Importance of Saliva
Dr Shamaz Mohamed Eiliyah Dental Care
Wilson garden, Bengaluru Feb 14, 2017
Saliva and Health
Saliva serves multiple functions, and the importance of saliva in the maintenance of oral health becomes evident when saliva flow is reduced. Impaired salivary secretion and altered salivary composition, which can be caused by a variety of medical conditions and medications, increase the risk of oral diseases such as dental caries, dental erosion and oral candidal infections. Furthermore, the sensation of oral dryness (xerostomia) can have a negative impact on the patient’s quality of life.
The salivary glands under normal physiological conditions we produce between 0.5-1.0 liter of saliva per day. The majority of saliva is produced by the three paired major salivary glands, that is, the parotid glands, the submandibular glands and the sublingual glands. The minor salivary glands, which are widely distributed on the inner mucosal surface of the lips, cheeks, palate and glossopharyngeal area, contribute about 10% of the total volume of saliva.
However, the minor salivary glands play an important role in the lubrication of the oral mucosa, since they secrete about 70% of the total volume of salivary proteins. The mixed fluid that covers the teeth and oral mucosa is designated whole saliva, and apart from saliva from both the major and minor salivary glands, it also contains gingival crevicular fluid, micro-organisms from dental plaque, leucocytes, discarded epithelial cells from the oral mucosa and food debris.
In healthy, non-medicated persons the unstimulated (resting) whole saliva flow rates usually vary within the range of 0.3-0.5 ml/min and the stimulated whole saliva flow rates within the range of 1.0-1.5 ml/min. The salivary flow and thereby the composition of saliva display large variations during the day. The salivary flow rate typically increases during the day and peaks around noon, and is almost zero during sleep. In addition, several factors influence the salivary flow rate including the nature and duration of stimulus, the emotional state and water balance of the body.
The composition and functions of saliva Saliva not only protects the teeth and oral mucosa, facilitates mastication, swallowing and speech, but also acts in the initial digestive processes that take place in the upper parts of the gastrointestinal tract.
Normally saliva is composed of more than 99% water and less than 1% of solutes such as proteins and salts. These salts primarily comprise sodium, potassium, chloride, bicarbonate, calcium, phosphate and magnesium.
One of the important functions of saliva is maintenance of oral hard tissue integrity by providing mechanical cleansing, buffering effect, antimicrobial actions and maintaining saturation with respect to hydroxyapatite (calcium and phosphate) in the saliva. Saliva dilutes and eliminates dietary sugars and acids as well as oral micro-organisms from the mouth by a process referred to as oral clearance
This process is dependent on the salivary flow rate and the swallowing frequency. Thus, if the salivary flow rate is reduced, the oral clearance rate is also reduced. In the latter case, the elimination of food substances including acids is prolonged resulting in a more acidic environment in the oral cavity and in
the dental plaque leading to an increased risk of dental erosion and caries.
Saliva buffer capacity
Another important function of saliva is its ability to buffer acids, which is essential for maintaining pH values in the oral environment above the critical pH for hydroxyapatite. The saliva buffer capacity includes the bicarbonate, phosphate and protein systems. The concentration of bicarbonate in saliva and the salivary pH are highly dependent on the salivary flow rate, and the pH can under normal physiological conditions vary from 6.0 to 7.5. The bicarbonate concentration and thereby the pH in saliva increases when the salivary flow rate increases and vice versa.
The concentration of bicarbonate is highest in parotid saliva and lowest in the minor salivary glands. In unstimulated state, the bicarbonate and phosphate contribute almost equally to the buffer capacity in whole saliva. In stimulated whole saliva, the bicarbonate buffer system is responsible for approximately 90% of the buffer capacity. At very low salivary flow rates and salivary pH below 5, the main contribution to the buffer capacity derives from the proteins. Salivary pH and salivary concentrations of calcium and phosphate are significant factors for the maintenance of saturation with regard to hydroxyapatite in the saliva. Saliva also contains proline-rich proteins and statherins, which prevent precipitation of calcium phosphate salts from saliva and keep the environment around the teeth supersaturated with regard to calcium phosphate salts.
