Xylitol and Oral Health

ADVANCES

IN FOOD RESEARCH. VOL.

25

XYLITOL AND ORAL HEALTH KAUKO K. MAKINEN* Department of Biochemistry, Institute of Dentistry, University of Turku, Turku, Finland

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Xylitol and Dental Caries.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

111. IV. V.

VI. VII. VIII.

A. Properties of Xylitol and Cariogenic Sugars in Relation to Dental Caries B. Lowered Cariogenicity of Foods Containing Xylitol . . . . . . . . . . . . . . . . . Microbiological Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . In Vitro Plaque Studies.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . Xylitol and the Exocrine Glands . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . A. Preliminary Findings in Feeding Studies . . . . . . . . . . . . . . .. . . . . . . . . . B. Sialic Acid and Sialoproteins . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . Xylitol and Periodontal Diseases . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . Mechanism of Action of Xylitol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Research Needs.. . . . . . . . . . . . . . . . . . . . . . . . . . ............ .......... . . . . . . . . . . . .. . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I.

I37 139 139 139 147 149 149 149 150 152 153 156 156

INTRODUCTION

The health of the oral cavity is an inseparable part of the general health of man. Physicians, dieticians, and nutritionists should take a serious look at the effects of improper nutrition on oral tissues, viz, the teeth, periodontal tissues, oral mucosa, salivary glands, etc., because in all peroral nutrition the oral tissues are first to react. It has been customary to show sucrose to be a decisive etiological factor in dental caries, but it has also been shown to contribute indirectly to the development of other pathological processes in the mouth (for example, in periodontal diseases). Other dietary carbohydrates (glucose, fructose, starch, etc.) are also cariogenic but the consumption of any of these carbohydrates per se should not be considered detrimental to the oral health. A decisive factor is the *Present address: Department of Biochemistry and Biophysics, Texas A. & M. University, College Station, Texas 77843.

137 Copyright 0 1979 by Academic Press. Inc All nghb of reproduction in any form reserved ISBN 0-12-016425-6

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KAUKO K. MAKINEN

manner in which they are consumed: the frequency and form of intake and the properties of other dietary ingredients. The periodontal diseases most likely result from the overgrowth of the bacterial plaque and no specific pathogens have yet been shown to cause these diseases, although plaque organisms such as Bacteroides melaninogenicus may be particularly damaging (Socransky, 1970). The periodontal tissues are under a continuous repair process and disease may be a simple accumulative process. In dental caries several pathogens have been demonstrated. They belong to the genera Streptococcus, Lactobacillus, and Actinomyces. A particular organism, S . mutans, has been the focus of active research and has been suggested to be associated with smooth surface caries. Each tooth site (interdental spaces, fissures, gingival margin, etc.) forms its own local ecosystem characterized by special microbial and biochemical determinants. Consequently, caries at these tooth sites are most likely initiated as a result of the action of different microorganisms and it is quite possible that the role of S. mutans has been exaggerated. In all oral diseases, however, man’s innate defense mechanisms play a very important role (MacFarlane and Mason, 1972). Impaired protective ability of these mechanisms, which are mostly related to salivary components, may lead to an unbalanced situation between the host and plaque bacteria with concomitant pathological changes in the former. The innate defense mechanisms may not be able to maintain the required balance if dietary habits are poor or if the oral hygiene methods are insufficient. Both anatomical and genetic factors are also involved and an insufficient supply of fluorine during fetal and early childhood development also contributes to the final resistance of the oral tissues. The preceding comments thus indicate that at least dental caries, but to a certain extent periodontal diseases as well, are multifactorial diseases. The causal factors include diet, oral hygiene, fluorine prophylaxis, dental plaque, saliva with its defense mechanisms, hereditary factors, and the oral tissues themselves. The current use of sucrose and certain other fermentable carbohydrates has made it necessary to search for substitutes that would not be cariogenic. As the main chemical properties of sucrose that make it damaging under certain conditions are indirectly related to bacterial adhesion and bacterial growth, any substitute should have properties that influence these phenomena. In addition, direct stimulatory effects on saliva should also be considered. Recent extensive clinicochemical human trials suggest that xylitol is a promising sucrose substitute. Sorbitol, mannitol, and hydrogenated hydrolysis products of starch have been used for some years for this purpose but they have not been totally effective. The present review deals with results obtained in recent clinicochemical xylitol studies on humans and experimental animals. The article also elucidates the physiological and chemical factors that underlie the encouraging results obtained with this sucrose substitute.

XYLITOL AND ORAL HEALTH

139

II. XYLITOL AND DENTAL CARIES A.

PROPERTIES OF XYLITOL AND CARIOGENIC SUGARS IN RELATION TO DENTAL CARIES

The biological processes of bacterial plaque that contribute to the incidence of dental caries include glycolysis, which produces, from suitable hexose-based sugars, lactic acid. Other bacterial fermentations can also be involved and the dietary carbohydrates may additionally yield, via bacterial syntheses, extracellular polysaccharides which contribute to the adhesion of bacteria to surfaces. This in turn leads to plaque growth. These factors are shown in Table I. The acids formed at the plaque interface may cause demineralization of the hard tissue. The chemical reactions shown in Table I chiefly require six-carbon skeletons as substrate. This fact stems from the evolutionary preference of human plaque microorganisms for this type of carbohydrate. Xylitol, being a sugar alcohol of the pentitol type, has quite different effects on the physiology of oral microorganisms, as presented in Table 11. The net effect of the bacteriophysiological properties of xylitol is that the time periods with very little or no risk of caries initiation are noticeably prolonged during and immediately after peroral administration of xylitol in a suitable form. Although pentitols are not effectively metabolized by plaque bacteria, xylitol, for example, is utilized by humans through normal preexisting pathways. This is an important difference between oral microorganisms and the animal organism. The latter benefits from the caloric value of xylitol while to most bacteria xylitol is an inert or inhibitory compound (Makinen, 1978a).

