Xylitol molecule

Exerpt from:
Xylitol and its effect on oral ecology
clinical studies in children and adolescents
by
Pernilla Lif Holgerson

Xylitol is a natural carbohydrate like substance. The industrially produced xylitol molecules have the exact same structure as the molecules that occur naturally and in humans. Xylitol is hydrophilic and may compete with water molecules for the hydration layers that surround protein molecules in biological environments. Furthermore, xylitol may form complexes with inorganic ions such as Ca2+, stabilizing the calcium phosphates in saliva (Makinen and Soderling, 1984).

Growth inhibition of bacteria by xylitol in vitro has been observed in many different bacterial species: Escherichia coli, some strains of Lactobacillus casei, Streptococcus pneumoniae, several Actinomyces species, food-spoilage organisms, and several strains of Streptococcus mutans (Knuuttila and Makinen, 1975; Vadeboncour et al., 1983; Beckers, 1988). The hampered growth is based on metabolic reactions. Most microorganisms do not incorporate xylitol because they do not have any suitable transport mechanism. Oral bacteria use the PTS system (phosphotransferase system) for sugar transport (Birkhed et al., 1985; Assev et al., 1996; Touger-Decker and van Loveren, 2003). Xylitol is incorporated with help from the fructose specific PTS and phosphorylated to xylitol-5-phosphate (Waler et al., 1984; Waler, 1992; Trahan 1995; Trahan et al., 1996; Roberts et al., 2002; Tanzer et al., 2006).

This substance inhibits further intracellular metabolism of the bacterial cell and the process consumes energy. The phosphate can also be dephosphorylated and transported back to saliva or plaque (xylitol futile cycle) (Soderling and Pihlanto- Leppala, 1989; Pihlanto-Leppala et al., 1990; Kakuta et al., 2003). An electron microscopic study showed that growth inhibition of MS by xylitol resulted in degraded cells, autolysis, and the formation of vacuoles (Tuompo et al., 1983). These changes reduce the possibility for the cell to adhere to a surface. After exposure for xylitol, increased bacterial tolerance to xylitol cannot be ruled out (Roberts et al., 2002). A shift towards xylitol resistant strains of MS has been shown in saliva. It has been suggested that these strains have a reduced ability to adhere to tooth surfaces (Trahan et al., 1992; Trahan et al., 1996; Soderling et al., 1997).

The Turku sugar studies were initiated in the beginning of 1970s. At first, there were two clinical studies, one feeding study and one chewing gum study (Scheinin and Makinen, 1975). In the feeding study, almost complete substitutions of dietary sucrose by fructose or xylitol were accomplished. The study included 115 subjects divided into three groups. They received sucrose, fructose, or xylitol as dietary sweeteners. Normal consumption of xylitol was 50-67g per day. The study showed an 85% reduction of dental caries in the xylitol group compared to the sucrose group, whereas a 30% caries reduction was shown in the fructose group compared to the sucrose group. The chewing gum study included 100 dental students that were divided into a sucrose group and a xylitol group. The daily dose of each sugar was 6.7g. After one year, the reduction of caries increment in the xylitol group was 82% compared to the sucrose group. It was concluded that a complete substitution of sucrose not was needed to reduce caries increment (Scheinin and Makinen, 1975).

After the Turku sugar studies, many clinical trials have been initiated. Studies with caries increment as outcome measure include the “Belize study”, which was performed during 1989-1993 in Belize in Central America. Over 1200 children participated. They were divided into nine treatment groups: one control group (no supervised gum use); four xylitol groups (range of supervised xylitol consumption: 4.3 to 9.0g/day); two xylitol-sorbitol groups (range of supervised consumption of total polyols: 8.0 to 9.7g/day); one sorbitol group and one sucrose group
(supervised consumption: 9.0g/day respectively). The results suggested that systematic use of polyol-based chewing gums reduces caries rates in young subjects; xylitol gums were more effective than sorbitol gums (Makinen et al., 1995). In the “Michigan xylitol programme”, the participants were from 6 years old to geriatric ages. They were provided with saliva stimulants, mostly chewing gums, for 2 weeks to 56 months. The clinical and microbiological results suggested that xylitol was more effective than sorbitol in preventing caries (Makinen et al., 1996). Alanenet al. (2000) demonstrated in a field study from Estonia that a supervised use of both xylitol-containing candy and chewing gums reduced caries incidence in schoolchildren compared with corresponding control groups. In contrast, Machiulskiene et al. (2001) performed a study in Lithuania with more than 600 participating schoolchildren. They were divided into different groups, and after three years there were no differences between the xylitol, sorbitol, and control group. The study concluded that the chewing process reduced caries increment.

Other areas where xylitol seems to be useful are in preventing the transmission of Streptococcus mutans from mother to child (Soderling et al., 2000; Peldyak and Makinen, 2002; Thorild et al., 2005) and in preventing acute otitis media (Uhari et al., 2000).

Studies have shown that the intake of xylitol has to be at least 5-10g/day in fractioned doses to have an anticaries effect; studies with lower doses show generally a less favourable outcome (Isokangas et al., 1988; Petersen and Razanamihaja, 1999; Alanen et al., 2000; Makinen et al., 2000; Machiulskiene et al., 2001). There is, however, also a question whether or not xylitol can cause side effects when being used in larger quantities (Scheie et al., 1998; Storey et al., 2006; Vernacchio et al., 2006). A debate is also ongoing whether it is the xylitol itself that gives the oral health effects or whether it is the effect from the saliva stimulation (Scheie and Fejerskov, 1998; Hayes 2001; van Loveren, 2004).