Research suggests that xylitol produces its anti-cavity benefits via more than one mechanism.
How does xylitol provide its protection?
There doesn't seem to be an overwhelming consensus regarding the precise way by which xylitol creates its anti-cavity effect. No doubt this is in part due to the fact that this effect is produced by a couple of different mechanisms.
Discussed on this page are some of the theories that studies have proposed to explain how the presence of xylitol can affect both the bacteria that cause tooth decay (primarily Streptococcus mutans and Streptococcus sobrinus) and the oral environment as a whole.
Cariogenic bacteria cannot metabolize xylitol.
When xylitol (a five-carbon molecule) is present in the oral environment cariogenic bacteria (the bacteria that cause tooth decay) import it through their cellular walls via a transport system designed for fructose (a five-carbon sugar). As it happens, during this uptake process xylitol gets transformed into xylitol phosphate, a compound that these bacteria cannot metabolize. The net effect is the creation of a "starvation" effect on the bacteria.
The benefits of this starvation effect are easy to imagine. Because the ingested xylitol is not metabolized, the total amount of acidic byproducts that the bacteria create is reduced. (A condition for tooth decay formation is an acid environment at the interface where dental plaque lies on a tooth's surface.)
This same starvation effect can also be used to explain the fact that some studies have reported both a reduction in the level of cariogenic bacteria found in the mouths of those who use xylitol, as well as a general reduction in the amount of accumulated plaque on tooth surfaces. (Since the bacterial colonies are not obtaining an adequate level of nutrition, they do not thrive and readily proliferate.)
Xylitol may alter the types of bacteria that live in dental plaque.
It has also been suggested that the long-term use of xylitol produces a shift in the type of bacteria that live in dental plaque.
A selection process seems to occur where the growth of "xylitol-sensitive" cariogenic bacteria (bacteria that cannot metabolize xylitol) is suppressed and these bacteria are subsequently replaced by strains of "xylitol-resistant" ones (bacteria that have an altered fructose transport mechanism and therefore are not affected in the same manner by the presence of xylitol).
To our great fortune in regards to cavity prevention, xylitol-resistant strains of bacteria seem to be less virulent than their counterparts. The exact reason why has yet to be determined but some theories have been suggested.
One of these is that xylitol-resistant bacteria appear to have impaired adhesion properties. This allows them to be shed more easily from dental plaque (where they can cause tooth decay) to saliva (where their presence is much less capable of causing tooth damage).
Other theories suggest that xylitol is metabolized by xylitol-resistant bacteria but only slowly, or else the presence of xylitol inhibits their metabolism of dietary sugars. (These latter scenarios would serve to both inhibit the growth of the bacteria and also decrease the overall levels of acidic byproducts that they create.)
The presence of xylitol promotes tooth remineralization.
Some researchers have determined that xylitol, on its own as a molecule, produces an effect that helps to inhibit the advancement of the cavity formation process. These studies have found that demineralized tooth enamel samples immersed in a xylitol solution showed greater middle and deep remineralization than non-xylitol samples.
This finding suggests that the xylitol molecule can facilitate the movement and accessibility of calcium ions, thus assisting the tooth enamel remineralization process (tooth decay reversal).
Other benefits of using xylitol products.
There are some mechanisms associated with tooth decay prevention that are not so much attributed directly to xylitol but instead the oral environment that its presence promotes.
Choosing xylitol products that require chewing helps to stimulate saliva production.
An especially beneficial way to achieve an exposure to xylitol is through the use of chewing gum. The reason for this is as follows.
The act of chewing stimulates the production of saliva. This increased flow of saliva allows for a greater degree of buffering and dilution of the acidic bacterial byproducts present in dental plaque. This reduced acidity helps to promote an environment where enamel remineralization is the dominant process.
An especially good time to choose to use xylitol chewing gum is right after a meal (when acid production is typically at its peak).
Xylitol use influences the pH of dental plaque in a way so remineralization is more likely to occur.
When the environment at the tooth-dental plaque interface is acidic (pH 5.5 and below), tooth enamel demineralization (tooth decay formation) will occur. Less acidic environments allow enamel remineralization (a natural "healing" process and reversal of tooth demineralization) to take place. What a person needs, in terms of cavity prevention, are factors that help to perpetually tip the scales so a less acidic environment exists. This way remineralization will be the dominantly occurring process.
Since in the presence of xylitol decay-forming bacteria are less capable of producing acidic byproducts, the overall pH of the dental plaque in which they live will be less acidic too. This helps to tip the scales in favor of the predominance of the remineralization process.