Dental caries is an infectious disease. Most children appear to acquire the acid-producing mutans streptocci (MS) bacteria, including Streptococcus mutans, from their mothers.1 Transmission occurs through salivary contact and the transmission rate can be affected by the frequency of contact and the MS bacterial levels in the mother’s saliva. The earlier in infancy that a high level of MS colonization is attained, the more severe the caries disease will be in the primary dentition.2 Therefore, products that delay or reduce colonization of cariogenic bacteria in the child might also provide protection against dental caries.
Xylitol, which has 40% fewer calories than sugar,3 has been a popular sugar substitute in Europe since the 1970s. Xylitol is approved in more than 35 countries, including the United States, for use in food products, pharmaceuticals, and oral health products. Because of the undeniable role of fermentable carbohydrates in dental caries, sugar substitutes are always examined for their ability to reduce or eliminate dental caries. A considerable body of research indicates that substituting sucrose with xylitol reduces the incidence of dental caries in rats and humans.4-6 Xylitol, however, is superior to other sugar substitutes because it can also prevent mother-to-child transmission of cariogenic bacteria.4,7
Xylitol-containing products are more expensive than products with other sugar substitutes. In this country, xylitol is mainly used in combination with less expensive sugar alcohols, such as sorbitol or mannitol, to improve the flavor of the product, reduce calories, and reduce caries. Often, these products do not contain enough xylitol to prevent dental caries through its specific antimicrobial properties.8 However, the use of sugar substitutes instead of sugar still results in reduced demineralization and caries prevention. When recommending a product with xylitol, the dental hygienist must determine whether xylitol is being used as a sugar substitute or at therapeutic doses for a prescribed length of time in order to produce additional protective effects.
Scientists are currently researching efficient and less expensive ways to produce xylitol from plant materials.9 Xylitol may be more affordable in the near future as a byproduct of ethanol production. Ethanol production facilities are increasing in the United States and research into manufacturing processes that provide useful coproducts, like xylitol, is ongoing.10
Sixteen separate studies (length of xylitol use ranging from 25 days to 40 months) show the protective effects of xylitol use by either reducing S. mutans numbers in plaque and saliva or reducing DMFS (decayed, missing, filled surfaces) scores following xylitol use.5 The recommended dose for xylitol ranges from 6 g to 10 g daily, to be administered three to five times. Gum products in these studies were chewed for 5 minutes each time. Therefore, most studies required a long-term period of daily chewing at a certain dose to see the antimicrobial effects of xylitol. Lozenges or mints sweetened with the appropriate dose of xylitol are also appearing on the market and may be more readily accepted by the public for use multiple times per day but, to date, most of the research has focused on chewing gum products.
In addition to reducing S. mutans levels in both oral biofilm and saliva, xylitol use can reduce the level of lactic acid produced by these bacteria.11 Xylitol may also decrease the rate of transmission of S. mutans from mothers to their infants. These benefits appear to last for years after the mother has stopped xylitol use. In children examined 2.5 years12 after the mother stopped using xylitol gum, the mean DEFS (decayed, extracted, filled surfaces in primary teeth) scores in the group whose mothers chewed gum with xylitol only were lower than the mean DEFS of the group whose mothers chewed gum containing sodium fluoride/xylitol/sorbitol mix. In another study,7 the children were examined as long as 4 years following cessation of xylitol use in the mother, and the salivary MS levels were significantly lower in the xylitol chewing gum group than in the groups that were treated with chlorhexidine or fluoride varnishes. The mothers in these studies did chew a certain dosage of xylitol two to three times daily for 12 or 21 months.
Xylitol’s Dual Action
Xylitol provides benefits in two ways. First, xylitol is a sugar substitute.5,8 Its action is similar to other sugar substitutes that are nonfermentable by oral bacteria. Bacteria break down a variety of carbohydrates (glucose, fructose, sucrose, maltose, mannose) for use as sources of energy and carbon. The bacteria have multiple systems in their cell wall that are each responsible for the transport of a specific carbohydrate into the cell. Bacteria cannot use a carbohydrate as a food-source if it does not have a compatible system to transport the sugar into the cell. Xylitol, while not used as a food source, can still be transported into some types of bacteria but the enzymes are not present for breaking it down. Thus, no lactic acid is produced from xylitol. This is why xylitol is an excellent sugar substitute and does not promote demineralization.
Second, when used at high enough doses, xylitol is an antimicrobial.8 Inside the bacterial cell, most of the different transport systems are linked through a common set of enzymes in the phosphorylation pathway. Bacteria that have a fructose transport system are able to use it to transport xylitol into the cell but then xylitol locks the shared phosphorylation pathway enzymes in a nonproductive loop. During this process, xylitol is not broken down and the cell cannot effectively use other carbohydrate transport systems. So xylitol uptake by a cell affects cell-wide energy use and slows down metabolism and bacteria’s survival.13 In other words, xylitol prevents the susceptible bacteria from simultaneously using other available sources of nutrients and directly leads to the death of the bacteria. This can explain an immediate decrease in S. mutans numbers and, thus, decreased transmission from a mother to a child4,7 if xylitol is used at appropriate dosages and for an extended time.
The benefits associated with xylitol appear to last for years after use has been discontinued.7,12 Prolonged use of xylitol appears to select for xylitol-resistant mutants of the MS bacteria.14 Some of these have been analyzed and have a mutation in their fructose transport system that prevents them from transporting xylitol into the cell. These same mutant bacteria appear to be shed more readily into the saliva than the parent strains.15 Xylitol-resistant S. mutans detection coincides with a reduction of S. mutans in plaque,16 which may hamper the transmission and/or colonization of S. mutans from mother to child. More recently, research involving S. gordonii has shown that mutation of its fructose transport system leads to defective biofilm formation.17 The exact mechanism is not yet understood but the evidence suggests that a functional fructose transport system has a role in proper biofilm formation and that prolonged exposure to xylitol can impair this system.
In conclusion, an abundant amount of evidence exists to support the benefits of using xylitol to prevent caries. Affordable xylitol-containing chewing gum products and permission from the dentist to chew might be effective ways to reduce childhood caries. Xylitol use presents an exciting solution for those in the dental profession who wish to provide an acceptable alternative to patients at high risk for caries. However, dental hygienists need to remember that to gain the antimicrobial benefits, xylitol needs to be used at an appropriate dose over long periods of time (6 g to 10 g daily, administered three to five times, chewed for 5 minutes each time).
Diana M. Catt, PhD, is an instructor and a postdoctoral fellow in the Department of Preventive and Community Dentistry at Indiana University School of Dentistry, Indianapolis. Her research interests include biofilm colonization by S. mutans , cell-to-cell communications within a biofilm, and the mechanism of xylitol resistance.
Margherita Fontana, DDS, PhD, a native Venezuelan, moved to the United States to pursue her research interests. She has received research grants from the National Institutes of Health, the American Association of Pediatric Dentistry, the Delta Dental Fund, and private industry. An international speaker on cariology, Fontana currently lectures on cariology in the dental and dental hygiene programs at Indiana University School of Dentistry where she is a tenured assistant professor in the Department of Preventive and Community Dentistry. She is the director of the Microbial Caries Model Facility and the new Oral Biofilms in Caries Assessment and Management Research Program.
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From Dimensions of Dental Hygiene. March 2007;5(3): 24, 26.