B.

LOWERED CARIOGENICITY OF FOODS CONTAINING XYLITOL

I.

Turku Sugar Studies

Perhaps the best proof of the extremely low or nil cariogenicity of xylitolcontaining foods so far obtained has been in the Turku sugar studies (Scheinin and Mikinen, 1975). This was a series of collaborative studies camed out at the University of Turku, Institute of Dentistry, Turku, Finland, in which human volunteers were placed on strict xylitol, fructose, or sucrose diets for 2 years. The caries incidence in the xylitol group was reduced very strongly, suggesting approximately 90% reduction compared with the sucrose group (Fig. 1). This same reduction in the incidence of dental caries, associated with xylitol, was, however, also obtained in a 1-year chewing gum trial (Fig. 2). In other words, these two studies showed that both the full substitution, involving consumption of approximately 70 gm of xylitol per day and partial substitution of sucrose with

140

KAUKO K. MAKINEN

TABLE 1 BIOCHEMICAL PROPERTIES OF DENTAL PLAQUE INVOLVED IN THE ETIOLOGY OF DENTAL CARIES Cariogenic traits of plaque and carbohydrates

Significance of the microbial products formed

I . Glycolysis C,H,zO,

1 . Dissolution of the hydroxyapatite structures

+ CH,CH(OH)COOH

Glucose or other n(CG)sugars ( n

3

1)

2. Other fermentations Propionic acid fermentation Acetic acid fermentation Mixed-acid fermentation Fermentations forming carboxylic acids capable of chelation 3 . Synthesis of extracellular polysaccharides CizHmOit Sucrose" GTb

I

O-W2

0-CH)

"

OH

oH

~OH

H

z

0-CH

OH

I

"-*"_

L ^.. " .

OH

W

1. Adhesive factors (to enamel, cement, etc.,

and to each other) 2. Diffusion barriers (against the rinsing effects of saliva)

forn other organisms ~ 3. Nutrients ~ ~

4. Protective layers against oral defensive

O-CH2

DylHH W OH I

1. As above 2. Chelation of hard-tissue metal ions

a'$oH

H ~ C H z H OHH H

I

of hard tissues 2. Providing an acid environment for organisms favoring low pH values 3. Action as a nutrient substrate for other plaque bacteria

factors %H

O

H

5 . Inflammatory to periodontal tissues

(C& @,)n dextrans

+

fructose or

(continued)

XYLITOL AND ORAL HEALTH

141

TABLE I-(cuntinued) C b o g e n i c traits of plaque and carbohydrates

Significance of the microbial products formed

4. Synthesis of intracellular polysaccharides

Takes place intracellularly following sugar transport

5 . Formation of microbial intracellular and

extracellular enzymes

Form reserve energy sources during periods of low supply of nutrients (sugars and amino acids). Typical at the interphases of olderdental plaque. The final products differ very much in molecular weight branching points, etc., depending on the specificity of the enzyme, bacterial strain, chemical and physical environment, acceptor molecule involved at the start of the synthesis, substrate concentration. etc. Some of the enzymes may be important in the breakdown of covalent chemical bonds in the organic structure of hard tissues during pathogenesis

Glucose, fructose, and other carbohydrates may partly replace sucrose. G T = glycosyltransferase.

xylitol (6-7 gm per day and subject, or 3 to 4 chicles per day) produced essentially the same result. The 2-year feeding study showed that the consumption of a fructose diet reduced the incidence of caries by approximately 30% (Scheinin and Mainen, 1975). Some scientists have been under the impression that this reduction was primarily on smooth surface caries comprising precarious lesions not recognized as such by the U.S. dental profession, although recognized as caries in several other countries. It has to be emphasized, however, that in the above studies the caries incidence was expressed in several quantitative and qualitative terms, including the conventional ADMSF-index (increment in the number of decayed, missing, and filled tooth surfaces). Irrespective of the way of expressing the

TABLE I1 COMPARISON O F PROPERTIES OF XYLITOL AND SUCROSE THAT ARE IMPORTANT IN DENTISTRY AND NUTRITION PropeflY

Sucrose 1.o

I . Relative sweetness 2. Organoleptic properties

Good

3. Solubility in water (20°C) 4. Combustion value

199.4 gm/100 gm H,O 4.06 kcaVgm

5 . Occurrence in nature

Widespread

6. Fermentation in dental plaque 7. Production of organic acids in plaque

Rapid and virtually total Variety of acids via bacterial fermentations

Xylitol I .o Good (the strongly endothermic reaction when crystalline xylitol dissolves in saliva, i.e., cooling effect, is considered an advantage) 168.8 gm/100 gm H,O 4.06 kcaVgm (The true value is most likely about 5% lower, which would mean an advantage) Widespread in the plant kingdom but at lower concentrations than sucrose: values up to 0.3-1.0 g d 1 0 0 gm dry matter. Also occurs in certain mammalian organs (e.g., liver) Very rare Nil or extremely low rate of formation

