Milk and dairy products: possible effects on dental health

Milk and dairy products have a low cariogenic potential, but they are also claimed to be cariostatic. Thus, bovine milk and cheese provide protection against caries in rats and in situ, even in caries-susceptible conditions. The mechanisms involve several milk components and effects. Caseins and peptides thereof, i.e. casein glycomacropeptide (CGMP) and casein phosphopeptide (CPP), reduce the adhesion of cariogenic mutans streptococci in situ and seem to reduce colonization in the rat. They block adhesion in solution (clearance), as well as when bound to tooth surfaces. CGMP binds to the tooth as micelle-like structures, which do not bind bacteria. CPP binds as minor complexes with calcium and phosphate, buffering calcium and phosphate when hydroxyapatite solubility increases by decreasing pH, and possibly explaining the acid-buffering effect from milk and cheese. Further, peptides in the whey fraction, i.e. proteose-peptones, provide protection against tooth tissue demineralization, and other milk peptides, such as kappacin (a k-casein-derived peptide), lactoferrin, lactoperoxidase and lysozyme, possess innate immunity-like functions. Using milk components as a caries-prophylactic measure has not been studied in humans, but epidemiological studies con rm associations between milk:cheese intake and protection against caries. However, the impact of possible confounders cannot yet be fully evaluated.


Association of milk and cheese with low caries development
Milk and dairy products comprise a substantial part of the food consumed in Sweden (1), but even though overall mean intake is high substantial individual variations are seen in the population. Milk intake has been negatively correlated with the development of dental caries in children (2) and adolescents (3). Thus, those with low caries development drank more milk than those with high caries development. The strongest association was among children with high sucrose intake (2). Similarly, root surface caries development was halved in elderly people who ate cheese ve or more times per week compared with those with one or fewer intakes per week (4). The aim of the present review is to describe the state of the art of possible anticariogenic properties of milk and cheese.

Effect of milk and cheese on demineralization and remineralization of tooth tissues
A nimal studies and in situ human studies In 1959 Shaw et al. reported that milk, ice cream and cheese reduced caries development in rats (5). Several investigators have since con rmed that milk, cheese, caseins and whey proteins reduce caries development from sucrose even in highly susceptible rats, such as after desalivation (6)(7)(8)(9). These results are supported by a large number of in situ studies, e.g. using enamel discs carried in the human mouth (10). Using this model cheese even restores demineralized enamel in head-and neck-irradiated xerostomic patients (11).

Prevention of fall in pH in tooth bio lms
Milk contains 4 -5% of the disaccharid e lactose, which can be fermented by oral bio lm bacteria; however, unless the bacteria are adapted to lactose, fermentation is signi cantly less than from sucrose ( Fig. 1a) (12,13). Normally, sucrose lowers pH below 5.0, whereas lactose lowers it to around 6.0. With some individual variations, pH lower than 5.5 is hazardous to tooth enamel and lower than 6.2 to root tissues. Thus, under normal conditions the carbohydrate content of milk confers a low cariogenic potential to milk and dairy products. However, drinking milk (12) or chewing on cheese (10,14) counteracts the acidi cation induced by sucrose in the dental bio lm (Fig. 1b). The clinical impact of this is unknown but, theoretically, diminished periods of acidi cation and demineralization may contribute to tooth preservation.

Prevention of tooth demineralization
Removal of casein, fat or lactose from the milk does not affect its protective capacity on demineralization (15). Free calcium and phosphate have some effect, but after removal of all these components milk still contains powerful protective factors identi ed as water-soluble proteose-peptones, amounting to 1 g l ¼ 1 milk (15).

Prevention by milk of the adhesion of cariogenic Streptococcus m utans bacteria to tooth surfaces
Bio lm formation on teeth involves three distinct   (12) and (14), respectively. phases ( Fig. 2): (i) pellicle formation, i.e. adhesion of salivary proteins:polypeptides (or from other sources, such as serum or milk); (ii) attachment of bacteria to pellicle proteins through the matching of bacteria adhesins to host receptors; and (iii) multiplication of the attached bacteria (environmentally driven, such as pH and access of nutrients). Colonization is established.
No single species of bacteria fully explains caries development, but mutans streptococci are frequently found in higher numbers on caries-active than non-active sites, and Streptococcus mutans adhesion is associated with caries susceptibility (16). Milk abolishes the adhesion of S. mutans to saliva-coated hydroxyapatit e (17 -20), possibly through binding of a -, b -, and k -casein to S. mutans in solution (Table 1), and blocking by large micelle-like k -casein structures bound onto hydroxyapatite (21). Peptides derived from k -and b -casein fractions, i.e. casein glycomacropeptide (CGMP) and caseinophosphopeptid e (CPP), respectively, incorporated into the salivary pellicle by displacing albumin, confer adhesion inhibition (22). In addition, milk and k -casein signi cantly reduce the adsorption to teeth of glucosyltransferas e (GTF, a polysaccharide-producin g enzyme from, for instance, S. mutans (23).  (17) and (18). »: inhibited adhesion; ¼: inhibition or no effect on binding to hydroxyapatite (HA).

Antibacterial peptides in milk
Oral epithelial cells and secretory cells in the salivary glands produce antibacterial peptides, refereed to as innate immunity peptides, acting immediately in the mouth upon challenge (24). In general, these peptides are formed by proteolytic cleavage from larger precursor proteins, such as the fungiostatic and bacteriostatic lactoferricin from lactoferrin, and a bacteriostatic and adhesion-preventing RGRPQ-pentapeptide released from salivary acidic proline-rich proteins (25). Recently, caseino-macropeptide (CMP), a C-terminal fragment from bovine k -casein, was shown to inhibit the growth of S. mutans and Porfyromonas gingivalis (periodontal disease) (26). Lactoperoxidase and lysozyme are other milk polypeptides that reduce the metabolism of S. mutans. Notably, an inhibitory effect of bovine milk on saliva aggregation of S. mutans is ascribed to the interference by milk lactoferrin with a streptococcal surface protein antigen (27).

Lactobacilli in fermented milk products do not colonize the mouth
At present, there are no indications that the bacteria used in yoghurt and fermented products colonize the mouth. Thus, neither Lactobacillus casei nor L. acidophilus was established in the mouth after yoghurt consumption (28). Lactobacilli-containing products are reported to reduce the number of mutans streptococci in saliva (marker for bio lm content), but not endogenous lactobacilli or Candida albicans (29,30).

Milk with saliva-mimicking effects for caries prevention and symptom relief
In contrast to common belief, dental caries is far from eradicated in children and adolescents living in Sweden: 10 -15% have high disease activity due to genetic or environmental susceptibility or excessive cariogenic challenge. In these groups, which include children with high numbers of mutans streptococci in early childhood, the present preventive measures are insuf cient. Using selected milk components (i.e. peptides) to prevent the colonization of mutans streptococci at young ages may enhance caries resistance as the child grows older. Furthermore, dry mouth conditions, e.g. due to the adverse effects of medication, head and neck irradiation, or primary and secondary Sjö gren's syndrome, are associated with oral discomfort and rampant caries, with elevated numbers of mutans streptococci. Milk or milk component-based drinks seem to possess many of the biological and physical properties desired for a saliva substitute. Thus, the need for frequent sipping of liquid to lubricate and moisture the mouth is met at the same time as providing caries protection to highly susceptible individuals. Since hyposalivation is often associated with eating problems and inadequate energy and nutrient intake, a drink based on milk or milk components might be used, serving both purposes.