Inulins are a group of naturally occurring polysaccharides (several simple sugars linked together) produced by many types of plants. 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.
Inulins are named in the following manner, where n is the number of fructose residues and py is the abbreviation for pyranosyl:
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%.
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, iso-hexanol, the radioisotope Chromium51 (chelated with EDTA) and creatinine, have had their utility confirmed in large cohorts of patients with chronic kidney disease.
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 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. 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 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.
Inulin is also 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, but dietary inulin has small amounts of bacteria and fungal spores and this case is most likely to represent a reaction to one of these contaminants: every day billions of people eat inulin-containing foods, e.g. onions, without any suggestion of allergy.
About 30–40% of people suffer from fructose malabsorption. Since inulin is a fructan, excess dietary intake may lead to minor side effects such as increased flatulence and loosened bowel motions 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.