Catherine Hayes, D.M.D., D.M.Sc.:
Dental caries continues to be a significant public health problem, affecting a majority of the world�s population. The role of sucrose and other fermentable carbohydrates in the etiology of dental caries has been well established, and the use of sugar substitutes in candy, food, and gum and their effects on dental caries have been investigated in several studies.
It is believed that the benefits of sugar-free gum may be twofold. First, since sugars are not available for fermentation, lactic acid is not produced. Therefore, the pH of the oral cavity is not lowered to a range that would increase the risk for dental caries. Second, the use of chewing gum is believed to stimulate salivary flow, thus providing caries-preventive benefits, such as the buffering of acids in plaque formed from dietary carbohydrates, increased supersaturation of dental tissue with mineral ions leading to enhanced remineralization, and enhanced clearance of sugars from the mouth. Thus, sugar substitution and salivary stimulation could be equally responsible for the noncariogenicity of sugar-free chewing gum (Edgar, 1998).
The majority of sugar-free gums have been sweetened with sorbitol, a sugar alcohol derived from glucose. Xylitol, a sugar alcohol derived from the pentose sugar xylol, is another sweetener and has been the subject of many studies. Xylitol is nonacidogenic and is phosphorylated to an inhibitory compound upon entering cells. Other substitutes include mannitol, saccharin, and aspartame, which enhance shelf life and product taste (Edgar, 1998).
Studies of the relationship of sugar substitutes to dental caries have included both clinical trials and community-based observational studies. Although clinical trials are considered the "gold standard" of clinical research, it is important to consider information from observational studies as well. Information from multiple studies of both types points to the protective effect of xylitol against dental caries.
One clinical trial investigated the effect of sugar-free gum on the incidence of dental caries in 2,601 male and female schoolchildren in grades 5-7 in three communities in Puerto Rico. This population had a high prevalence of caries, low levels of professional dental care, and drinking water with negligible amounts of fluoride. Participants were assigned to either a no-gum group or a sugar-free gum group. Subjects in the gum group had a significantly smaller increase in caries rates than those in the no-gum group (Beiswanger BB, Boneta EA, Mau MS, et al., 1998).
Another study involved patients in the VA system who were enrolled in a randomized clinical trial. Patients with exposed root surfaces were randomly assigned to either sorbitol or xylitol chewing gum and were then followed for 1.8 years. Neither subjects nor examiners knew which patients got which type of gum. There were 40 subjects in each of the intervention groups. The relative risk for caries incidence in the xylitol versus sorbitol group was 0.19 (Makinen KK, Pemberton D, Makinen PL, et al., 1996a). A longitudinal study in Finland also demonstrated a decreased rate of caries among schoolchildren in an xylitol group (Isokongas, 1987).
In a double-blind cohort study conducted in Belize, 1,277 schoolchildren were randomly assigned (by school) into nine treatment groups: one control group (no gum), four xylitol groups (4.3-9.0 g/day), two xylitol-sorbitol groups (8.0-9.7 g/day), one sucrose group (9 g/day), and one sorbitol group (9 g/day). The largest reduction in caries occurred in the four xylitol groups and was significant in comparison to reductions in the sorbitol and sucrose groups (Makinen KK, Bennett CA, Hujoel PP, et al., 1995a).
A 5-year followup study of Estonian schoolchildren to evaluate the effect of xylitol gum or candy on caries rates was recently reported. In this study, the effects of xylitol consumption by 740 10-year-old children in 12 schools over a 2-year period were evaluated. Children using either xylitol gum or candy experienced a significant reduction in caries incidence (53.5 percent and 59 percent) compared to those in a control group (Alanen P, Isokangas P, Gutmann K, et al., 2000).
Another study in Belize with 6-year-old subjects found a lower rate of caries in xylitol or sorbitol groups as compared to a group of children not assigned to a chewing group, with relative risks reported as 0.35 (.21-.59) and .44 (.30-.63), respectively (Makinen KK, Hujoel PP, Bennett CC, et al., 1996b). Another analysis by Makinen and colleagues (1995b) of arrested or nonprogressed lesions also found a significant improvement in the xylitol group.
Studies of Streptococcus Mutans
Changes in streptococcus mutans levels as a result of sugar-free chewing gum have also been investigated. One study reported significant decreases in streptococcus levels in subjects using xylitol gum for 3 months as compared to subjects in a placebo or no-gum group. All subjects in that study rinsed daily with chlorhexidine for 2 weeks and were later randomized into three treatment groups and evaluated after 3 months. Streptococcus levels were no different in the three groups at baseline or after the chlorhexidine rinse period. The increase in streptococcus levels 3 months after rinsing was fortyfold in the placebo group, twenty-five fold in the control group, and eightfold in the test group (Hildebrand, Sparks, 2000).
A study in Finland examined the influence of maternal xylitol use on streptococcus levels in infants. Mothers participating in a postnatal oral health program were randomly assigned to xylitol chewing gum, chlorhexidine varnish, or fluoride varnish, and evaluated at 6, 12, and 18 months after delivery. Plaque samples were taken from the children, and saliva samples were taken from the mothers. The level of streptococcus did not differ significantly among the three groups at baseline, but the children of the mothers in the xylitol group had significantly lower levels of streptococcus than either of the other two groups after 18 months (Soderling E, Isokangas P, Pienih�kkinen K, et al., 2000). A third study also demonstrated a decrease in strep mutans levels in children in a chewing gum group (Makinen KK, Soderling E, Isokangas P, et al., 1989).
The long-term effects of sugar-free gum have been reported in a single study in which children were reexamined 5 years after a 2-year study ended. Comparisons were made between sorbitol, xylitol, xylital-sorbital, and no gum. The sorbitol group did not show a significant long-term reduction in caries, but the xylitol and xylitol/sorbitol groups demonstrated significant long-term caries reductions, with relative risks of 0.41 (0.23, 0.75) and 0.56 (0.36,0.89) respectively. The protective effect of xylitol depended on when teeth erupted. Children whose teeth erupted after 1 year of gum chewing or after the 2-year period had ended demonstrated the most significant long-term caries reductions (93 percent and 88 percent, respectively).
The use of xylitol as a sugar substitute in chewing gum has been evaluated in several observational studies as well as clinical trials, with results consistently demonstrating that xylitol had a protective effect against caries incidence. Limitations of the studies included small sample sizes, lack of radiographs for caries diagnosis, high loss of subjects to follow-up, potential confounding, and bias due to the nature of long-term community intervention studies. In order to effectively evaluate the effect of xylitol chewing gum on caries incidence, well-controlled double-blind clinical trials are needed with careful attention to study power, compliance, reliable caries assessments, and retention of participants.
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