Role of fermentable carbohydrates (sugar and starch)
A diet rich in fermentable carbohydrates (frequent sugar intake) is indisputably a very
powerful external RF and PRF for dental caries in populations with poor oral hygiene
habits and an associated lack of regular topical fluoride exposure from toothpaste.
However, in populations with good oral hygiene and daily use of fluoride toothpaste,
sugar is a very weak RF and PRF, because clean teeth never decay, and fluoride is a
unique preventive factor. The biochemical role of fermentable carbohydrates such as
sucrose in the development of an enamel caries lesion on a plaque-covered tooth
surface is illustrated in Fig 2 (see chapter 1).
Figure 49 depicts the most important variables and the interactions determining an
eventual acid attack on enamel after dietary intake. The final (eventually cumulative)
effect of single dietary intakes is dependent on their frequency. During and following
consumption, depending on the quality of salivary gland function, a certain amount of
saliva is stimulated by particular characteristics of the food, such as taste, acid
content, and surface texture, and by the intensity of mastication. Together with the
volume of saliva secreted, other substrate qualities, such as solubility, stickiness, and
an individual food- and host-dependent intraoral distribution, will determine the
specific oral clearance of the item in question.
Thus, after each intake, distinct amounts of dietary fermentable carbohydrates, acids,
and neutralizing agents will be present and capable of influencing the pH of the
surface of the tongue, of plaque, and of saliva for a given time. The resolution of the
interactions among these three factors, which is greatly influenced by the thickness
and diffusion characteristics of the dental plaque on the specific tooth surface,
determines the severity (fall in pH) and duration of the acid attack on the tooth
surface. The "true" plaque acid production, potentially dangerous to the tooth, should
therefore be measured at the enamel surface beneath the undisturbed plaque.
Methods for measuring the pH of plaque will be discussed later in this chapter.
Categories of fermentable carbohydrates
Box 2 shows the fermentable carbohydrates, ranked in order of complexity. All can be
fermented to acids by the plaque bacteria. In addition, the sugars may influence the
quantity and quality, and thus the cariogenicity, of microbial plaque on the teeth.
For several reasons, sucrose is regarded as the arch-criminal in dental caries. Sucrose
refined from sugar cane or beet is the most common dietary sugar and is largely
responsible for the above-described effects. Apart from familiar sweet products, such
as candy, cakes, desserts, jam, dried fruits, and soft drinks, a surprisingly large variety
of other everyday foods contains added sucrose: most breakfast cereals, many milk
products, some meat and fish products, salad dressings, ketchup, etc. Sucrose also
occurs naturally in fruit.
The dietary sugars all diffuse rapidly into the plaque and are fermented to lactic and
other acids or can be stored as intracellular polysaccharides by the bacteria,
prolonging the fall in pH and promoting a suitable environment for other aciduric and
acidogenic bacteria (see chapter 1). Sucrose, however, is unique because it is the
substrate for production of extracellular polysaccharides (fructan and glucan) and
insoluble matrix polysaccharides (mutan). Thus, sucrose favors colonization by oral
microorganisms and increases the stickiness of the plaque, allowing it to adhere in
larger quantities to the teeth.
Figure 50 (a and b) shows free plaque accumulation on the same tooth during a week
on a "sugar-free" diet and a week of high sugar intake, respectively. The sugar-free
diet is associated with a thick pellicle and a homogenous, relatively thin plaque, while
frequent sugar intake promotes the development of a thick, sticky, sugar plaque with a
high percentage of extracellular polysaccharides. Because of this effect on the quality
of plaque, sucrose is considered to be somewhat more cariogenic than other sugars.
Regardless of possible minor differences between the caries-inducing potential of
sucrose and that of other sugars, for practical purposes all dietary monosaccharides
and disaccharides are regarded as powerful risk factors: All are rapidly fermented on
plaque-covered tooth surfaces. Glucose, fructose, maltose, and sucrose give identical
curves for falls in the pH of plaque; for lactose, the fall in pH is somewhat smaller
(Neff, 1967).
Sucrose constitutes the bulk of dietary sugar. Lactose is present in milk, and maltose
is derived mainly from hydrolysis of starch. Glucose and fructose occur naturally in
fruit and honey and are also formed by acid hydrolysis of sucrose during the
manufacture and storage of soft drinks, marmalade, and other acidic products. Some
foods (Swedish baby food) are produced with invert sugar, which is hydrolyzed
sucrose. In industrial food processing, the use of glucose is increasing, produced by
hydrolysis of starch from cereals or potatoes and declared in the contents as dextrose,
corn syrup, or glucose syrup. Therefore, a decrease in national sucrose consumption
may not necessarily reflect a drop in sugar consumption.
Starch, the major storage polysaccharide of plants, is the major dietary carbohydrate.
In many countries, cereals, such as wheat, rice, maize, oats, and rye, provide about
70% of the calories; in the United States and Western Europe, the corresponding
figure is 25%. Other important sources of starch are root vegetables (potato, sweet
potato, cassava, yams, taro) and pulses (beans, lentils, and peas). Starch is a
polysaccharide of glucose.
The starch granules in plants are only slowly attacked by salivary amylase because the
starch is in an insoluble form and protected by cellulose membranes. Heating at
temperatures used in cooking and baking, however, causes partial degradation to a
soluble form, which can be further broken down by salivary and bacterial amylases to
maltose, maltotriose, dextrins, and small amounts of glucose. Although
polysaccharide molecules are too large to diffuse into the plaque, low-molecular
weight carbohydrates (released in saliva or at the plaque surface) become available for
bacterial fermentation.
Consumption of raw starch has little effect on the pH of plaque. The fall in pH
following consumption of soluble (cooked) starch and starch-containing foods, such
as bread or crackers, while not as pronounced as for sugars, may easily reach pH 5.5
to 6.0, levels which may be critical for initiation of root caries. A combination of
soluble starch and sucrose would be expected to be a more powerful caries risk factor
than sucrose alone, because the increased retention of the food on the tooth surfaces
would prolong sugar clearance time.