8. Effect on plaque pH values

9 . Effect on whole saliva (oral fluid) pH values 10. Stimulatory effect on flow of saliva 11. Relation to periodontal tissues

12. Effect on the levels of salivary lactoperoxidase 13. Effect on levels of salivary proteins 14. Effect on the levels of HCO, ions in saliva

Usually produces acidic plaque rapidly (pH below the critical point of 5.5 as regards hydroxyapatite dissolution) Small initial increase (0.2-0.3 units), often during 5-10 minutes, then leveling off to starting values Moderate amounts stimulate as xylitol

May be associated with overgrowth of bacteria at the gingival margin Affects as the regular diet Affects as the regular diet

No remarkable effect

The pH values virtually never fall below 5 . 5 , but rather stay above 6.0 Rather strong rise (even 0.4-0.8 units); the leveling off takes place more slowly Moderate amounts (10-30 g d d a y in unadapted subjects) stimulate as sucrose. In adapted subjects, up to 70 gm may be regarded as moderate D o e s not promote the overgrowth mentioned. No irritation of periodontal tissues found May, under certain conditions, be associated with elevated levels As above May be associated with higher levels, whichindicates better buffering capacity in saliva

144

KAUKO K. MAKINEN

X W

n

z

-

l2

~d

lo

t

8 -

2

6 -

k l-

2

t .SUCROSE FRUCTOSE XYLITOL

m

wa

5

4

2

0

6

8

10 12

14

16

18 20 22 24 MONTHS

FIG. 1. Turku sugar studies. Development of Canes Activity Index in man during consumption of sucrose, fructose, or xylitol diets for 2 years. The Index equals all clinical and radiographic quantitative and qualitative changes, including the cumulative development of DMF (decayed, missing, and filled) tooth surfaces, all the secondary caries reversals, and all qualitative changes in the size of the caries lesions. Xylitol was shown to be essentially noncariogenic. From Scheinin et al. (1975a).

canes increment rate, the reduction in the canes incidence in the xylitol group as compared to the sucrose group exceeded 85%, and a corresponding 30% in the fructose group. Furthermore, although smooth surface lesions would not be included in calculations of caries activity, the appearance and disappearance of such lesions as a function of the sweeteners used, viz sucrose, fructose, and xylitol, should be considered. A smooth surface lesion may lead to more irreversible tissue destruction. It was also shown in a recent article (Mikinen, 1978c) 5 4

-

W SUCROSE CHEWING GUM O - - O XYLITOL CHEWING GUM

3 -

2 -

_-_-_---------*

-0

1

2

3

4

5

6

7

8

9

101112 MONTHS

FIG. 2. Turku sugar studies. Total canes activity following 12-month use of either sucrose or xylitol chewing gum. The ordinate gives the increment of decayed, missing, and filled tooth surfaces, all new secondary caries reversals, and the increment in lesion size of primary and secondary caries reversals. The subjects consumed 4.0pieces of chewing gun1 per day in the sucrose group and 4.5 in the xylitol group. Other diet factors were maintained normal. Even partial substitution of the dietary sucrose with xylitol strongly reduced the incidence of dental caries. From Scheinin er n l . (1975b).

XYLITOL AND ORAL HEALTH

I45

that the strong reduction in the incidence of caries observed at Turku was not caused by the inclusion of precarious lesions in the calculations. The fact that an intermittent exposure of three to four times a day of xylitol chewing gum accomplished the same as a total replacement of sucrose by xylitol in the diet shows that a full substitution is unnecessary in the raajority of cases. These results also suggest that xylitol would act ideally when used in a slowly soluble form (in chewing gum) which simultaneously enables active mastication and salivary flow. In a full substitution much of xylitol would be swallowed without causing these long-lasting local effects. The two studies thus suggested that it was not the total amount of xylitol which was decisive, but rather the continuous, immediately after meal use of chewing gum at a constant frequency. Basing on the results of the 2-year feeding study and the 1-year chewing gum trial, the authors of these studies suggested that xylitol may under certain circumstances be considered therapeutic (Scheinin and Makinen, 1975). These views were supported by biochemical and microbiological results obtained in the 2-year trial: the consumption of a xylitol diet reduced the concentration of lactic acid in dental plaque and mixed saliva (oral fluid) and virtually removed from mixed saliva an invertase-like enzyme group which liberates reducing sugars from sucrose (Mikinen and Scheinin, 1975). The first of these findings indicated reduced chances for dissolution of hydroxyapatite, the main tooth mineral. The second suggests that during the course of the consumption of a xylitol diet, the plaque’s ability to attack sucrose was reduced. For example, dental plaque obtained from xylitol-consuming subjects was characterized by increased nitrogen metabolism compared with plaque obtained from sucrose-consuming subjects (Miikinen and Scheinin, 1975). In the latter case the metabolism of carbohydrates was more prevalent than in the former. The above chemical effects can be regarded as normal consequences in many bacterial cultures (mixed or pure) in which the microbial cells are deprived of their best source of energy. In the case of dental plaque one of the best energy sources is sucrose. Under such circumstances microorgani;ms search for other nutrients available, e.g., proteins, peptides, and amino acids of the medium, with a concomitant increase in bacterial transaminations, peptide bond hydrolysis, and related traits of the nitrogen metabolism. The microorganisms then also may increase the breakdown of salivary glycoproteins and mucins secreted from the salivary glands. It has been observed that the consumption of a xylitol diet is associated with slightly increasing levels of the activity of salivary glycosidases. The enzymes whose levels have been found to increase include a-fucosidase, a-glucosidase, and Pgalactosidase (Makinen et al., 1975b). This is in full accord with the fact that the salivary glycoproteins contain fucose, glucose, and galactose in the prosthetic groups of the glycoproteins. An important function of the salivary glycoproteins is to protect the enamel in the form of a thin (0.001 mm) layer which is adsorbed and/or precipitated onto the enamel surface. This film is called acquired pellicle (Dawes, 1968). For the

