Xylitol is a natural substance appearing in many forest and agricultural materials containing hemicellulose.
These materials rich in hemicellulose have been used as a raw material in xylitol manufacturing. Hemicellulose is chemically a xylan, a long polysaccharide molecule consisting of D-xylose units. Xylans (which in turn are examples of so-called pentosans) are typically present in certain hardwoods (such as birch and beech), rice, oat, wheat and cotton seed hulls, various nut shells, straw, corn cobs and stalks, sugar cane bagasse, etc.
According to this terminology, pentosans are polysaccharides consisting of five-carbon pentose sugars, such as D-xylose. (Glucans consist of six-carbon D-glucose units, and represent spesific hexosans, important in the growth of dental plaque.) In the manufacturing process of xylitol (2), the xylan molecules are first hydrolyzed into D-xylose. The latter is chemically reduced to xylitol which can be separated by large-scale column chromatography. Xylitol is finally crystallized. The entire process is complicated and demands great engineering skills and experience.
The amounts of xylitol present freely in plants are too low for industrial exploitation. Xylitol can, of course, be synthesized by means of organic chemical procedures, but the usage of D-xylose as a starting material is currently more feasible. Xylitol can also be made by means of bacterial fermentations which utilize D-xylose, D-glucose, or other suitable raw materials as substrates. These processes have not been economically feasible.
The chemical profile of xylitol; terminology
Xylitol is a natural sugar alcohol of the pentitol type, i.e. the xylitol molecule contains five carbon atoms and five hydroxyl groups (Fig. 1). Therefore, xylitol can be called a pentitol. Xylitol belongs to the polyalcohols (polyols) which are not, strictly speaking, "sugars" which traditionally include certain nutritive carbohydrate sweeteners (sucrose, corn sugar, corn syrup, invert sugar, D-fructose, D-glucose, etc.; in some reports the term "sugars" is collectively used to refer to mono- and disaccharides). However, the legitimacy for including polyols in the sugar field results from biochemical relationships; polyols are formed from, and can be converted to, sugars (i.e. aldoses and ketoses). Some chemical encyclopedias define sugars as crystalline, sweet carbohydrates. The sugar alcohols thus fall in this category.
To fully understand the dental effects of xylitol, it is important to refer to the structural differences between various dietary polyols (3). Sorbitol is another sugar alcohol, a hexitol type of polyol, owing to its 6-carbon structure. Because of this, sorbitol can support the growth of cariogenic mutans streptococci and other oral bacteria which are not normally able to utilize xylitol for growth. Because of evolutionary expediency, cariogenic organisms prefer 6-carbon ("hexose-based") structures, such as D-glocose, as an energy source. Therefore, it is important to akcnowledge the inevitable biochemical differences between xylitol (a pentitol and pentose-derived) and sorbitol (a hexitol and hexose-derived), and to understand the nomenclature-related definitions described above.
In spite of the existence of some differences between the various sugar alcohols, xylitol and most other polyols also display dentally interesting common properties: they can form certain type of complexes with calcium and certain other polyvalent cations. Such Ca-xylitol complexes can be present, for example, in the oral cavity and in the intestines. In the former, such complexes may contribute to the remineralization of demineralized enamel and dentine caries lesions observed in subjects who habitually consume xylitol.
In the intestines, those complexes can facilitate the absorption of calcium through the gut wall; this effect has been suggested to play a role in the xylitol-associated prevention of osteoporosis in experimental animals (4). From the dental point of view, the role of xylitol (and certain other polyols) as stabilizers of the salivary calcium and phosphate ions may be important. It is possible that xylitol stabilizes the calcium phosphate system present in saliva in the same manner some salivary peptides (such as statherin) do (5).
Professor Kauko K. Mäkinen, Institute of Dentistry, University of Turku, Finland
References 1. Mäkinen KK. Biochemical principles of the use of xylitol in medicine and nutrition with special consideration of dental aspects. Birkhäuser Verlag, Basel, 1978.
2. Aminoff C. New carbohydrate sweeteners. In "Sugars in Nutrition" (Sipple HL, McNutt KW, eds), Chapter 10, Academic Press, New York 1974.
3. Mäkinen KK. Latest dental studies on xylitol and mechanism of action of xylitol in caries limitation. In "Progress in Sweeteners" (Grenby TH, ed.), Chapter 13, Elsevier, London 1989.
4. Svanberg M, Knuuttila M. Dietary xylitol prevents ovariectomy-induced changes of bone inorganic fraction in rats. Bone Miner (1994) 26:81-88.
5. Mäkinen KK, Söderling E. Solubility of calcium salts, enamel, and hydroxyapatite in aqueous solutions of simple carbohydrates. Calcif Tissue Int (1984) 36:64-71.
6. Mäkinen KK. Dietary prevention of dental caries by xylitol - clinical effectiveness and safety. J Appl Nutr (1992) 44:16-28.
7. Uhari M, Kontiokari T, Koskela M, Niemelä M. Xylitol chewing gum in prevention of acute otitis media: double blind randomised trial. Br Med J (1996) 313:1180-1184.
The author is a Professor in the Institute of Dentistry,
University of Turku, Finland.