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ChEMBL | |
ChemSpider |
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DrugBank | |
ECHA InfoCard | 100.029.701 |
KEGG | |
PubChem CID | |
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CompTox Dashboard (EPA) | |
Properties | |
C6nH10n+2O5n+1 | |
Molar mass | Polymer; depends on n |
Pharmacology | |
V04CH01 ( WHO ) | |
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NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Inulins are a group of naturally occurring polysaccharides produced by many types of plants, [1] industrially most often extracted from chicory. [2] The inulins belong to a class of dietary fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes. Most plants that synthesize and store inulin do not store other forms of carbohydrate such as starch. In 2018, the United States Food and Drug Administration approved inulin as a dietary fiber ingredient used to improve the nutritional value of manufactured food products. [3] Using inulin to measure kidney function is the "gold standard" for comparison with other means of estimating glomerular filtration rate. [4]
Inulin is a natural storage carbohydrate present in more than 36,000 species of plants, including agave, wheat, onion, bananas, garlic, asparagus, Jerusalem artichoke, and chicory. For these plants, inulin is used as an energy reserve and for regulating cold resistance. [5] [6] 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. [7]
Inulin was discovered in 1804 by German scientist Valentin Rose. He found "a peculiar substance" from Inula helenium roots by boiling-water extraction. [7] [8] In the 1920s, J. Irvine used chemical methods such as methylation to study the molecular structure of inulin, and he designed the isolation method for this new anhydrofructose. [8] [9] 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. [10] [11] A. N. Richards introduced inulin because of its high molecular weight and its resistance to enzymes. [10] Inulin is used to determine glomerular filtration rate of the kidneys. [12]
Inulin is a heterogeneous collection of fructose polymers. It consists of chain-terminating glucosyl moieties and a repetitive fructosyl moiety, [13] 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. [5] [6] Some articles considered the fractions with DP lower than 10 as short-chained fructo-oligosaccharides, and only called the longer-chained molecules inulin. [7]
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. [14] It can also improve the stability of foams and emulsions. [6]
Chicory root is the main source of extraction for commercial production of inulin. The extraction process for inulin is similar to obtaining sugar from sugar beets. [5] After harvest, the chicory roots are sliced and washed, then soaked in a solvent (hot water or ethanol); [15] the inulin is then isolated, purified, and spray dried. Inulin may also be synthesized from sucrose. [5]
Inulin received no-objection status as generally recognized as safe (GRAS) from the U.S. Food and Drug Administration (FDA), [16] including long-chain inulin as GRAS. [17] In the early 21st century, the use of inulin in processed foods was due in part to its adaptable characteristics for manufacturing. [18] It is approved by the FDA as an ingredient to enhance the dietary fiber value of manufactured foods. [3] 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). [19] [20] In addition to being a versatile ingredient, inulin provides nutritional advantages by increasing calcium absorption [21] and possibly magnesium absorption, [22] while promoting the growth of intestinal bacteria. [18] Chicory inulin is reported to increase absorption of calcium in young women with lower calcium absorption [23] and in young men. [1] In terms of nutrition, it is considered a form of soluble fiber and is sometimes categorized as a prebiotic. [18] Conversely, it is also considered a FODMAP, a class of carbohydrates which are rapidly fermented in the colon producing gas. [18] 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. [24] [25] [26]
Due to the body's limited ability to process fructans, inulin has minimal increasing impact on blood sugar, and may potentially have use in managing blood sugar-related illnesses, such as metabolic syndrome. [27]
Inulin and its analog sinistrin 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 per unit time. [28]
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. [29] To determine the glomerular filtration rate in humans, a large initial dose of inulin is injected, which is followed by a constant infusion of inulin at a rate which compensates for its loss in the urine, thus maintaining a reasonably constant level in the plasma. [30] : 228 In the United States, creatinine clearance is more widely used to estimate GFR. [31]
A 2017 systematic review of low-to-moderate quality clinical trial research showed that dietary supplementation with inulin-type fructans reduced blood levels of low-density cholesterol, a biomarker of cardiovascular disease. [27]
In doses of 15–50 grams per day, dietary inulin increases the frequency of defecation. [32] Possible side effects of regularly using inulin in the diet include gastrointestinal discomfort, bloating, flatulence, diarrhea, and stomach inflammation in people with allergies to inulin. [32] [33] [34]
Nonhydrolyzed inulin can also be directly converted to ethanol in a simultaneous saccharification and fermentation process, which may have potential for converting crops high in inulin into ethanol for fuel. [35]
This section needs additional citations for verification .(February 2021) |
Inulins are polymers composed mainly of fructose units (fructans), and typically have a terminal glucose. 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. [36] : 58 In general, plant inulins contain between 2 and 70 fructose units [36] : 58 or sometimes as high as 200, [37] : 17 but molecules with less than 10 units are called fructo-oligosaccharides, the simplest being 1-kestose, 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. [37] : 17
Inulins are named in the following manner, where n is the number of fructose residues and py is the abbreviation for pyranosyl:
Hydrolysis of inulins may yield fructo-oligosaccharides, which are oligomers with a degree of polymerization (DP) of 10 or less.