Salivary proteins and the tooth pellicle
In addition to the oral clearance, the saliva buffer capacity and maintenance of saturation with regard to mineral, the salivary proteins also play a role in relation to protection of the teeth against caries and dental erosion. Several saliva proteins such as proline-rich proteins, cystatins, MG1, lactoferrin, lysozyme and amylase take part in the formation of the acquired pellicle that covers the teeth. The protein composition of the pellicle seems to determine the initial colonisation of bacteria to the tooth surface and thereby the risk of developing caries. Similarly, the salivary protein composition of the pellicle as well as the thickness of the pellicle seems to be important in relation to the risk of developing dental erosion. Thus a thick pellicle seems to be more protective against acid challenge on the hard dental tissue than a thin one.
Growth factors in saliva
The duct cells produce and secrete certain growth factors like epidermal growth factor (EGF) and fibroblast growth factor, which enhance healing of ulcers in the mouth and play a role in the protection of oesophageal mucosa. Salivary EGF interacts with the innate salivary protein defence mechanisms to form a mucosal defence barrier. Nerve growth factor is important for among others the development of sympathetic nerves. Chewing- and acid (pepsin) exposure to the oesophageal mucosa lead to increased secretion of epidermal growth factor from the salivary glands.
Saliva and the initial digestive processes
Both α-amylase, which are being secreted from the serous acinar cells of the parotid gland and lipase, being secreted from the lingual (von Ebner’s) glands, take part in the initial digestion of starch and triglycerides, respectively, in the oral cavity. However, the salivary α-amylase and lipase are considered to be of minor significance in the polysaccharide digestion and lipolysis of healthy individuals, but may be of particular importance to patients suffering from chronic pancreatic insufficiency and patients with cystic fibrosis due to the lack of pancreatic amylase and lipase activity. The concentration of salivary α-amylase increases with increasing salivary flow rate. Salivary α-amylase also appears to have antimicrobial properties like inhibitory effects on the bacterial adhesion on the teeth and oral mucosa.
Salivary gland function and aging
It is still controversial whether xerostomia and reduced salivary flow are natural consequences of age-related degeneration of the salivary gland tissue or rather results of an increased incidence of systemic diseases and a more extensive intake of medication among the elderly.
Oral dryness and reduced salivary secretion
Subjective oral dryness is a common problem to many people. It is assumed that at least 10% of an adult population suffers from oral dryness. The prevalence of oral dryness increases with age which is primarily ascribed to a higher incidence of systemic diseases and intake of medications among the elderly. Approximately 30% of persons aged 65 years and above is assumed to suffer from oral dryness. The sensation of oral dryness (xerostomia) is often related to a severely reduced salivary unstimulated flow rate. However, xerostomia may also occur without objective evidence of reduced salivary flow (hyposalivation). Accordingly, the sensation of oral dryness is apparently closely related to the composition of saliva, in particular to the content of glycoproteins (mucins). The moistening and lubricating properties of glycoproteins are believed to minimize the sensation of mucosal dryness. In addition, it has been shown that the symptoms of oral dryness first occur when the unstimulated salivary flow rate falls to about 50% of its normal value.
Consequently, the detection of salivary gland hypofunction requires, in addition to a thorough medical and oral history taking and a clinical examination, measurement of the salivary flow rate (sialometry). Do visit if you have dryness of mouth due to insufficiency of saliva to you near by dentist or physician for through check up and evaluation of the cause and subsequent treatment.
Medications and salivary gland function
Intake of medications is the most common cause of xerostomia and salivary gland hypofunction. There is also a relationship between the number of medications taken on a regular daily basis and xerostomia and salivary gland hypofunction in such a way that when the number of medications taken on daily basis is increased, the complaints of oral dryness become more pronounced and the salivary flow rate decreases. The medication induced side effects are often reversible meaning that salivary gland function will recover after withdrawal of the pharmacotherapy. Medications can affect the salivary glands and their secretions in several ways. Benzodiazepines, for example, affect the central neural regulation of salivary secretion. Others exert actions on the pheripheral neural regulation via interaction with the binding of neurotransmitters and peptides to receptors on the plasma membranes of the salivary gland cells. These medications include among others atropine, which binds to muscarinic cholinergic receptors, and α- and β-blockers which bind to adrenergic receptors on the plasma membranes of the salivary gland cells. In addition, medications like diuretics can indirectly affect the salivary secretion mainly via their diuretic effect.
