Sorbitol

Sorbitol (Template:IPAc-en), less commonly known as glucitol (Template:IPAc-en), is a sugar alcohol with a sweet taste which the human body metabolizes slowly. It can be obtained by reduction of glucose, which changes the converted aldehyde group (−CHO) to a primary alcohol group (−CH₂OH). Most sorbitol is made from potato starch, but it is also found in nature, for example in apples, pears, peaches, and prunes.<ref name=Teo>Template:Cite journal</ref> It is converted to fructose by sorbitol-6-phosphate 2-dehydrogenase. Sorbitol is an isomer of mannitol, another sugar alcohol; the two differ only in the orientation of the hydroxyl group on carbonTemplate:Nbs2.<ref name="Kearsley, M. W. pp 249-249">Kearsley, M. W.; Deis, R. C. Sorbitol and Mannitol. In Sweeteners and Sugar Alternatives in Food Technology; Ames: Oxford, 2006; pp 249-249-261.</ref> While similar, the two sugar alcohols have very different sources in nature, melting points, and uses.

As an over-the-counter drug, sorbitol is used as a laxative to treat constipation.<ref name="drugs">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Synthesis

Sorbitol may be synthesised via a glucose reduction reaction<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> in which the converted aldehyde group is converted into a hydroxyl group. The reaction requires NADH and is catalyzed by aldose reductase. Glucose reduction is the first step of the polyol pathway of glucose metabolism, and is implicated in multiple diabetic complications.

Template:Chem2

The mechanism involves a tyrosine residue in the active site of aldehyde reductase. The hydrogen atom on NADH is transferred to the electrophilic aldehyde carbon atom; electrons on the aldehyde carbon-oxygen double bond are transferred to the oxygen that abstracts the proton on tyrosine side chain to form the hydroxyl group. The role of aldehyde reductase tyrosine phenol group is to serve as a general acid to provide proton to the reduced aldehyde oxygen on glucose.

Mechanism of glucose aldehyde reduction into sorbitol

Glucose reduction is not the major glucose metabolism pathway in a normal human body, where the glucose level is in the normal range. However, in diabetic patients whose blood glucose level is high, up to 1/3 of their glucose could go through the glucose reduction pathway. This will consume NADH and eventually leads to cell damage.

Uses

Sweetener

Sorbitol is a sugar substitute, and when used in food it has the INS number and E number 420. Sorbitol is about 60% as sweet as sucrose (table sugar).<ref>Sugar substitute</ref>

Sorbitol is referred to as a nutritive sweetener because it provides some dietary energy. It is partly absorbed from the small intestine and metabolized in the body, and partly fermented in the large intestine. The fermentation produces short-chain fatty acids, acetic acid, propionic acid, and butyric acid, which are mostly absorbed and provide energy, but also carbon dioxide, methane, and hydrogen which do not provide energy. Even though the heat of combustion of sorbitol is higher than that of glucose (having two extra hydrogen atoms), the net energy contribution is between 2.5 and 3.4 kilocalories per gram, versus the approximately 4 kilocalories (17 kilojoules) for carbohydrates.<ref>Template:Cite journal</ref> It is often used in diet foods (including diet drinks and ice cream), mints, cough syrups, and sugar-free chewing gum.<ref>Template:Cite book</ref> Most bacteria cannot use sorbitol for energy, but it can be slowly fermented in the mouth by Streptococcus mutans, a bacterium that causes tooth decay. In contrast, many other sugar alcohols such as isomalt and xylitol are considered non-acidogenic.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

It also occurs naturally in many stone fruits and berries from trees of the genus Sorbus.<ref name=Teo/><ref>Template:Cite book</ref>

Medical applications

Laxative

As is the case with other sugar alcohols, foods containing sorbitol can cause gastrointestinal distress. Sorbitol can be used as a laxative when taken orally or as an enema.<ref name=drugs/> Sorbitol works as a laxative by drawing water into the large intestine, stimulating bowel movements.<ref name=drugs/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Sorbitol has been determined safe for use by the elderly, although it is not recommended without the advice of a physician.<ref name=drugs/><ref>Template:Cite journal</ref>

Sorbitol is commonly used orally as a one-time dose of Template:Convert 70% solution.<ref name=drugs/> It may also be used as a one-time rectal enema.<ref name=drugs/>

