Compliance and success with daily toothbrushing of pets are sufficiently uncommon amongst owners that the diet remains an essential component of dental homecare. There are several important mechanisms by which the diet can modify dental disease, only some of which have been properly investigated, and few of which are completely understood.
Physical Abrasion and Food Deposition
Periodontitis will develop in almost all animals fed commercial diets, which do not have other regular measures to prevent the disease. Even more so, softer diets appear convincingly to be worse. Cats fed dry food develop less calculus and gingivitis than cats fed exclusively canned food.1 In a large survey of domestic cats in Japan, calculus was more common in cats fed canned or home-cooked meals than cats fed dry foods (41% vs. 25%).2 Similarly, it was shown many years ago that soaking dry food prior to feeding is a reliable method of inducing the rapid formation of plaque, calculus, gingivitis, and eventually periodontitis in dogs.3 In a study of dogs in Brazil, those that were fed diets of home-prepared foods and scraps had significantly more dental disease than those fed commercial dry diets.4
Additionally, when commercial diets are supplemented with more abrasive, "natural" ingredients, the development of periodontitis is retarded or even prevented. Feeding raw oxtails as a supplement to a dry food has been shown to be effective in minimising the development of gingivitis and periodontitis in long-term (> 6 years) studies in beagles.5 Once-weekly feeding of oxtails was shown to remove existing calculus to 5% of previous amounts within 2 weeks and to maintain them at that level for years. Further, a diet consisting of raw bovine trachea and attached tissues was much more effective in reducing plaque, calculus and gingivitis than the same diet when fed minced.6
In a large study of several thousand dogs and cats in Poland, animals fed home-prepared diets (HPDs) had significantly poorer oral health than animals fed commercial canned food, which were worse than animals fed dry food.7 This is consistent with the abrasive and reduced deposition effect of harder food. However, the difference between typically soft, HPDs and commercial canned food also illustrates that there are factors other than the physical effects that influence oral health.
Thus commercial diets are associated with periodontal disease, and softer diets are worse than dry diets, though perhaps there is less difference between the two types of commercial diet than one might expect. In addition, the supplementation of commercial diets with "natural" chews, such as oxtails, dramatically improves oral health. And finally, there is a difference between a commercial complete and balanced diet and a HPD that is not explained by its physical characteristics.
Chelation of Calcium
Sodium hexametaphosphate (HMP) forms soluble complexes with most cations and reduces the availability of calcium for incorporation into plaque to form calculus. It has been proposed that the HMP-calcium complexes are then "washed away" in the saliva. The calcium then disassociates within the acid environment of the stomach, and HMP does not reduce dietary calcium bioavailability. The addition of an HMP solution to a dry diet reduced calculus formation in dogs by almost 80% when softened biscuits were fed.8 However, no long-term studies have been performed to demonstrate that reducing calculus formation in such a manner has any effect on the long-term production of gingivitis or periodontitis in dogs or cats.
Modification of Saliva and Microflora
Normal saliva contains lysozyme, myeloperoxidase, lactoperoxidase, lactoferrin, and histatin, a group of small peptides that bind to hydroxyapatite and can kill Candida albicans, and Streptococcus mutans, and are inhibitory for Porphyromonas gingivalis, Prevotella, and Bacteroides spp. Saliva also contains IgA and leukocytes, and probably has a "flushing" effect, inhibiting the attachment of bacteria to the gingival tissues. In dogs, synthetic, topically applied histatin preparations can significantly inhibit the development of experimental gingivitis.
Animals maintained on liquid diets develop salivary gland atrophy within days. The atrophy is rapidly reversible once a hard diet is reintroduced. The saliva secreted by such animals has a 50% reduction in the protein content. These findings have been observed in animals from rodents to man. So food consistency affects the synthesis of salivary proteins, and the volume of saliva produced. In humans, an inverse relationship exists between the lysozyme concentration in stimulated parotid saliva and the mass of plaque that develops in 48 hours.
In humans, modification of the diet to be of a firmer texture resulted in a 40% increase in the flow rate of stimulated parotid saliva, as well as an increased plaque pH, and salivary flow rates were significantly correlated with the bite force required to consume the diet.9 The flow rate of saliva is significantly increased when human subjects chewed four sticks of sugar-free gum per day for eight weeks.
Ascorbate
High concentrations of ascorbate inhibits the growth of several species of oral bacteria in vitro. A stable form of vitamin C (sodium ascorbyl phosphate) has been added to an experimental diet and fed to cats. After 28 days, the development of gingivitis was almost completely prevented compared with the control group, although plaque and calculus were only reduced slightly. Similarly, the application of zinc ascorbate gel to the mouths of cats caused a significant decrease in gingivitis, and anaerobic periodontal pathogens and plaque. However, the amount of calculus was not significantly reduced.