Management of oral dryness and salivary gland hypofunction
It is important to identify the etiological factor, if possible, to the oral dryness and salivary gland hypofunction in order to make appropriate decisions regarding therapeutic interventions for the patient. It is also important to inform the patient about the relationship between hyposalivation and the increased risk of developing oral diseases. In case of medication-induced xerostomia and salivary gland hypofunction, change in dose or alteration of dose schedules of the causative (xerogenic) medications may occasionally prove to be effective. Thus many patients complain of oral dryness at night and change in dose schedules can result in reduced maximal blood levels of the drug during the night and thereby minimize the symptoms of oral dryness. Substitution of the causative drug for another with fewer side effects on the salivary secretion, but with similar clinical effect, if possible, may also prove to be useful. It must be stressed that changes in the patient’s medication should always be carried out in collaboration with the patient’s physician and dentist.
In cases where change in dose, alteration of dose schedules and substitution of a drug is not possible, the therapeutic approaches are aimed at alleviating the symptoms of oral dryness and limiting the consequences of salivary gland hypofunction.
Patients with salivary gland hypofunction should reduce their consumption of fermentable carbohydrate (sucrose) due to the increased risk of caries. They should also be advised to sip water when eating and swallowing, and they should rinse their mouths thoroughly with water after eating. One of the key elements in maintaining a functional dentition often includes a strict individual oral hygiene regimen with regular follow-up at a dental clinic at least every 3 months. Topical application of fluoride may be useful in periods.
In patients with residual functioning salivary gland tissue, stimulation of the salivary reflex is highly recommended. Patients with natural dentition are recommended to use sugar-free chewing gum (including chewing gum containing fluoride), while patients with full dentures are recommended use of sugarless sour sweets. Pharmacological sialogogues such as pilocarpine, a muscarinic cholinergic agonist, may also provide an increase in salivary secretion.
Common causes to oral dryness, salivary gland hypofunction and altered salivary composition.
|Intake of medications||Such as antihypertensives, antidepressants, antipsycotics and antihistamines|
|Systemic diseases||Sjögren´s syndrome, rheumatoid arthritis, dysregulated diabetes mellitus, HIV-infection and conditions causing dehydration|
|Irradiation||Radiation therapy of tumours in head- and neck region|
|Psychogenic disorders||Such as depression, anxiety and stress|
|Neurological conditions and factors affecting the autonomic outflow pathway||Such as cerebral-vascular diseases, brain tumours and neurosurgical traumas affecting the peripheral nerves and the central nervous system (e.g. the trigeminal, facial or glossopharyngeal nerves and salivation centre, respectively)|
|Local factors||Salivary gland infection and obstruction|
|Dryness of the lips, oral mucosa and pharynx|
|Soreness, itching and burning sensation of the oral mucosa|
|Problems with eating spicy and acidic foods|
|Altered/impaired taste perception|
|Difficulties in chewing, swallowing and speech Very viscous and foaming saliva Gastrointestinal reflux, heartburn|
|Increased frequency of mucosal ulcerations|
|Difficulty in wearing removable dentures|
|Increased caries activity and atypical caries pattern (carious lesions on cervical and incisal tooth surfaces)|
|Atrophy of the filiform papillae, red or fissured appearance of the tongue|
|Pale and atrophic oral mucosa|
|Increased frequency of oral candidosis|
|Soreness and enlargement of the major salivary glands|
|Altered and impaired diet due to the reduced salivary secretion, causing malnutrition or weight loss|
Consequences of permanently reduced salivary secretion
|Functions ||Component in saliva|
|Protection of teeth and oral, pharyngeal and oesophageal mucosa|
|Mechanical cleansing of teeth and mucosa ||Water|
|Lubrication of teeth and mucosa||Water, mucins|
|Keep oral mucosa intact, soft and moistened ||Water, mucins, salts, epidermal growth factor, fibroblast growth factor, nerve growth factor|
|Prevent tooth demineralisation ||Proline-rich proteins, statherins cystatins, histatins, calcium and phosphate|
|Buffer capacity ||Bicarbonate, phosphate and protein|
|Antibacterial functions ||Amylases, cystatins, histatins, mucins, peroxidase, lysozyme, lactoferrin, calprotectin, immunoglobulins, chromogranin A.|
|Fungicidal functions ||Histatins, immunoglobulins, chromogranin A|
|Antiviral functions|| Cystatins, mucins, immunoglobulins|
|Formation of food bolus||Water, mucins|
|Facilitation of mastication and swallowing||Wand, mucins|
|Initial digestion ||α-amylases, lipases, ribonucleases, proteases, water, mucins|
|Dissolution of taste compounds ||Gustin (carbonanhydrase), zinc (Zn2+), water|
|Facilitation of speech ||Water, mucins|