Inulin is uniquely treated by nephrons 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.[ citation needed ]
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%. [38]
The measurement of GFR by inulin or sinistrin is still considered the gold standard. 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, 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.[ citation needed ]
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).[ citation needed ]
Inulin is indigestible by the human enzymes ptyalin and amylase which are adapted to digest starch and as a result, it passes through much of the digestive system intact. Only in the colon do bacteria metabolise inulin with the release of significant quantities of carbon dioxide, hydrogen, and/or methane. Inulin-containing foods can be rather gassy, especially for those unaccustomed to inulin and these foods should be consumed in moderation at first.[ citation needed ]
Inulin is a soluble fiber, one of three types of dietary fiber including soluble, insoluble and resistant starch. Soluble fiber dissolves in water to form a gelatinous material. Some soluble fibers may help lower blood cholesterol and glucose levels. [39]
Because normal digestion does not break inulin down into monosaccharides, 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. [5] Inulin passes through the stomach and duodenum undigested and is highly available to the gut bacterial flora. This makes it similar to resistant starches and other fermentable carbohydrates.[ citation needed ]
Some traditional diets contain over 20g 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. [40] Many foods naturally high in inulin or fructo-oligosaccharides such as chicory, garlic, and leek have been seen as "stimulants of good health" for centuries. [41]
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 and it does not affect triglyceride levels or development of fatty liver. It may also help to prevent travelers' diarrhea and may help increase calcium absorption in adolescents. [18]
Plants that contain high concentrations of inulin include:
A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cm(H2O)n, which does not mean the H has covalent bonds with O. However, not all carbohydrates conform to this precise stoichiometric definition, nor are all chemicals that do conform to this definition automatically classified as carbohydrates.
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 water (hydrolysis) using amylase enzymes as catalyst, which produces constituent sugars. They range in structure from linear to highly branched. Examples include storage polysaccharides such as starch, glycogen and galactogen and structural polysaccharides such as hemicellulose and chitin.
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 absorbed by the gut directly into the blood of the portal vein during digestion. The liver then converts most fructose and galactose into glucose for distribution in the bloodstream or deposition into glycogen.
Dietary fiber or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition and can be grouped generally by their solubility, viscosity and fermentability which affect how fibers are processed in the body. Dietary fiber has two main subtypes: soluble fiber and insoluble fiber which are components of plant-based foods such as legumes, whole grains, cereals, vegetables, fruits, and nuts or seeds. A diet high in regular fiber consumption is generally associated with supporting health and lowering the risk of several diseases. Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, pectins, beta-glucans, and oligosaccharides.
The glycemic (glycaemic) index is a number from 0 to 100 assigned to a food, with pure glucose arbitrarily given the value of 100, which represents the relative rise in the blood glucose level two hours after consuming that food. The GI of a specific food depends primarily on the quantity and type of carbohydrate it contains, but is also affected by the amount of entrapment of the carbohydrate molecules within the food, the fat and protein content of the food, the amount of organic acids in the food, and whether it is cooked and, if so, how it is cooked. GI tables, which list many types of foods and their GIs, are available. A food is considered to have a low GI if it is 55 or less; high GI if 70 or more; and mid-range GI if 56 to 69.
