The term sugar alcohol does not truly fit sorbitol and mannitol: neither is as sweet as sugar and neither will get you drunk. These two, nearly identical twin sugar substitutes are close enough to sugar, however, to allow you to satisfy your sweet tooth without rotting it away. Both can be part of a low-sugar, low-calorie diet for diabetics and weight watchers. If you are a diabetic, consult your physician about the best way to use sweeteners like sorbitol and mannitol to keep your blood sugar under control.
Sorbitol and mannitol are isomers: substances that have the same chemical formula and molecular weight, but may differ in chemical properties and physical structure. Both sugar alcohol molecules consist of six carbon, 14 hydrogen and six oxygen atoms. Both have the same molecular weight of 182.17176 g/mol. If you compared a model of the molecular structure of sorbital to that of mannitol, the two would be identical -- except one hydrogen atom on mannitol would be pointed in a different direction in space.
There are some differences, however, in chemical properties of the two. Mannitol has a melting point of 327 to 336 degrees Fahrenheit, whereas sorbitol will melt at between 201 to 208 degrees Fahrenheit. In pure form, both are white, odorless powders, but mannitol sometimes forms granules. Sorbitol is a slightly more dense molecule than mannitol. Mannitol's appearance and less pronounced taste makes it a popular diluting agent for illegal drugs such as cocaine.
Occurence in Nature
Both sorbitol and mannitol are naturally occurring chemical compounds in plants, algae and bacteria. Both are derived from the sugar fructose. Sorbitol is frequently found in fruits like apples, pears and prunes. The leaves of the flowering ash tree and several members of the olive family exude mannitol from their leaves: in fact, mannitol derives its name from this similarity to the Biblical Manna from heaven.
Comparison to Sugar
Mannitol is about half as sweet as sugar: sorbitol is about 60 percent as sweet. As a group, sugar alcohols are not as well absorbed into the body and converted into glucose and caloric energy as sugar is. Mannitol has 1.6 calories per gram and sorbitol has 2.6 calories per gram. In comparison, sugar has 4 calories per gram. Like sugar, sorbitol can be used in baked goods. Mannitol, on the other hand, is often used to dust hard candy, dried fruit and chewing gum in much the same way you would use powdered sugar.
Benefits and Risks
Both sorbitol and mannitol can be safe sugar substitutes for diabetics. Neither causes the rise in blood glucose seen with sugar. Neither are digested by bacteria in the mouth to form tooth enamel-destroying acids.
Too much sorbitol or mannitol, however, may cause diarrhea. More sorbitol is absorbed from your intestine that mannitol, but the unabsorbed residual of either sugar alcohol is digested by bacteria in your intestine. This fermentation action by bacteria may produce intestinal gas, abdominal bloating and loose stools. The US. Food and Drug Administration requires manufacturers of foods containing sorbitol to include a laxative effect warning on their labels.