146

KAUKO K. MAKINEN

subsequent discussion it is important to note that the carbohydrate side chains in the salivary glycoproteins often bear sialic acid (N-acetylineuraminic acid) as the terminal carbohydrate. Although sucrose seems to be most effective in producing caries, glucose and fructose also cause some damage, although usually at a lower rate. The particular chemical properties which make sucrose especially cariogenic are in part related to the special energy of hydrolysis of this disaccharide. The enthalpy of the reaction, AH, is about -28 kJ per mole. Other common disaccharides yield lower values. The energy mentioned is built in the glycosidic bond of the sucrose molecule and is partly utilized by cariogenic microorganisms in their production of extracellular polysaccharides. This biosynthesis thus proceeds without phosphorylated energy-rich intermediates. Extracellular sucrase enzymes may in turn facilitate the formation of glucose and fructose, which enter glycolysis. Xylitol does not offer the above energetic and other advantages to cariogenic microorganisms. The strong reduction in the incidence of dental caries in the 2-year study is easy to understand in view of the bacteriophysiological aspects mentioned. Because the same clinical result was achieved in the chewing gum trial as well, additional explanations will be required. These will be dealt with in subsequent paragraphs, and the previously mentioned glycoproteins may play an important role in this context. Numerous in vitro plaque and microbiological studies and in vivo animal studies on xylitol have been carried out. The results of a single in vitro experiment of the above type should not be extrapolated to show any anticariogenic effects of xylitol in humans, but as the number of supporting in vitro and animal studies is rather high, the accumulated total evidence strongly indicates the noncariogenicity, nonacidogenicity, and even anticariogenicity of xylitol under certain fixed conditions. Virtually all published xylitol studies, along with a few unpublished ones, have been reviewed and listed elsewhere (Mikinen, 1976a, 1978a,b). A few pertinent investigations will be mentioned in the subsequent sections. A detailed description of the Turku studies has already appeared (Scheinin and Miikinen, 1975), supplemented by a few generalizing articles (Makinen, 1976b-d; Scheinin, 1976a,b). 2 . pH Telemetry Muhlemann and his co-workers have observed the nonacidogenic nature of xylitol in human use by pH telemetry, which involves pH registrations of interdental areas with a microelectrode placed on a desired tooth site. Xylitol has acted in these experiments as an inert carbohydrate that is not to any significant extent, or at all, attacked by plaque bacteria. Xylitol does not cause pH drops in this type of experiment (Hassel, 1971; Miihlemann et al., 1977).

147

XYLITOL AND ORAL HEALTH FISSURE

CONTROL

P

(BD)

B D + 2000 S ED + SUC.

CARIES

CHOCOLATE

ED + FRU. CHOCOLATE

BD + SOR. CHOCOLATE BD+ XYL. CHOCOLATE

YIG. 3. Effect of various sugars on rat caries. Caries incidence in the lower jaws of rats after Irogrammed feeding o f various chocolates for 6 weeks. The concentrations of sucrose and sucrose substitutes were 10-30% in the basal diet. The arithmetic means and the standard deviations are indicated. BD = Basal Diet; 2000 S = a cariogenic diet; SUC = sucrose; FRU = fructose; SOR = sorbitol; XYL = xylitol. From Gehring and Karle (1974).

3 . Studies on Rat Gehring and co-workers have clearly demonstrated the noncariogenicity of xylitol in rat experiments (Gehring and Karle, 1974). For example, rats fed a cariogenic diet (Fig. 3) developed the expected caries incidence. When xylitol chocolate was added to the basal diet, the caries incidence remained unchanged.

111.

MICROBIOLOGICAL ASPECTS

To date the only long-term human clinical trial involving an in vivo microbiological follow-up study during continuous consumption of xylitol was carried out in connection with the Turku sugar studies. The consumption of xylitol did not affect the major microbial categories occurring in dental plaque and mixed saliva (Lamas et al., 1975). On the other hand, the mean values of viable S. mutans in plaque were lower in the xylitol group than in the sucrose or fructose groups throughout the study (Gehring et al., 1975). Furthermore, the geometric and arithmetic means of the colony-forming units on selective Rogosa S.L. agar were significantly lower in the xylitol group than in the fructose and sucrose groups, A reduction of the acidogenic oral flora was observed particularly in the xylitol group. During the course of the study, no evidence was obtained of microbial adaptation or mutation enabling acidogenic decomposition of xylitol