Renal functions include maintaining an acid–base balance; regulating fluid balance; regulating sodium, potassium, and other electrolytes; clearing toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
An oligosaccharide is a saccharide polymer containing a small number of monosaccharides. Oligosaccharides can have many functions including cell recognition and cell adhesion.
Fructose malabsorption, formerly named dietary fructose intolerance (DFI), is a digestive disorder in which absorption of fructose is impaired by deficient fructose carriers in the small intestine's enterocytes. This results in an increased concentration of fructose. Intolerance to fructose was first identified and reported in 1956.
Fructooligosaccharides (FOS) also sometimes called oligofructose or oligofructan, are oligosaccharide fructans, used as an alternative sweetener. FOS exhibits sweetness levels between 30 and 50 percent of sugar in commercially prepared syrups. It occurs naturally, and its commercial use emerged in the 1980s in response to demand for healthier and calorie-reduced foods.
Prebiotics are compounds in food that foster growth or activity of beneficial microorganisms such as bacteria and fungi. The most common environment concerning their effects on human health is the gastrointestinal tract, where prebiotics can alter the composition of organisms in the gut microbiome.
Polydextrose is a synthetic polymer of glucose. It is a food ingredient classified as soluble fiber by the US FDA as well as Health Canada, as of April 2013. It is frequently used to increase the dietary fiber content of food, to replace sugar, and to reduce calories and fat content. It is a multi-purpose food ingredient synthesized from dextrose (glucose), plus about 10 percent sorbitol and 1 percent citric acid. Its E number is E1200. The FDA approved it in 1981.
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, leeks, garlic, onions, yacón, jícama, barley and wheat.
Agave syrup, also known as maguey syrup or agave nectar, is a sweetener commercially produced from several species of agave, including Agave tequilana and Agave salmiana. Blue-agave syrup contains 56% fructose as a sugar providing sweetening properties.
Levan is a naturally occurring fructan present in many plants and microorganisms. This polymer is made up of fructose, a monosaccharide sugar, connected by 2,6 beta glycosidic linkages. Levan can have both branched and linear structures of relatively low molecular weight. Branched levan forms a very small, sphere-like structure with basal chains 9 units long. The 2,1 branching allows methyl ethers to form and create a spherical shape. The ends of levan also tend to contain a glucosyl residue. Branched levan tends to be more stable than linear polysaccharides. However, the amount of branching and length of polymerization tends to vary among different species. The shortest levan is 6-kestose, a chain of two fructose molecules and a terminal glucose molecule.
FODMAPs or fermentable oligosaccharides, disaccharides, monosaccharides, and polyols are short-chain carbohydrates that are poorly absorbed in the small intestine and ferment in the colon. They include short-chain oligosaccharide polymers of fructose (fructans) and galactooligosaccharides, disaccharides (lactose), monosaccharides (fructose), and sugar alcohols (polyols), such as sorbitol, mannitol, xylitol, and maltitol. Most FODMAPs are naturally present in food and the human diet, but the polyols may be added artificially in commercially prepared foods and beverages.
Xylooligosaccharides (XOS) are polymers of the sugar xylose. They are produced from the xylan fraction in plant fiber. Their C5 structure is fundamentally different from other prebiotics, which are based upon C6 sugars. Xylooligosaccharides have been commercially available since the 1980s, originally produced by Suntory in Japan. They have more recently become more widely available commercially, as technologies have advanced and production costs have fallen. Some enzymes from yeast can exclusively convert xylan into only xylooligosaccharides-DP-3 to 7.
Isomaltooligosaccharide (IMO) is a mixture of short-chain carbohydrates which has a digestion-resistant property. IMO is found naturally in some foods, as well as being manufactured commercially. The raw material used for manufacturing IMO is starch, which is enzymatically converted into a mixture of isomaltooligosaccharides.
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 storage molecule in plants.
Kestose is a class of sugars that belongs to a group of fructooligosaccharides.