Other medical applications

Sorbitol is used in bacterial culture media to distinguish the pathogenic Escherichia coli O157:H7 from most other strains of E. coli, because it is usually unable to ferment sorbitol, unlike 93% of known E. coli strains.<ref name=wells_1983_101>Template:Cite journal</ref>

A treatment for hyperkalaemia (elevated blood potassium) uses sorbitol and the ion-exchange resin sodium polystyrene sulfonate (tradename Kayexalate).<ref>Template:Cite journal</ref> The resin exchanges sodium ions for potassium ions in the bowel, while sorbitol helps to eliminate it. In 2010, the U.S. FDA issued a warning of increased risk for gastrointestinal necrosis with this combination.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Sorbitol is also used in the manufacture of softgel capsules to store single doses of liquid medicines.<ref name="Catalent Softgel website">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Health care, food, and cosmetic uses

Sorbitol often is used in modern cosmetics as a humectant and thickener.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is also used in mouthwash and toothpaste. Some transparent gels can be made only with sorbitol, because of its high refractive index.

Sorbitol is used as a cryoprotectant additive (mixed with sucrose and sodium polyphosphates) in the manufacture of surimi, a processed fish paste.<ref name="MedinaGarrote2002">Template:Cite journal</ref> It is also used as a humectant in some cigarettes.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Beyond its use as a sugar substitute in reduced-sugar foods, sorbitol is also used as a humectant in cookies and low-moisture foods like peanut butter and fruit preserves.<ref name=":0">Template:Cite book</ref> In baking, it is also valuable because it acts as a plasticizer, and slows down the staling process.<ref name=":0" />

Miscellaneous uses

A mixture of sorbitol and potassium nitrate has found some success as an amateur solid rocket fuel. It has similar performance to sucrose-based rocket candy, but is easier to cast, less hygroscopic and does not caramelize.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Sorbitol is identified as a potential key chemical intermediate<ref>Template:Cite journal</ref> for production of fuels from biomass resources. Carbohydrate fractions in biomass such as cellulose undergo sequential hydrolysis and hydrogenation in the presence of metal catalysts to produce sorbitol.<ref>Template:Cite journal</ref> Complete reduction of sorbitol opens the way to alkanes, such as hexane, which can be used as a biofuel. Hydrogen required for this reaction can be produced by aqueous phase catalytic reforming of sorbitol.<ref>Template:Cite journal</ref>

19 C₆H₁₄O₆ → 13 C₆H₁₄ + 36 CO₂ + 42 H₂O

The above chemical reaction is exothermic, and 1.5 moles of sorbitol generate approximately 1 mole of hexane. When hydrogen is co-fed, no carbon dioxide is produced.

Sorbitol based polyols are used in the production of polyurethane foam for the construction industry.

It is also added after electroporation of yeasts in transformation protocols, allowing the cells to recover by raising the osmolarity of the medium.

Medical importance

Aldose reductase is the first enzyme in the sorbitol-aldose reductase pathway<ref>Template:Cite journal</ref> responsible for the reduction of glucose to sorbitol, as well as the reduction of galactose to galactitol. Too much sorbitol trapped in retinal cells, the cells of the lens, and the Schwann cells that myelinate peripheral nerves, is a frequent result of long-term hyperglycemia that accompanies poorly controlled diabetes. This can damage these cells, leading to retinopathy, cataracts and peripheral neuropathy, respectively.

Sorbitol is fermented in the colon and produces short-chain fatty acids, which are beneficial to overall colon health.<ref name=":2">Template:Cite journal</ref>

Potential adverse effects

Sorbitol may cause allergic reactions in some people.<ref name=drugs/> Common side effects from use as a laxative are stomach cramps, vomiting, diarrhea or rectal bleeding.<ref name=drugs/>

Compendial status

• Food Chemicals Codex<ref name=rffs>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

• European Pharmacopoeia<ref name=dsbtl>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> 6.1<ref name=ipe>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

• British Pharmacopoeia 2009<ref name=ib29>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

• Japanese Pharmacopoeia 17 <ref name=nihs>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

See also

• Sorbitan
• Isosorbide

External links

• Template:Commons category-inline