Etiology of Caries

The causes of dental caries and factors influencing their formation have been the subject of research for more than 100 years. The importance of oral bacteria was discovered well before the specific influence of sugars derived from the diet became known in around 1950. While the protective effect of fluoride has also been known for more than 50 years, the mechanism of this effect is still a subject of debate.

Different approaches have been used to try to understand the caries process. Experimental studies have either induced clinically apparent caries or attempted to model the early stages of caries. Ethical limitations on studies that might cause caries in humans and increasing resistance to animal experimentation have stimulated a great deal of imaginative recent work with laboratory modelling.

Direct studies of caries induction are rarely conducted nowadays. But, in the past, important evidence in this field has come from experiments in which caries were induced in laboratory animals and, in one important instance, from a similar experiment in human subjects. The animal experiments are now regarded with some suspicion, since the information gained cannot be readily interpreted in terms of human risk. The animals used differ appreciably from their human counterparts in the structure of their teeth, their way of eating, and other factors such as saliva and oral bacterial populations. These animal experiments have been useful, however, in establishing that all fermentable carbohydrates are capable of inducing caries under appropriate conditions.

A key human experiment was conducted in the 1950s, before it was entirely clear that sugar is capable of causing caries. It was important in that it demonstrated conclusively that the consumption of a large amount of sugar does not necessarily have a discernable influence on caries risk, provided it is eaten at mealtimes, whereas frequent consumption of quite small amounts of sugar had a marked influence. Subjects given 340gday_1 of sugar at meal times showed no increase in caries incidence, while subjects given 50gday_1 or 100gday_1 between meals showed an increase. Typical European intakes of sugar are less than 100gday_1.

The subjects in this study had little or no oral hygiene and no access to fluoride. It can therefore be readily concluded that the amount of sugar consumed in the diet, even in countries with high consumption, is unlikely to influence caries risk, especially with the regular use of fluoride toothpaste for oral hygiene. Whether frequent consumption of sugar will influence caries risk in an individual who cleans his or her teeth regularly with a fluoride toothpaste is more controversial. But, given the current state of knowledge, it seems unwise to assume that any dietary behavior would be safe, however outlandish. The current fashion of eating and drinking perpetually and of sipping sugar-containing drinks from a can over long periods seems designed to cause caries and cannot be recommended.

Research into the causes of dental caries has addressed a number of questions. These include why there are large differences in the disease experience of individuals within the same population or even family group, why the prevalence and severity of the disease are so different in different populations, and why these can change so dramatically with time. Entirely satisfactory answers to these questions are still being sought, but much has been learned over the last 100 years about the contributing factors and protective measures that determine the likelihood of this disease developing. This knowledge has been synthesized into the currently held view that clinically significant caries will develop only when a number of circumstances occur simultaneously. Inappropriate dietary habits (frequent consumption of sugars or starches) will allow the selective proliferation of bacteria attached to the tooth surface that are capable of metabolizing sugars to organic acids (especially lactic acid). These acids will facilitate dissolution of the tooth enamel whenever their production is sufficient to lower the local pH below a critical level. The presence of saliva or of other components of the food matrix will influence the pH attained and also the rate at which mineral is lost from the tooth surface.

The formation of dental caries is not, however, a simple unidirectional process of demineralization. Some tooth mineral may be removed almost every time something is eaten or drunk, but this loss will generally be made good by the subsequent accretion of mineral from saliva. Thus, a cavity develops only when the balance of repeated cycles of demineraliza-tion and remineralization results in localized overall mineral loss. It is for this reason that caries are most likely to occur at sites where food residues are likely to be trapped and access for saliva is limited (for example, between two closely abutting teeth).

The presence of fluoride not only radically alters both demineralization and remineralization but may also inhibit the activity of the acid-generating bacteria. To date, the most effective methods of reducing the incidence of dental caries have involved the use of fluoride either (at low concentrations) in community water supplies or (at higher concentrations) in toothpaste.

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