There is a trend, therefore, that reducing or even preventing the development of dental calculus is insufficient to prevent gingivitis, and is presumably insufficient to prevent the development of periodontitis in either dogs or cats. This applies to experimental means and the "natural" diet.
Plaque, Calculus, and Periodontitis are Not Necessarily Linked
There does not seem to be a clear or simple relationship between the amount of plaque, and especially the amount of calculus on a tooth and the severity of gingivitis associated with it. Some studies have shown a reduction in gingivitis in the absence of calculus reduction by adding dental chews, whilst others have demonstrated a reduction in calculus and plaque with no appreciable reduction in gingivitis.
Therefore, mechanical debridement or simply reducing plaque or calculus formation is unlikely to be the panacea for the prevention of dental disease. So do all these approaches fall short because they are unable to effectively remove plaque, or because simply removing or preventing plaque is insufficient to prevent gingivitis and periodontitis? Mechanical stimulation of the gingivae by toothbrushing enhances proliferation of fibroblasts, collagen synthesis, and a reduction in gingivitis in dogs. When daily toothbrushing, including brushing of the gingivae, was compared with removal of plaque with a supragingival curette in dogs fed a softened diet, toothbrushing reduced inflammatory cell infiltration into the gingival tissues, increased fibroblast proliferation and collagen synthesis after 5 weeks.10 In addition, a greater effect of brushing was seen with twice-daily than once-daily brushing. This difference was apparent despite the complete prevention of plaque accumulation by use of the curette.
Mechanical stimulation by tooth brushing has been found to enhance pocket oxygen tension, decrease exudation, increase microcirculation in gingivae, and increase saliva flow following the induction of gingivitis in dogs. If the rate of gingival tissue turnover and desquamation is increased, access to the gingival tissues of the sulcus may be reduced. Thus the mechanical stimulation by brushing contributes to the prevention of periodontal pocket formation, and can promote epithelial reattachment.
This is a central, important point to recognise in dental home care: prevention of dental calculus formation is insufficient to prevent periodontitis.
Dental Diets
Several food companies have produced diets with claims of benefits to dental health. Such diets are either formulated to abrade the teeth (Hill's, Nestle-Purina, Iams), or contain additives that function as inhibitors of calculus formation (Waltham), as antibacterials (Royal Canin), and as plaque retardants. None of the studies of commercial dental diets have shown 100% efficacy, and some do not demonstrate any benefit in scores of gingivitis. None of the dental diets have been trialled for long enough for us to know their efficacy of preventing periodontitis in the long term. In the longest trial to date, feeding Hill's t/d® for 6 months reduced plaque by 39% and gingivitis by 36% in 20 mixed-breed dogs.11 Although most "dental diets" have some proven efficacy, they are probably inadequate to completely prevent periodontitis in the long term, despite significantly reducing calculus. For the foreseeable future, unless an owner can effectively brush their pet's teeth, effective dental home care requires a multimodal approach.
References
1. Studer E, Stapley RB. The role of dry foods in maintaining healthy teeth and gums in the cat. Veterinary Medicine, Small Animal Clinician. 1973;68:1124–1126.
2. Anon. Survey on the health of pet animals, 2nd report. Japan Small Animal Association. 1985:25.
3. Burwasser P, Hill TJ. The effect of hard and soft diets on the gingival tissues of dogs. Journal of Dental Research. 1939;18:389–393.
4. Domingues LM, Alessi AC, Canola JC, et al. Type and frequency of dental diseases and disorders in dogs in the region of Jaboticabal, SP. Arquivo Brasileiro De Medicina Veterinaria E Zootecnia. 1999;51:323–328.
5. Brown MG, Park JF. Control of dental calculus in experimental beagles. Laboratory Animal Care. 1968;18:527–535.
6. Egelberg J. Local effect of diet on plaque formation and development of gingivitis in dogs. I. Effect of hard and soft diets. Odontologisk Revy. 1965;16:31–41.
7. Buckley C, Colyer A, Skrzywanek M, et al. The impact of home-prepared diets and home oral hygiene on oral health in cats and dogs. The British Journal of Nutrition. 2011;106 Suppl 1:S124–127.
8. Stookey GK, Warrick JM, Miller LL. Effect of sodium hexametaphosphate on dental calculus formation in dogs. American Journal of Veterinary Research. 1995;56:913–918.
9. Yeh CK, Johnson DA, Dodds MW, et al. Association of salivary flow rates with maximal bite force. Journal of Dental Research. 2000;79:1560–1565.
10. Horiuchi M, Yamamoto T, Tomofuji T, et al. Toothbrushing promotes gingival fibroblast proliferation more effectively than removal of dental plaque. Journal of Clinical Periodontology. 2002;29:791–795.
11. Logan EI, Finney O, Hefferren JJ. Effects of a dental food on plaque accumulation and gingival health in dogs. Journal of Veterinary Dentistry. 2002;19:15–18.
Additional references are available upon request.