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Polyols are primarily organic alcohol that has multiple hydroxyl groups. The name polyol is comprised of 2 terms, “poly” refers to multiple and –“ol” indicates alcohol group.
What are Polyols?
Polyols like other organic compounds comprise C-C and C-H covalent chemical bonds. Sugar alcohols are a group of polyols that are derived from sugars produced naturally or synthetically. Even though they are formed by the hydrogenation of sugars, they are not true sugars.
Sugars are comprised of monosaccharides and disaccharides having a formula Cn (H2O) n, where n may range from 3 – 7.
The group sugar alcohols are similar in structure to sugars except for the fact that they have additional hydroxyl groups and have the formula (CHOH)nH2. They are less sweet and can act as an energy source.
Glucose is converted into fructose in the polyol pathway through a biological process. In this pathway, glucose is first reduced to polyol sorbitol that is oxidized to fructose. The first reaction is catalyzed by aldose reductase whereas the oxidation of sorbitol occurs in presence of the enzyme sorbitol dehydrogenase.
This pathway also employs cofactors, NADPH and NAD+. Dysfunctions in this pathway can lead to microvascular damage to kidney cells, retina, or nerves that are insulin-independent leading to type-II diabetes complications.
If glucose has not been phosphorylated by hexokinase then instead of the glycolytic pathway, it enters the polyol pathway where it is converted to fructose.
This occurs when there is a surplus of glucose due to which enzyme hexokinase becomes saturated and the excess glucose is diverted to the polyol pathway instead. Here it is reduced sorbitol and utilizes NADPH and NAD+.
These co-factors become less available for other significant pathways and metabolic reactions like glutathione and nitric oxide production. It also increases free radical oxygen species which can be damaged cells.
Biological Importance of Polyols
The most important function of polyols is to provide an energy source and it is involved in the polyol pathway where glucose is reduced. In this pathway, excess glucose is reduced to fructose which is required for processes like glycation and fructolysis.
Natural polyols occur naturally in fruits like apples, prunes, peaches, and in prunes, in forms like sugar alcohol sorbitol. Another naturally occurring polyol is found in bacterial cell walls as sugar alcohol ribitol.
Polyols may also act to maintain high intracellular osmolality as osmolytes in the kidney cells of higher organisms; some also act as antifreeze molecules.
Artificially polyols can be synthesized starch and sugar. They are used commercially as an alternative for sugar or as additives. They are employed in food products that are categorized as having no sugar content or low calorie. These sugar sorbitols except erythritol provide about 2.4 kilocalories/g.
They are not labeled as essential nutrients. But for diabetic patients, it can help them in regulating glucose levels in the blood. They can also act as a substitute for sucrose that is also a natural sugar. It is synthesized by plants like beets and sugarcane.
It is extracted from these sources and commercially processed as sugars utilized in various preparations. But unlike polyols, it is an essential carbohydrate that provides both fructose and glucose.
One of their disadvantages is that it has a high GI index of 65 when compared to sugar alcohols that have a GI of less than 10.2. GI of glucose is 100, while fructose has a GI of 25.
A high glycemic index is not preferred as it means that the substance can cause a substantial peak in blood glucose levels as in conditions of obesity and diabetes mellitus.
Sugar alcohols with a low GI are preferred and they do not promote tooth decay as they are not metabolized by mouth oral microbes. They are used commercially in the production of medicinal syrups, lozenges, and toothpaste as they do not produce any sort of acids that might trigger tooth decay.
One of their downsides is that they are poorly digestible which can lead to irritation and increased fermentation by gut microbe leading to bloating, flatulence, and diarrhea. These conditions can be triggered even in smaller amounts for sensitive individuals.
Sugar alcohols class includes maltitol, sorbitol, xylitol, isomalt, erythritol, glycerol, lactitol, hydrogenated starch hydrolysates, and mannitol.
The most common polyol is sorbitol that is found naturally in fruits like pears. It can also be produced artificially for commercial purposes for the production of sugar-free food products like sugar-free ice cream.
Maltitol is another commonly used alternative polyol that can replace sugars that are employed in manufacturing sugar-free products like baked goods. Another polyol that is commonly used in candies is isomalt that is a disaccharide.
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