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The benefits of salivary stimulation

This article was previously published from a supplement for Dentistry magazine from the UK.

The value of sugarfree chewing gum, like Wrigley’s ORBIT, as a safe, effective and convenient means of saliva stimulation is well recognized. In this article, Dr Ronnie Levine investigates the dental benefits for your patients.

Many of us have suffered from a dry mouth at some time from your lives. It might have been during a viva examination, or on a first date using an attractive partner. Fortunately, these episodes are transitory, however xerostomia, a more persistent impairment of salivary secretion, can not only be distressing but can have the most profound effects from the mouth (Levine RS, 1989).

In xerostomia, the mouth and throat become dry and sore, eating and speaking become a difficult and painful process and the sensation of taste is reduced. For those using dentures there can be a constant discomfort. The effect on the natural dentition can be rapid and destructive. Plaque deposits build up quickly, leading to gingivitis, and toothbrushing can become a painful ordeal.

Effects on the teeth can be even more dramatic. The desalivation that can occur as a result of radiotherapy for tumours of the head and neck can produce a rapidly developing caries activity. The rate of progress of ‘radiation caries’ will surprise anyone who has not witnessed it before.

The secretion and composition of saliva

Saliva is produced by three pairs of major and many minor salivary glands scattered throughout the mouth and together they are capable of producing up to one litre of saliva a day. Flow rates can vary by a factor of 50 up to 7ml per minute when stimulated.

The composition of saliva can vary rapidly according to flow rate, the type of stimulation and the time of day. Apart from a circadian rhythm during the day, flow rate falls to almost zero during sleep, an important consideration for bedtime snacks and drinks (Levine RS and Stillman-Lowe CR, 2004).

To make research into the composition of saliva even more complicated, the secretions from the three major salivary glands are all different and from themselves vary from their basal and stimulated states.

The contribution made by the minor glands, which are scattered throughout the mouth, also affect the overall composition of pooled saliva, to which we must add the products of the plaque bacterial metabolism, the bacterial cells themselves, shed epithelial cells and the gingival crevicular fluid. It is only when we add all these ingredients together that we arrive at the composition of  ‘whole’ saliva.

Whole saliva is about 99% water and contains a mixture of inorganic ions, including calcium, phosphate, sodium, potassium, chlorine, bicarbonate, and magnesium, together using some minor ionic components, including fluoride. The resting pH is between 6.7 and 7.4, but the parotid gland tends to produce a more acidic secretion.

Apart from these inorganic components, pooled saliva also contains a very wide range of organic molecules. Some of these are simple proteins such as the enzyme albumin, together using free amino acids. However the bulk of the organic component is made up of a group of complex glycoproteins, the mucins. These large molecules are essential to the physical properties of saliva (Edgar WM & O’Mullane DM, 1990).

The role of saliva

In recent years we have gained a much better understanding of both the mechanical and chemical functions of saliva from the mouth.

The first and most obvious role of saliva for most people is its value as a lubricant. This property is largely due to the mucins, which are characterised by low solubility, high viscosity, elasticity and adhesion to the oral mucosa, which enables them to lubricate and protect the epithelial cells lining the mouth. It is these properties that make speech and eating possible.

A second mechanical property of saliva is its binding ability, which enables food to be formed into a bolus for swallowing. In addition, the salivary mucins absorb water and coat the oral mucosa, effectively waterproofing the surface and preventing loss of fluid from the epithelial cells below.

An important role of salivary mucins, which is often overlooked, is their ability to protect the epithelial surface from the many noxious substances that may enter the mouth, including nicotine, alcohol and other chemicals that we may wittingly or unwittingly ingest. They also protect the oral epithelium from the proteolytic enzymes, largely produced by organisms of the dental plaque and those excluded from the gingival crevice if periodontal disease is present.

However, the greatest testimony to the protective effect of mucins is their ability to shield the stomach lining from gastric acid secretion and the digestive enzyme pepsin. Without this protective action, much of the digestive tract would self-destruct.

But there are limits to the protective effect of mucin. Chemicals such as strong alkalis, acids and other caustic chemicals can rapidly destroy the protective barrier and attack the underlying epithelium. A good example is the effect of an aspirin when placed next to a tooth from the hope of relieving toothache.

Finally, one of the most important roles of saliva is its ability to dilute and buffer the acid generated from the plaque from dietary sugars, the first stage from the carious process. The buffering capacity of saliva stems largely from the bicarbonate ions and varies both between individuals and within an individual mouth, depending on the time of day and increasing greatly using flow rate.

