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Except where noted otherwise, 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, industrially most often extracted from chicory. 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.
Origin and history
Inulin is a natural storage carbohydrate presenting in more than 36,000 species of plants including wheat, onion, bananas, garlic, asparagus, and chicory. For these plants, inulin is used for reserving energy as well as regulating cold resistance. It is osmotically active for it is soluble in water. The plants can change the osmotic potential of cells by changing the degree of polymerization of inulin molecules with hydrolysis process. Being able to change 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. The substance was named inulin because of Inula helenium, but it also called helenin, alatin, meniantin, etc. Indigestible polysaccharides were of great scientific concern in the beginning of twentieth century. Irvine used chemical method like methylation to study the molecule structure of inulin, and designed the isolation method for this new anhydrofructose. During the studies of renal tubule in 1930s, researches searched for a substance which can be a biomarker that is not absorbed or diffused after introducing into the tubule. Richards introduced inulin because of its high molecular weight and its resistance to enzymes. Today, inulin is provided by industry as active ingredient for functional foods; and it is also used for determination of the glomerular filtration rate.
Chemical structure and properties
Inulin is a heterogeneous collection of fructose polymers. It consists of glucosyl moiety and 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 fructooligosaccharides, 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, Inulin 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. Tri-dimensional gel network, consisting of insoluble sub-micron crystalline inulin particles, immobilizes large amount of water, assuring its physical stability. It can also improve the stability of foams and emulsions.
Inulin is increasingly used in processed foods because it has unusually adaptable characteristics. Its flavour ranges from bland to subtly sweet (approx. 10% 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 has many health benefits. Inulin increases calcium absorption and possibly magnesium absorption, while promoting the growth of beneficial intestinal bacteria. Chicory inulin is reported to increase absorption of calcium in girls with lower calcium absorption and in young men. In terms of nutrition, it is considered a form of soluble fiber and is sometimes categorized as a prebiotic. Conversely, it is also considered a FODMAP, a class of carbohydrates which are problematic for some individuals through causing overgrowth of intestinal methanogenic bacteria. The consumption of large quantities (in particular, by sensitive or unaccustomed individuals) can lead to gas and bloating, and products that contain inulin will sometimes include a warning to add it gradually to one's diet.
Due to the body's limited ability to process fructans, inulin has minimal increasing impact on blood sugar. It is considered suitable for diabetics and potentially helpful in managing blood sugar-related illnesses.
Nonhydrolyzed inulin can also be directly converted to ethanol in a simultaneous saccharification and fermentation process, which may have great potential for converting crops high in inulin into ethanol for fuel.
Inulin and its analog sinistrin are used to help measure kidney function by determining the glomerular filtration rate (GFR). GFR is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule per unit time. Inulin is of particular use as it is not secreted or reabsorbed in any appreciable amount at the nephron, allowing GFR to be calculated. However, due to clinical limitations, inulin and sinistrin, although characterised by better handling features, are rarely used for this purpose and creatinine values are the standard for determining an approximate GFR.
Inulin benefits the immune system by targeting gut-associated lymphoid tissues and especially Peyer’s patches, thus reduces risk of gastrointestinal dysfunction. In mice experiment, mice supplemented with inulin showed reduced carcinogen-induced aberrant crypt foci in the distal colon during exposure to pathogens or tumor inducers. Inulin-supplemented mice also showed lower mortality rate in exposure with Listeria. Research on Crohn’s disease patients suggested daily intake of inulin significantly decreases disease activity and significantly increases the amount of IL-10-positive mucosal dendritic cells and Toll-like receptors 2 and 4 of these cells. In experiment on infants at high risk of atopy, formula with galacto-oligosaccharides and inulin significantly decrease incidence of atopic dermatitis, which suggests inulin and galacto-oligosaccharides promote immune development.
Inulin enhances the growth and activities of selected beneficial bacteria or inhibits growth or activities of certain pathogenic bacteria, hence promoting colonic health. In vitro inulin was found to selectively stimulate the growth of Bifidobacteria and Lactobacilli, which are health beneficial bacteria. This phenomenon is probably attributable to that inulin affected short-chain fatty acids(SCFA) concentrations in the lumen. Butyrate, one of the well-studied SCFA, regulates many genes related to proliferation procedure, differentiation and apoptosis of colonic epithelial cells. And butyrate might stimulate growth of Bifidobacteria and Lactobacilli by reducing apoptosis.