148

KAUKO K. MAKINEN

(Scheinin and Makinen, 1975), nor have any phenomena been observed after 4.5 years of continuous consumption of xylitol (Mikinen and Virtanen, 1978). One of the working hypotheses of the Turku sugar studies was that the human oral microorganisms would gradually become adapted to xylitol use. However, no adaptation to xylitol use was detected. During consumption of higher amounts of xylitol in place of sucrose or other fermentable sugars, the oral micoorganisms started to use extracellular proteins, peptides, and amino acids as a source of energy, with concomitant changes in the nitrogen metabolism, as mentioned previously. As discussed elsewhere (Mikinen, 1976b, 1978a), adaptation, from the microorganisms' point of view, may not be necessary as long as easily available and water-soluble hexose-based sugars are available in the diet. The nonfermentability of xylitol in human dental plaque seems to be genetically fixed, for reasons of evolutionary expediency. An illustrative example of the strong diet-dependent changes in the incidence of certain microorganisms in the human oral cavity is shown in Fig. 4. The subject, who first consumed a fructose diet for 1 year, showed the colony5000

4000

3000

20

2000

; 1000

15

v

z

0

v

0

cn

LL

500 400 300

10

Q

200 100 0

2

5

-

0

5

10

FRUCTOSE -XYLITOL-

(---

)

15

20 MONTHS

0

(- - - --)

FIG. 4. Turku sugar studies. The relationship between diet and the incidence of salivary microorganisms and dental caries. A female subject was on a strict fructose diet for 1 year, followed by the consumption of a strict xylitol diet for another year. The figure gives the total salivary colonyforming unit values (CFU) on Rogosa S. L. agar on a logarithmic scale, and clinically and radiographically detected dentine caries lesions, as well as newly filled surfaces. The change of diet resulted in a steep decrease in the levels of the parameters investigated. From Lamas ef d.( I 975).

XYLITOL AND ORAL HEALTH

149

forming unit values indicated. When fructose was replaced with xylitol, there was a very sharp decrease in the incidence of microorganisms in saliva. It is interesting to observe that when the diet was changed, the incidence of dental caries also began to decrease. The DMFS values finally showed a clear tendency to decrease below the maximum previously attained. These types of changes led the authors of the Turku studies to ascribe therapeutic and anticariogenic claims to xylitol (Scheinin and Miikinen, 1975). Other microbiological studies have shown that Actinornyces viscosus, a typical oral microorganism which may be associated with root caries, does not ferment xylitol (Noguchi and Muhlemann, 1976). Another microbiological investigation showed that, of more than 200 oral bacterial strains, representing 10 oral genera, none used xylitol (Havenaar et al., 1978). Xylitol was not shown to promote the growth of oral Candida (Makinen et al., 1975~). After a series of mutations, microorganisms outside of the human oral ecosystem may use xylitol even as a novel carbon. In these cases xylitol often uses transport systems initially developed for the transport of other carbohydrates across cell membranes (Makinen, 1978a).

IV. IN VITRO PLAQUE STUDIES Labeled xylitol binds to human dental plaque only to a very limited extent (Mikinen, 1976a; Makinen and Rekola, 1976). This may be an important finding, as the sugar transport across cell membranes usually requires binding of the carbohydrate molecule to the specific recognition sites of a cell wall (Kaback, 1970). The human oral microorganisms possess such sites to a very small extent only. It can furthermore be assumed that the observed low binding of xylitol resulted from nonspecific reactions. The activity levels of plaque xylitol dehydrogenase are constantly very low or nil, whereas those of sorbitol dehydrogenase are understandably higher (Makinen and Scheinin, 1975; Makinen and Virtanen, 1978). These enzymes are often regarded as the first enzymes in the possible decomposition of the polyols in dental plaque. However, certain oral bacteria may, as the first step, phosphorylate mannitol and sorbitol (Stegmeier et al., 1971). Gulzov (1976) has also shown the low ability of model systems to produce acids from xylitol.

V. A.

XYLITOL AND THE EXOCRINE GLANDS

PRELIMINARY FINDINGS IN FEEDING STUDIES

The first finding that suggested that sugar alcohols, compared with sucrose, would cause different effects on the biochemistry and physiology of the exocrine glands was obtained in the Turku sugar studies: a xylitol diet was associated with

