Mannitol: Definition, Types, Structure, & Examples

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Mannitol Definition

Mannitol is a sugar alcohol with a sweetish flavour that occurs naturally in certain plants and algae and is chemically produced by reducing mannose for use as a low-calorie sweetener in the food industry and as a diuretic in medicine.

What is Mannitol?

One of the sugar alcohols is mannitol. Sugar alcohols are white, water-soluble organic molecules with the typical chemical formula (CHOH)nH2 that belong to the polyol class. The hydrogenation of sugars can result in sugar alcohols.

Mannitol is a polyol having the same chemical formula as sorbitol, which is C6H14O6. Isomers of each other include mannitol and sorbitol. The direction of the hydroxyl group on C-2 distinguishes them structurally.

They have various melting points as well. Sorbitol has a melting point of 94–96 °C, while mannitol has a melting temperature of 166–168 °C. Both are found in nature in plants. Chemically, they’re also made. Because of their sweet flavour, they are utilised as an alternative sweetener.

In medicine, however, they are employed in a different way. Mannitol is mostly used as a diuretic, whereas sorbitol is primarily utilised as a laxative. Sorbitol is a hygroscopic sugar, whereas mannitol is a non-hygroscopic sugar. Mallard responses do not occur with mannitol.

Mannitol, like sorbitol, is used as a sucrose substitute (table sugar). Mannitol has a milder flavour than sucrose. It has a 50-60% sweetness level when compared to sucrose and a significant cooling impact. It has a glycemic index of 0 compared to 65 for sucrose.

History of Mannitol

Joseph Louis Proust (1754–1826) is credited with discovering mannitol in 1806. He was a chemist from France. In 1819, he was also credited for discovering leucine. Julije Domac (1853-1928), a Croatian scientist, discovered the molecular structure of mannitol. Mannitol was derived from manna, a naturally sweet secretion of the manna ash.

Mannitol is now manufactured chemically using a nickel catalyst to hydrogenate fructose (hydrolyzed from starch or sucrose). The US Food and Drug Administration has declared mannitol to be generally safe, similar to sorbitol (another sugar alcohol).

Mannitol Function

D-mannitol is the most prevalent polyol found in nature. Many species, including bacteria, fungus, algae, lichens, and vascular plants, biosynthesize it, while Archaea and mammals do not. Many plants, such as celery and privet, produce it as one of their primary photosynthetic products.

Mannitol is biosynthesized in bacteria by a variety of fermentation processes. Certain lactic acid bacteria, for example, may convert three fructose molecules (or two fructose and one glucose) into two mannitol molecules, one lactic acid, one acetic acid, and one carbon dioxide molecule.

Mannitol biosynthesis is a two-step process in algae and Apicomplexa: the first step involves the enzyme mannitol-1-phosphate dehydrogenase converting fructose-6-phosphate to mannitol-1-phosphate, and the second step involves the enzyme mannitol-1-phosphatase converting mannitol-1-phosphate to D-mannitol.

This has also been observed in fungi. The “mannitol cycle” refers to the mannitol metabolic process in fungi. Mannitol biosynthesis is a three-step process in vascular plants.

An isomerase enzyme converts fructose-6-phosphate to mannose-6-phosphate in the first step. The second step involves the NADPH-dependent enzyme mannose-6-phosphate reductase converting mannose-6-phosphate to mannitol-1-phosphate. The conversion of mannitol-1-phosphate to D-mannitol is the final (and irreversible) step.

Mannitol may be synthesised chemically in addition to being generated naturally. The hydrogenation of fructose over a nickel catalyst, for example, produces mannitol (along with sorbitol). Mannitol, like sorbitol, is used in meals and special dietary items as a low-calorie sweetening ingredient.

Mannitol is absorbed in the gut at a slower rate in humans. When it is absorbed, it is released into the circulation, where it is taken up by liver cells, where it is turned into fructose and processed by dehydrogenases (just as sorbitol may be transformed into fructose).

Dehydrogenases, on the other hand, convert sorbitol more efficiently than mannitol. As a result, a substantial portion of mannitol may be eliminated in the urine rather than digested.

Mannitol and Health Issue

The US Food and Drug Administration has declared mannitol to be generally regarded as safe (GRAS). It may induce stomach discomfort, excessive gas (flatulence), loose stools, or diarrhoea if used in doses more than the recommended 50 mg/kg body weight.

Biological Importance of Mannitol

Mannitol is an important carbon source for species that are capable of spontaneously generating it. It is one among the most important products of fructose hydrogenation in plants. Plants that produce mannitol can survive in salty environments.

Mannitol appears to aid in the prevention of water loss and the maintenance of a healthy salt balance without interfering with regular metabolism. It’s also been suggested that it acts as a scavenger of reactive oxygen species, which might assist prevent lipid peroxidation, which causes cell damage.

For usage in the food sector, mannitol has been isolated and processed. Ice cream, confections, chewing gum, butter, pre-cooked pasta, and baby formula have all utilised it as a sweetener, anti-caking agent, thickening, or emulsifier.

Mannitol is a sugar replacement that does not cause blood sugar spikes and does not promote tooth damage. Mannitol, like the other sugar alcohols, is not generally fermented or digested by oral bacteria, thus it isn’t transformed into acids or other decay-causing byproducts.

It has a GI of 0, which is relatively low compared to sucrose, which has a GI of 65. The GI of glucose is 100, while that of fructose is 25. A high GI indicates that it has the potential to increase blood glucose levels.

Diabetes mellitus and obesity are linked to a persistently high blood glucose level. In terms of nutritional energy, mannitol offers 1.6 kcal per gramme, which is similarly less than sucrose. Mannitol, like other sugar alcohols, has laxative properties, especially when eaten in large amounts.

Mannitol is a sugar that is used to treat or control a variety of diseases in medicine. Its FDA-approved medicinal applications include briefly breaking the blood-brain barrier before certain types of chemotherapy and inducing diuresis.

Mannitol can aid in the elimination of harmful metabolites and chemicals. It works by removing accumulated fluid from the brain and eyes, lowering intracranial pressure and intraocular pressure (for example, in glaucoma). It is given as an injection.

Sorbitol is also employed as a growth substrate for bacteria in various experiments, and it’s also utilised to keep low-ionic-strength media tonic.

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