| Inulin | |
|---|---|
| Identifiers | |
| CAS number | 9005-80-5 |
| PubChem | 24763 |
| UNII | JOS53KRJ01 |
| DrugBank | DB00638 |
| ChEMBL | CHEMBL1201646 |
| Properties | |
| Molecular formula | C6nH10n+2O5n+1 |
| Molar mass | Polymer; depends on n |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) | |
| Infobox references | |
Inulins are a group of naturally occurring polysaccharides produced by many types of plants.[1] 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. Most plants that synthesize and store inulin do not store other materials such as starch.
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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).[2] 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).[3] While inulin is a versatile ingredient, it also has health benefits. Inulin increases calcium absorption[4] and possibly magnesium absorption,[5] while promoting the growth of intestinal bacteria. In terms of nutrition, it is considered a form of soluble fiber and is sometimes categorized as a prebiotic. Due to the body's limited ability to process fructans, inulin has minimal increasing impact on blood sugar, and—unlike fructose—is not insulemic and does not raise triglycerides,[6] making it considered suitable for diabetics and potentially helpful in managing blood sugar-related illnesses. 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.
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.[7]
Inulin is 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.[8] Inulin is of particular use as it is not secreted or reabsorbed in any appreciable amount at the nephron allowing GFR to be calculated, rather than total renal filtration. However, due to clinical limitations, inulin is rarely used for this purpose and creatinine values are the standard for determining an approximate GFR. It is also used for rehydration and remineralization following important loss of water, like diarrhea and diaphoresis. Inulin can also be used as a vaccine adjuvant.[9]
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:
Hydrolysis of inulins may yield fructooligosaccharides, which are oligomers with a degree of polymerization (DP) of <= 10.
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 actually RPF by about 10%.[10]
The measurement of GFR by inulin 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, iso-hexanol, the radioisotope Chromium51 (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.)
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.
There are three types of dietary fiber; soluble, insoluble, and resistant starch. Insoluble fiber increases the movement of materials through the digestive system and increases stool bulk; it is especially helpful for those suffering from constipation or stool irregularity. Soluble fiber dissolves in water to form a gelatinous material. Some soluble fibers may help lower blood cholesterol and glucose levels. Inulin is considered a soluble fiber.
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.[6] 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.[11] Many foods naturally high in inulin or fructooligosaccharides, such as chicory, garlic, and leek, have been seen as "stimulants of good health" for centuries.[12]
Inulin is also used in medical tests to measure the total amount of extracellular volume and determine the function of the kidneys.[13]
Inulin is generally recognized as safe (GRAS) by the U.S. Food and Drug Administration (FDA).[14]
There is a single report of what is claimed to be an allergic reaction to inulin in the literature,[15] but dietary inulin is accompanied by small amounts of bacteria and fungal spores and so this case could represent a reaction to one of these contaminants.
About 30–40% of people in Central Europe suffer from fructose malabsorption.[16] 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.[17] It is recommended that fructan intake for people with fructose malabsorption be kept to less than 0.5 grams/serving.[17]
Plants that contain high concentrations of inulin include:
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