Aging and the Salivary Gland
Concern about salivary gland function is frequent among patients with Sjogren's syndrome (SS), as the disease itself leads to destruction of secretory cells in these glands. Since many SS patients are also middle-aged or older, and peri- or post-menopausal women, their concern may be compounded because of the common belief that salivary gland function is reduced with age or following menopause. This article provides a brief, general review of salivary gland performance in the aging adult.
About Salivary Glands
Salivary glands are composed of two parts, the acinar region or secretory endpiece and the ductal region. Acinar cells are the only site of fluid secretion in these glands. They are also responsible for making, and secreting, more than 85% of the proteins that mediate the functions of saliva. While the ducts primarily serve as a conduit to carry the forming salivary fluid into the mouth, they are hardly inactive.
Ductal cells play an extremely important role in determining the salt concentrations in saliva, and in making the remainder (~15%) of salivary proteins. Ductal cells, however, cannot secrete fluid. In SS, it is the acinar cells that are the principal targets of destruction, hence SS patients can't secrete salivary fluid.
The Role of Saliva
Saliva is the main protective factor for all the exposed tissues of the upper gastrointestinal tract -- i.e., the mouth, pharynx and esophagus. Salivary proteins help to keep your teeth hard, to repair wounds or ulcers in the soft mucosal tissues, to kill bacteria, certain fungi and some viruses, and to allow the formation of a food bolus and permit its proper swallowing. Saliva also facilitates one's ability to taste and to speak. Patients with a lack of saliva, like persons with SS, have a significant change in their quality of life because these functions are severely reduced or lost.
Aging and Disease
Does aging itself, or does the associated natural occurrence of menopause, also lead to these problems? While many common beliefs and "folk tales" suggest that aging and menopause do cause decreased saliva production, objective research has shown quite clearly that this is not the case. To appreciate the full meaning of this statement, it is worthwhile to digress slightly in order to describe some fundamental issues associated with aging research. In any study of normal human functions, it is critical to separate out any influences of disease from the intended evaluations. In aging research, this is particularly critical since as we grew older we tend to experience more disease and are treated often with drugs, having side effects, for these diseases.
Healthy aging can be defined by the absence of disease. The results that are described below have utilized study populations that have been selected for the absence of disease by a variety of clinical criteria. Thus the elders in these groups may not be "typical" older persons in that, as far as modern technology can tell us, they are free from overt disease. However, such persons are appropriate to study if one wishes to understand human biology free from the influence of disease or its treatment -- i.e., "normal function".
Types of Aging Studies
There are two general types of aging studies that can be conducted. They are termed cross-sectional and longitudinal. A cross-sectional study is relatively easy to perform. For example, if you had a room with one hundred people in it, you could divide the people up by age group (e.g., less than 40 years old, 40-65 years old, older than 65 years). Next, you could measure any biologic function, say saliva output. You then would compare the values for this function in each of the three age groups. While such studies are useful, and indicate trends, different people with different life experiences (for example growing up during the depression or during World War II, which could have influenced dietary experiences) are being compared even though all may be "healthy". So a cross-sectional study has some inherent weaknesses that must be recognized. A longitudinal study examines the particular biologic function in the same individuals over a long time. Thus, you could measure saliva output in those one hundred people today and then ten or twenty years from now. Each person in this type of study serves as their own control, a powerful experimental situation. You still would have to be sure the person was still healthy at the second measurement, and address certain other concerns, but longitudinal results generally tend to give the most meaningful findings. For studies of salivary function during aging we are fortunate to have longitudinal as well as cross-sectional results. Both support the conclusion that salivary function does not decline markedly as a result of aging alone.
Studying the Salivary Function
When studying salivary function, there are also several variables that must be considered. First, it is best to study saliva directly from the glands (parotid, submandibular/sublingual, minor) rather than to study the mixture of secretions in the mouth, so called "whole saliva". The latter contains bacteria, food debris, and other non-salivary components and interpretations are often difficult to make. Second, saliva is secreted upon stimulation (during meals) or "at rest" (all other times). Both are extremely important and should be measured. The stimulated secretion helps us process food for swallowing while the unstimulated secretion provides most of the protective benefits. Finally, saliva is made up of many constituents (salts, proteins) dissolved in a solvent fluid (water). It is important, then, to know the status of both the secretion of constituents and the secretion of fluid in an individual. With this background in aging and salivary studies, it may be easier to appreciate the research findings.
Most studies of fluid secretion have been done with parotid glands as they are an easier source from which to collect saliva. