2.3. Other sugar alcohols
There are several naturally occuring sugar alcohols, including erythritol, D-arabitol, ribitol, xylitol, sorbitol, D-mannitol, galactitol and rhamnitol (for review see Mäkinen 1994). All the sugar alcohols are acyclic polyols, containing three or more hydroxyl groups. Within the scope of the present study were, besides xylitol, a four-carbon sugar alcohol (tetritol) erythritol, and the six-carbon sugar alcohols (hexitols) sorbitol and D-mannitol (Figure 2-2). Polyols occur naturally in many plants. Sorbitol is found in numerous berries and higher plants (Lohmar 1962). D-Mannitol is also widely distributed, being most frequently present in plant exudates (Lohmar 1962). Erythritol exists in fruits (Shindou et al. 1989) and mushrooms (Yoshida et al. 1986), but the amounts are extremely low. Like xylitol, sorbitol (Winegrad et al. 1972) and D-mannitol (Laker & Gunn 1979) are also endogenous metabolites in mammals.
Exogenous polyols are absorbed slowly, and metabolized to the corresponding 2-ketosugars by the action of nonspecific NAD-dependent polyol dehydrogenase mainly in the liver (McCorkindale & Edson 1954). Unlike xylitol, sorbitol and D-mannitol, erythritol is not a precursor of liver glycogen (Mäkinen 1994). Xylitol and sorbitol are metabolized completely after moderate administration, while D-mannitol is poorly utilized due to its low affinity for L-iditol dehydrogenase, causing an increased D-mannitol concentration in the urine (Dills 1989). Exogenous erythritol is very poorly metabolized, being excreted almost completely in urine without degradation (Noda et al. 1994).
Polyols are used as dietary sugar substitutes. They are also used as ingredients in the diets of diabetic subjects, and in infusion therapy solutions. Comparing to xylitol and sucrose, which are approximately of equal sweetness (Moskowitz 1971), erythritol is 75-80% (Kawanabe et al. 1992), D-mannitol 45-57% (Moskowitz 1974) and sorbitol 35-60% (Wright 1974) as sweet at equal weight. Like xylitol, sorbitol has an energy content similar to that of sucrose. D-Mannitol, when consumed as part of a mixed diet, has a reduced energy value (Dills 1989). Erythritol is a very low-energy sweetener, the available energy value being under 10% of that of sucrose (Noda & Oku 1992).
Other polyols share some properties with xylitol, also regarding their association with calcium metabolism. Sorbitol and D-mannitol increase calcium absorption and urinary calcium excretion in rats (Vaughan & Filer 1960, Hämäläinen & Mäkinen 1986, Knuuttila et al. 1989). Dietary sorbitol also increases the concentration of bone calcium, although less than xylitol (Knuuttila et al. 1989).