イヌリン

サプリメント > 食物繊維 > Inulin/ja

Inulin/ja

Identifiers
CAS Number	9005-80-5 Y
ChEMBL	ChEMBL1201646 Y
ChemSpider	none
DrugBank	DB00638 Y
KEGG	D00171
PubChem CID	24763
UNII	JOS53KRJ01 Y
Properties
Chemical formula	C6nH10n+2O5n+1
Molar mass	Polymer; depends on n
Pharmacology
ATC code	V04CH01 (WHO)
Hazards
NFPA 704 (fire diamond)	1 1 0

イヌリン(English: Inulin)は多くの種類の植物が生産する天然多糖類の一群で、工業的にはチコリから抽出されることが多い。イヌリンは、フルクタンとして知られる食物繊維の一群に属する。イヌリンは、一部の植物がエネルギーを貯蔵する手段として利用し、通常、根や根茎に含まれている。イヌリンを合成し貯蔵するほとんどの植物は、デンプンのような他の形態の炭水化物を貯蔵しない。米国では2018年、食品医薬品局はイヌリンを、製造された食品の栄養価を向上させるために使用される食物繊維成分として承認した。イヌリンを用いて腎機能を測定することは、糸球体濾過量を推定する他の手段と比較するための「ゴールドスタンダード」である。

起源と歴史

イヌリンは、リュウゼツラン、小麦、タマネギ、バナナ、ニンニク、アスパラガス、エルサレム・アーティチョーク、チコリなど、36,000種以上の植物に含まれる天然の貯蔵炭水化物である。これらの植物にとって、イヌリンはエネルギー備蓄として、また耐寒性を調節するために利用されている。イヌリンは水に溶けるため、浸透圧活性である。ある種の植物は、加水分解によってイヌリン分子の重合度を変えることで、細胞の浸透圧ポテンシャルを変えることができる。炭水化物の総量を変えずに浸透圧ポテンシャルを変えることで、植物は冬の寒さや乾燥に耐えることができる。

イヌリンは1804年、ドイツの科学者ヴァレンティン・ローズによって発見された。彼はイヌラヘレニウムの根から沸騰水抽出によって「特異な物質」を発見した。1920年代、J.アーバインはイヌリンの分子構造を研究するためにメチル化などの化学的方法を用い、この新しいアンヒドロフラクトースの単離方法を考案した。1930年代、腎尿細管の研究において、研究者たちは尿細管に導入された後、再吸収も分泌もされないバイオマーカーとなる物質を探した。A.N.リチャーズは、その高分子量と酵素に対する抵抗性からイヌリンを導入した。イヌリンは腎臓の糸球体濾過率を測定するために使用される。

化学構造と特性

イヌリンはフルクトースポリマーの異種集合体である。鎖末端のグルコシル部分と繰り返しのフルクトシル部分からなり、これらはβ(2,1)結合で結ばれている。標準的なイヌリンの重合度（DP）は2〜60である。製造工程でDPが10以下のフラクションを除去した後、残った製品が高性能イヌリンとなる。DPが10より低い画分を短鎖のフラクトオリゴ糖とみなし、より長鎖の分子だけをイヌリンと呼ぶ論文もある。

β(2,1)結合のため、イヌリンはヒトの消化器系では酵素によって消化されず、カロリー値の低減、食物繊維、プレバイオティクス効果といった機能的特性に寄与している。また、色や匂いもなく、食品の官能特性にはほとんど影響を与えない。オリゴフルクトースの甘味度はショ糖の35％で、甘味のプロファイルは砂糖に似ている。標準的なイヌリンはわずかに甘いが、高機能イヌリンは甘くない。溶解性は古典的な繊維よりも高い。液体と十分に混合すると、イヌリンはゲルを形成し、脂肪に似た白いクリーム状の構造を形成する。不溶性のサブミクロン結晶イヌリン粒子からなるその三次元ゲルネットワークは、大量の水を固定化し、その物理的安定性を保証する。また、フォームやエマルションの安定性を向上させることができる。

用途

収穫と抽出

チコリの根は、イヌリンの商業生産のための主な抽出源である。イヌリンの抽出プロセスは、テンサイから砂糖を得るのと似ている。収穫後、チコリの根をスライスし、洗浄した後、溶媒（熱水またはエタノール）に浸し、イヌリンを分離、精製し、噴霧乾燥する。イヌリンはスクロースから合成することもできる。

Processed foods

Inulin received no-objection status as generally recognized as safe (GRAS) from the US Food and Drug Administration (FDA), including long-chain inulin as GRAS. In the early 21st century, the use of inulin in processed foods was due in part to its adaptable characteristics for manufacturing. It is approved by the FDA as an ingredient to enhance the dietary fiber value of manufactured foods. 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). In addition to being a versatile ingredient, inulin provides nutritional advantages by increasing calcium absorption and possibly magnesium absorption, while promoting the growth of intestinal bacteria. Chicory inulin is reported to increase absorption of calcium in young women 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 rapidly fermented in the colon producing gas. 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.

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.

Medical

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.

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. 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. In the United States, creatinine clearance is more widely used to estimate GFR.

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.

Diet and side effects

The side effects of inulin dietary fiber diet, which may occur, usually in sensitive persons, are:

• Intestinal discomfort, including flatulence, bloating, stomach noises, belching, and cramping
• Diarrhea
• Inflammation – inulin can cause an allergy-related type of inflammation in the gut and lungs
• Anaphylactic allergic reaction (rare) – inulin is used for GFR testing, and in some isolated cases has resulted in an allergic reaction, possibly linked to a food allergy response.

Industrial use

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.

Biochemistry

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. In general, plant inulins contain between 2 and 70 fructose units or sometimes as high as 200, 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.

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 or less.

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, 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.

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).

Metabolism in vivo

Inulin is indigestible by the human enzymes ptyalin and amylase, which are adapted to digest starch. 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, 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.

Some traditional diets contain over 20 g 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. 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.

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, does not affect triglyceride levels or development of fatty liver, may help prevent travelers' diarrhea, and may help increase calcium absorption in adolescents.

Natural sources

Plants that contain high concentrations of inulin include:

• Agave (Agave spp.)
• Banana and plantain (Musaceae)
• Burdock (Arctium lappa)
• Camas (Camassia spp.)
• Chicory (Cichorium intybus)
• Coneflower (Echinacea spp.)
• Costus (Saussurea lappa)
• Dandelion (Taraxacum officinale)
• Elecampane (Inula helenium)
• Garlic (Allium sativum)
• Globe artichoke (Cynara scolymus, Cynara cardunculus var. scolymus)
• Jerusalem artichoke (Helianthus tuberosus)
• Jicama (Pachyrhizus erosus)
• Leopard's bane (Arnica montana)
• Mugwort root (Artemisia vulgaris)
• Onion (Allium cepa)
• Wild yam (Dioscorea spp.)
• Yacón (Smallanthus sonchifolius)