweet-tasting, tuberous roots. Their texture and flavour are ver ...
'' (''Smallanthus sonchifolius'')
References
{{carbohydrates
Polysaccharides
Food additives
Prebiotics (nutrition)