Saliva and the oral flora

The oral cavity is a micro-biologists’ delight. It contains a bewildering variety of organisms, which can be influenced from their proportions by your dietary and oral hygiene habits, from addition to general health and certain medications we might take. However, saliva is the main factor. It maintains control over the vying multitude of different organisms using an armoury of chemical weapons. These range from simple compounds such as urea and thiocyanate to enzymes and the complex proteins of the secretory immunoglobulins. Under normal conditions from a healthy mouth, all of these agents help to maintain the proper ecological balance of the oral flora. Urea has long been recognized as a powerful anti-bacterial agent and has even been incorporated into toothpaste as an aid to plaque suppression. Of the salivary enzymes involved from maintaining the ecology of the mouth, one of the first to be recognized was the enzyme lysozyme, which appears to work by destabilising the bacterial cell wall and causing cellular destruction. Gram-positive bacteria, including streptococcus mutans, are sensitive to this action. Another mechanism for controlling growth of bacteria is the clumping or aggregation of the bacterial cells to the point where they cannot function or cling to soft tissues. It would appear that some salivary glycoproteins can produce this effect. Finally, the large immunoglobulin molecules appear to have an important role from controlling the oral flora.

Why we need to stimulate salivary flow

Exercise is essential for any part of the body and this is true for the salivary glands.  A good salivary flow will help keep the secretory elements within the glands healthy and will also reduce the likelihood of infection from the gland and calcification within the ducts.

The benefits of a high salivary flow rate for the prevention of caries are well established. Caries occurs when there is an imbalance between the demineralization of the enamel surface following acid generation from the plaque and remineralization produced by the return of mineral ions into enamel from the plaque and saliva. The frequent stimulation of saliva, especially after the intake of sugars, will help to dilute and buffer plaque acid, bring extra mineral ions into the plaque fluid and thereby promote remineralization. It is the lack of remineralization due to a reduced salivary flow that produces the rapidity of attack seen from radiation caries.

Apart from these beneficial effects of salivary stimulation, there is some evidence that saliva may help protect the teeth from the erosive effect of acidic beverages and gastric reflux due to the dilution and neutralizing effects of saliva (Schewtzwl P, 1996).

With erosion now a major oral health concern, the benefit of stimulating saliva after the consumption of erosive drinks, from addition to sugar-sweetened foods and drinks, is becoming a useful aid to the maintenance of oral health.

Which factors reduce salivary flow?

It appears that the most common cause of reduced flow rate is medication and a wide range of common drugs, including antidepressants and antihistamines, have this potential.

Contrary to popular, belief, age is not a factor. Although many elderly people do suffer a reduced saliva flow, from most cases this is due to the medication they are taking rather than their age (Edgar WM & O’Mullane DM, 1990).

Smoking does appear to be a factor, while from a small number of cases xerostomia is due to blockages of the ducts, tumours, glandular atrophy or the rare but well known condition of Sjögren’s syndrome. In all these cases attempts should be made to stimulate saliva flow.

How can salivary flow be stimulated?

A number of drugs have been used as salivary stimulants, but only a few have proved of value from controlled clinical trials. Pilocarpine has been shown to improve symptoms of oral dryness and to increase salivary flow from patients using Sjögren’s syndrome and post-radiation xerostomia. Recently, cevimeline has helped patients using Sjögren’s syndrome (Macquire A & Rugg-Gunn AJ, 2003). Both of these drugs have a similar mechanism of action, side effects and duration of activity.

However, eating almost anything will help stimulate saliva but some things are more powerful stimulants than others. Lemon juice has long been recognized as a powerful and rapid salivary stimulant, but apart from certain flavors the mechanical act of chewing also has an important effect.

To this end, the value of sugarfree chewing gum, like ORBIT, as a safe, effective and convenient means of saliva stimulation is well recognized.

References

Edgar W M & O’Mullane D M (1990). Saliva and dental health. pp 1-17. London, BDJ Books

Fox PC (2004) Salivary enhancement therapies. Caries Res. 38: 241-246

Levine RS (1989) Saliva: 3. Xerostomia. Dent Update June: 197-201

Levine RS and Stillman-Lowe CR (2004) The scientific basis of oral health education. pp 23-24. London, BDJ Books

Macquire A and Rugg-Gunn   A J (2003) Xylitol and caries prevention – is it a magic bullet? British Dental Journal 194: 429-436

Schewtzwl P (1996) Aetiology of dental erosion – intrinsic factors. European J Oral Sci. 104: 178-190