150

KAUKO K . MAKINEN

clearly higher levels of the salivary lactoperoxidase than the sucrose or fructose diets (Makinen et al., 1975a, 1976). Lactoperoxidase is an oxidative enzyme which is a part of the body’s innate defense mechanisms. This enzyme may be important for the antimicrobial properties of such secretions as saliva, lacrimal fluid, and milk (Morrison and Steele, 1968). The enzyme needs thiocyanate ions (SCN-) and hydrogen peroxide as cofactors. These compounds are present in saliva and plaque at low concentrations and the enzyme system thus formed is capable of inhibiting pathogenic and apathogenic oral bacteria. For example, several species of Lactobacillus, Streptococcus, and Corynebacterium are sensitive to the lactoperoxidase system. The above finding suggested that the xylitolinduced elevated lactoperoxidase levels in saliva and the anticariogenic properties of xylitol are partly interrelated phenomena. The above studies in humans were followed by other experiments on monkeys, cows, and rats. The consumption of moderate amounts of xylitol for 3 days significantly increased the activity levels of monkey (Macacu mufutta) parotid and submandibular lactoperoxidase (Mikinen et al., 1978). Simultaneously, the levels of protein and a-amylase were also increased (Bird et al., 1977). Stimulation of parotid saliva with xylitol fruit pastils caused a slightly higher lactoperoxidase activity in human parotid saliva than the sucking of corresponding hexose-based pastils (Harper et al., 1977). Cows fed 0.5 kg/day of a polyol mixture containing 10% xylitol (w/w) showed slightly higher milk lactoperoxidase activity compared with feeding a molasses or control ration, but a 2-day intake of xylitol (0.5 gm per kg body weight and day) did not cause this effect in lactating mothers (unpublished results from this laboratory). Homogenates of the submandibular and lacrimal glands of the rat showed slightly higher lactoperoxidase activity following drinking of xylitol-sweetened (4%) water for 2 months compared with drinking a correspon2ing glucose solution (unpublished results from this laboratory). Due to the preliminary nature of these studies, verifying data are clearly required before further conclusions are drawn about the relationship between polyol feeding and exocrine gland function. The above results clearly suggest that the relation of sugars, sugar alcohols, and dietary ingredients in general to exocrine gland function is interesting and deserves investigation. It is most likely that the above selective enzyme changes only represent normal and physiological responses to the various dietary ingredients. It is unlikely that this line of research would reveal any pathologically alarming findings. Perhaps the best evidence for this was obtained in the 2-year study previously mentioned, which clearly demonstrated that the safety of moderate amounts of oral xylitol is indisputable. B.

SIALIC ACID AND SIALOPROTEINS

Sialic acids are a group of N - and 0-acyl derivatives of the 9-carbon 3deoxy-5-amino sugar neuraminic acid. The sialic acids are ubiquitously dis-

0

/CH3

\CH I H2N - C - H

c=o

e

ooc

I

c ooe

I L

HO-C-H I H-C-OH I

H - C - OH I CH20H

\

//

H2

OOOC

/c\

C HO-C-H

I

H2N-C-H HO - C

/

-

HO’I

‘C-H

0 H,r-C

I-H

‘CL I H-C-OH

H-C-OH

H - C -OH

I I

I

I

KETO

I

CHZOH

FORM PYRANOSE NEURAMINIC ACID

(I)

N -ACETYLacetylation J

H-C-OH

CH20H D -MANNOSAMINE

2; ‘C’

FORM

NE UR AM1NIC ACID (SIALIC ACID)

1

GLYCOPROTEINS MUCOPOLYSACCHARIDES

152

KAUKO K. M K I N E N

tributed in tissues in the form of mucopolysaccharides and mucoproteins, including salivary macromolecules. The sialic acid from human plasma has the structure shown in (I). The biosynthesis of N-acetylneuraminic acid takes place as an aldol condensation of the N-acetylhexosamine and pyruvic acid. Certain microorganisms produce enzymes (neuraminidases) which can effect hydrolysis to these products. Sialoproteins and sialopolysaccharides play an important role in the host defense mechanisms in the oral cavity. Sialoproteins contribute to the formation of the acquired pellicle and some of them may be involved in the elimination of microorganisms from the mouth. The study of the role of sialomacromolecules in oral and other diseases has intensified, and, as to the oral conditions, it has been suggested that dietary carbohydrates exert a selective effect on the secretion of such compounds. For example, polyol-stimulated whole saliva contained more sialic acid than paraffin-stimulated whole saliva. Because this type of effect is not readily seen in parotid saliva, it can be assumed that the submandibular and perhaps sublingual glands were specifically affected (unpublished results from this laboratory). Such findings should also be interpreted as normal physiological responses to changes in the diet, not as pathological consequences.

VI. XYLITOL AND PERIODONTAL DISEASES The consumption of a xylitol diet was not shown to cause any periodontal problems in man (Paunio et al., 1975). On the contrary, it can be assumed that xylitol would have advantageous indirect effects on the periodontal tissues in the form of enhanced host defense mechanisms (lactoperoxidase) and its inability to promote plaque overgrowth. As a matter of fact, xylitol has been shown to decrease the growth of dental plaque by approximately 50% compared with sucrose and fructose (Mainen and Scheinin, 1975). Of particular importance in relation to periodontal diseases are the properties of the crevicular exudate which is nearly always present in small amounts in the gingival pockets. Increased inflammatory changes in the periodontal tissues also increase the flow of the exudate. The exudate contains typical inflammatory cells and mediators. The mediators are responsible for the inflammatory reactions on the macroscopic, microscopic, and biochemical levels, and they affect, among other things, the microcirculation in capillaries. Gingival exudate, collected from subjects who were on a xylitol diet, displayed clearly smaller microcirculation velocity values in the hamster cheek pouch microvasculature than exudates obtained from subjects consuming sucrose or fructose (Fig. 5 ) . This finding should be interpreted as showing the insignificant inflammatory qualities of xylitol in peroral administration. The consumption of xylitol seemed to reduce the enzyme content of

153

XYLITOL AND ORAL HEALTH

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exudate compared with sucrose and fructose. The enzymes which were studied include peroxidase (Makinen el al., 1975a), glycosidases (Mikinen et al., 1975b), and aminopeptidase (Paunio et al., 1975). These enzyme findings should be interpreted as showing an increased clearance of inflammatory compounds and microbial enzymes (particularly glycosidases) from the exudate during xylitol consumption.

VII.