Inulin is reported to decrease amount of cholesterol and triglycerides, hence benefits lipidemia and cardiovascular system. Studies on rats demonstrated that inulin affects cholesterolemia and triglyceride levels by decreasing postprandial cholesterolemia and triglyceridemia. These effects are caused by a reduction in the number of very low-density lipoprotein (VLDL) particles and by reducing of lipogenic enzyme activity. Results of human studies are controversial. A study of healthy young man showed reduction for tryglcerides and cholesterol individually. However, there are no differences of tryglcerides and cholesterol in a study on 64 young women.
Inulin is a prebiotic bifidus factor (enhances the growth of bifidobacteria), but it's nonviscous; its value for infant nutrition, gastrointestinal health, colon cancer prevention, blood sugar and lipid metabolism, bone mineralization, fatty liver disease, obesity, and immunity is controversial.
In a study of type 2 diabetic women, 10 g/day of inulin was shown to have significant beneficial effects on glycemic indices, antioxidant capacity and body weight, compared with a control group that was fed an equal amount of maltodextrin.
Inulin is used in medical tests to measure the total amount of extracellular volume and determine the function of the kidneys.
There is a single report of what is claimed to be an allergic reaction to inulin in the literature.
About 30–40% of people in Central Europe suffer from fructose malabsorption. Since inulin is a fructan, excess dietary intake may lead to minor side effects, such as increased flatulence and loose stools in those with fructose malabsorption. It is recommended that fructan intake for people with fructose malabsorption be kept to less than 0.5 grams/serving.
- Intestinal discomfort, including flatulence, bloating, stomach noises, belching, and cramping
- Proliferation of harmful bacteria. As a prebiotic, there is no guarantee that harmful bacteria will not develop.
- Anaphylatic allergic reaction. (Rare) Inulin is used for Glomerular filtration rate (GFR) testing, and in some isolated cases has resulted in allergic reaction, possibly linked to a food allergy response.
Inulins are polymers composed mainly of fructose units, and typically have a terminal glucose. The fructose units in inulins are joined by a β(2→1) glycosidic bond. In general, plant inulins contain between 20 and several thousand fructose units. Smaller compounds are called fructooligosaccharides, the simplest being 1-kestose, which has 2 fructose units and 1 glucose unit.
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-[beta-D-fructofuranosyl](n-1)-D-fructofuranosides, abbreviated as GpyFn.
- Inulins without glucose are beta-D-fructopyranosyl-[D-fructofuranosyl](n-1)-D-fructofuranosides, abbreviated as FpyFn.
Calculation of glomerular filtration rate
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.
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. 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, 125 I-iothalamate, the chromium radioisotope 51Cr (chelated with EDTA), sodium radioiothalamate, 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.)
Fate in vivo
Inulin is indigestible by the human enzymes ptyalin and amylase, which are adapted to digest starch. As a result, inulin passes through much of the digestive system intact. It is only in the colon that bacteria metabolise inulin, with the release of significant quantities of carbon dioxide, hydrogen, and/or 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, 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 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. 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. This contrasts with proprietary probiotic formulations based on lactic acid bacteria (LAB) in which the bacteria have to survive very challenging conditions through the gastrointestinal tract before they are able to colonize the gut.
Some traditional diets contain over 20 g per day of inulin or fructooligosaccharides. 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 fructooligosaccharides, such as chicory, garlic, and leek, have been seen as "stimulants of good health" for centuries.
Plants that contain high concentrations of inulin include:
- Agave (Agave spp.)
- Burdock (Arctium lappa)
- Camas (Camassia spp.)
- Chicory (Cichorium intybus)
- Coneflower (Echinacea spp.)
- Costus Saussurea lappa
- Dandelion (Taraxacum officinale)
- Elecampane (Inula helenium)
- Garlic (Allium sativum)
- Jerusalem artichoke (Helianthus tuberosus)
- Jicama (Pachyrhizus erosus)
- Leopard's-bane (Arnica montana)
- Mugwort (Artemisia vulgaris)
- Onion (Allium cepa)
- Wild yam (Dioscorea spp.)
- Yacón (Smallanthus sonchifolius spp.)
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