Inulin

Inulins are a group of naturally occurring polysaccharides (several simple sugars linked together) 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 which synthesize and store inulin do not store other materials such as starch.

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Uses

Processed foods

Inulin is used increasingly in foods because it has unusual nutritional characteristics. It ranges from completely bland to subtly sweet and can be used to replace sugar, fat, and flour. This is particularly advantageous because inulin contains a third to a quarter of the food energy of sugar or other carbohydrates and a sixth to a ninth of the food energy of fat. It also increases calcium absorption^[2] and possibly magnesium absorption,^[3] while promoting intestinal bacteria. Nutritionally, it is considered a form of soluble fiber, and it is important to note that consuming large quantities (particularly for sensitive and/or unaccustomed individuals) can lead to gas and bloating. Inulin has a minimal impact on blood sugar, making it generally considered suitable for diabetics and potentially helpful in managing blood sugar-related illnesses.

Industrial use

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.^[4]

Medical

Inulin is used to help measure kidney function by determining something called 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.^[5]

Biochemistry

Inulins are polymers mainly comprised of fructose units and typically have a terminal glucose. The fructose units in inulins are joined by a beta-(2-1) glycosidic bond. Plant inulins generally contain between 20 to several thousand fructose units. Smaller compounds are called fructo-oligosacchrides, the simplest of these is 1-ketose, 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.

Hydrolysis of inulins may yield fructo-oligosaccharides, which are oligomers with a degree of polymerization (DP) of <= 10.

Calculation of Glomerular Filtration Rate (GFR)

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 percentage of plasma from the afferent arteriole that is filtered into Bowman's capsule.

It is useful to contrast the properties of inulin with those of para-aminohippuric acid (PAH). PAH is completely 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%.^[6]

The measurement of GFR by inulin is still considered the gold-standard. Practically, 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 and creatinine, have had their utility confirmed in large cohorts of patients with chronic kidney disease.

Health effects

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, particularly for those unaccustomed to inulin, and these foods should be consumed in moderation at first.

There are two types of dietary fiber, soluble and insoluble. 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 fibres 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.^{[citation needed]} Inulin passes through the stomach and duodenum undigested and is highly available to the gut bacterial flora. 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 up to 20g per day of inulin or fructo-oligosaccharides. 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.^[7]

Inulin is also used in medical tests to measure the total amount of extracellular volume and determine the function of the kidneys.^[8]

Inulin is generally recognized as safe (GRAS) by the FDA.^[9]

Contrary to the health benefits as mentioned above, allergic reactions to inulin in foods have been reported in a letter^[10] to the New England Journal of Medicine.

Between about 30-40% of the population suffers from fructose malabsorption.^[11] Since inulin is a fructan, it is problematic for people with fructose malabsorption.^[12] It is recommended that fructan intake for people with fructose malabsorption be kept to less than 0.5 grams/serving.^[12]

Natural sources of inulin

Plants that contain high concentrations of inulin include:

• Elecampane (Inula helenium)
• Dandelion (Taraxacum officinale)
• Wild Yam (Dioscorea spp.)
• Jerusalem artichokes (Helianthus tuberosus)
• Chicory (Cichorium intybus)
• Jicama (Pachyrhizus erosus)
• Burdock (Arctium lappa)
• Onion (Allium cepa)
• Garlic (Allium sativum)
• Agave (Agave spp.)
• Yacon (Smallanthus sonchifolius spp.)

References