MECHANISM OF ACTION OF XYLITOL

The mechanism of action of xylitol in dental caries prevention is rather well known. While a number of details still require intensive studies, the very near future will certainly reveal the most important remaining aspects. The following list presents the cornerstones which, according to the available literature, should be considered in the description of the xylitol effect: (a) The xylitol molecule is shorter than the hexitol molecules that are regularly metabolized by oral microorganisms. The difference of this molecular parameter

154

KAUKO K . MAKINEN

between xylitol and sorbitol is not big (Fig. 6), but in the chemistry of the active site of the microbial enzymes it is decisive. No matter what type of mechanism is involved in the substrate specificity of enzymes involved in the initial breakdown of hexitols, the improper length of the xylitol molecule makes it a poor substrate for most such enzymes. So, for example, xylitol may not be able to bring into effect such specificity mechanisms as lock and key, productive binding, and induced fit. The above not only concerns polyols, but the arrangement of Cs compounds versus C5 compounds is valid in many other cases in the biochemistry of carbohydrates as well. Consequently, the pentitol nature of xylitol is an important ecological chemodeteminant in plaque metabolism. The configurations of the molecules, however, also contribute to their suitability as substrates for bacterial metabolism. It is necessary to indicate that, unlike virtually all oral microorganisms by which xylitol is not metabolized, xylitol is metabolized by the human body. (b) The corollary to the above is: As a consequence of the inability of oral bacteria to metabolize xylitol effectively, it is virtually never converted to acid in human dental plaque: Consequently, the pH values attained at the plaque interface will most likely be on the safe side (above 5.5-6.0; Fig. 7) compared with the situation involving consumption of fermentable carbohydrates. The critical pH value (approximately 5 . 5 ) with regard to hydroxyapatite dissolution will not be reached readily during xylitol consumption.

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(c) Xylitol causes, as do the other sweet carbohydrates so far studied, increased salivary flow rates. With xylitol, however, the pH values reached are more advantageous (approximately 7.2-7.8) than with fermentable sugars and the salivary defense mechanisms act effectively. One of these mechanisms comprises the remineralizing ability of saliva. Saliva is normally supersaturated with regard to enamel (hydroxyapatite, or Caw and phosphate). The calcium and inorganic phosphate concentrations of paraffin-stimulated mixed saliva is usually 10-15 mg/l and 75-210 mg/l, respectively. These concentrations are sufficient to account for the supersaturated state mentioned. The maintenance of higher and constant pH values induced by xylitol (approximately 7.2-7.8) gives the supersaturated state possibilities for the repair of initial demineralized areas. (d) Xylitol can cause, under specified conditions, an elevation of the levels of other salivary defensive factors such as lactoperoxidase or glycoproteins, of which the latter are required in the formation of the acquired pellicle, and possibly other factors. The present data also suggest that the buffering capacity of saliva is maintained at a more effective stage following stimulation with xylitol compared with stimulation with sucrose, i.e., more HCO, ions would be present in the former case.

In summary, the mechanism of the xylitol effect is dual. It partly comprises the nonfermentability of xylitol in the human dental plaque, and partly the stimulation of a number of salivary host defense factors. The latter ones are also dual: there is the possibility of a direct stimulatory effect (e.g., via nerve impulses) and of a systemic effect via the following route: stomach (with possible release of

156

KAUKO K. MAKINEN

gastric hormones), circulation, salivary glands. Indications of both types of phenomenon hhve been obtained.

VIII.

RESEARCH NEEDS

1. Intestinalflora. The effect of the consumption of higher amounts of xylitol on

the composition of the intestinal flora and the biochemistry (vitamin synthesis, etc.) of the microorganisms involved should be studied. 2. Secretion of glycoproteins. The effect of xylitol (and dietary ingredients in general) on the secretion of glycoproteins and mucopolysaccharides from exocrine glands should be better elucidated. 3 . Lowest effective dose. It would be of great value to determine the minimum amount of xylitol, in the presence of various other carbohydrates that still produces a clear protective effect on the oral tissues and particularly dental caries. Severe clinical cases. Particular lines of research should be planned and pursued to demonstrate the possible benefits of xylitol in rampant caries, severe periodontal diseases, xerostomia, and in the treatment of the teeth of diabetic subjects, and as a salivary stimulator in gerontology. Xylitol products. Food technology and clinical sciences should cooperate in the planning of new xylitol products for health care. Such products would be combinations of xylitol with fluorine, vitamins (chew tablets or tonics), other pharmaceutical products, prophylactic tooth pastes, preparations used in endodontics (xylitol plus penetrating detergents in the treatment of carious dentine, etc.), and products aimed at enhancing the salivary flow.

REFERENCES Bird, J. L., Baum, B. J . , Makinen, K . K . , Bowen, W. H . , and Longton, R . W. 1977. Xylitol associated changes in amylase and protein content of monkey parotid saliva. J. Nutr. 107, 1763- 1767. Dawes, C . 1968. The nature of dental plaque, films, and calcareous deposits. Ann. N.Y. Acad. Sci. 153, 102-119. Gehring, F., and Karle, E. 1974. Tierexperimentelle Untersuchungen ubex Zuckeraustauschstoffe und Zuckerzusatzstoffe. Sonderforschungsber. 92 Univ. Wurzburg, B i d . Mundhohle, 1973 p. 192. Gehring, F., Makinen, K . K., Larmas, M., and Scheinin, A. 1975. Turku sugar studies. X. Occurrence of polysaccharide-forming streptococci and ability of the mixed plaque microbiota to ferment various carbohydrates. Acta Odontol. Scand. 33, Suppl. 70, 223-237. Gulzov, H.-J. 1976. Comparative biochemical investigations on the degradation of sugars and sugar alcohols by microorganisms of the oral cavity. I n t . J. Viram. Nutr. Res., Suppl. 15, 348-357.

XYLITOL AND ORAL HEALTH

I57

Harper, L. R., Poole, A. E., and Wolf, S. I. 1977. Xylitol stimulation of lactoperoxidase in human parotid saliva. J . Dent. Res. 56, Spec. Issue A , A62. Hassel. T. M. 1971. pH-Telemetrie der interdentalen Plaque nach Genuss von Zucker und Zuckeraustauschstoffen. Dtsch. Zahnaerrzl. 2. 26, 1 145-1 154. Havenaar, R., Huis in’t Veld, J. H. J . , Backer Dirks, 0.. and de Stoppelaar, J . D. 1978. Microbiological aspects of sugar substitutes. Caries Res. 12, 118. Kaback, H. R. 1970. Transport. Annu. Rev. Biochem. 39, 561-598. Larmas, M., Makinen. K . K., and Scheinin, A. 1975. Turku sugar studies. VIII. Principal microbiological findings. Acru Odonfol. Scund. 33, Suppl. 70, 173-216. Luostarinen, V., Paunio, K., Varrela, J . , Rekola, M., Luoma, S . , Scheinin, A., and Makinen, K . K . 1975. Turku sugar studies. XV. Vascular reactions in the hamster cheek pouch to human gingival exudate. Acfa Odontol. Scand. 33, Suppl. 70, 287-291. MacFarlane, T. W., and Mason, D. K . 1972. Local environmental factors in the host resistance to the commensal microflora of the mouth. I n “Host Resistance to Commensal Bacteria” (T. MacPhee, ed.), p. 64. Churchill-Livingstone, London. Makinen, K . K. 1976a. Microbial growth and metabolism in plaque in the presence of sugar alcohols. Microbiol. Abstr. 2, Spec. Suppl., 521-538. Makinen, K. K . 1976b. Dental aspects of the consumption of xylitol and fructose diets. I n t . Dent. J . 26, 14-28. Makinen, K. K. 1 9 7 6 ~ Long-term . tolerance of healthy human subjects to high amounts of xylitol and fructose: General and biochemical findings. Int. J . Vitam. Nutr. Res., Suppl. 15, 92-104. Makinen, K. K. 1976d. Possible mechanisms for the cariostatic effect of xylitol. I n t . J . Vitam. Nurr. Res., Suppl. 15, 368-380. Makinen, K. K. 1978a. Biochemical principles of the use of xylitol in medicine and nutrition with special reference to dental caries. Experientia, Suppl. 30, 1 - 160. Makinen, K. K. 1978b. Approaches to food modification: Xylitol. Proc., Workshop Cariogenicity of Food, Beverages, Confections, Chewing Gum, pp. 99-1 13. Am. Dent. Assoc. 1977. Makinen, K. K . 1978c. The use of xylitol in nutritional and medical research with special reference to dental caries. Proc., Sweeteners Denf. Caries, 1977. Feeding, Weight & Obesity Absfr. Spec. Suppl., 193-224. Makinen, K . K., and Rekola, M. 1976. Xylitol binding in human dental plaque. J . Dent. Res. 55, 900-904. Makinen, K. K . , and Scheinin, A. 1975. Turku sugar studies. VII. Principal biochemical findings on whole saliva and plaque. Acfa Odontol. Scand. 33, Suppl. 70, 129-171. Makinen, K. K., and Virtanen, K . 1978. Effect of 4.5-year use of xylitol and sorbitol on plaque. J . Dent. Res. 57, 441-446. Makinen, K . K . , Tenovuo, J . , and Scheinin, A. 1975a. Turku sugar studies. XII. The effect of the diet on oral peroxidases, redox potential and the concentration of ionized fluorine, iodine and thiocyanate. Acra Odontol. Scand. 33, Suppl. 70, 247-263. Makinen, K. K., Laikko, I., Scheinin, A,, and Paunio, K. 1975b. Turku sugar studies. XVII. The activity of glycosidases in oral fluids and plaque. Acta Odontol. Scand. 33, Suppl. 70,297-306. . of xylitol on the growth of three oral Makinen, K . K., Ojanotko, A , , and Vidgren, H. 1 9 7 5 ~Effect strains of Candidn albicans. J . Dent. Res. 54, 1239. Miikinen, K. K., Tenovuo, J., and Scheinin, A. 1976. Xylitol-induced increase of lactoperoxidase activity. J . Dent. Res. 55, 652-660. Makinen, K. K., Bowen, W. H., Dalgard, D., and Fitzgerdld, G. 1978. Effect of peroral administration of xylitol on exocrine secretions of monkeys. J . Nutr. 108, 779-789. Morrison, M., and Steele, W. F. 1968. Lactoperoxidase, the peroxidase in the salivary gland. I n “Biology of the Mouth” (P. Person, ed.), p. 89. Am. Assoc. Adv. Sci., Washington, D.C.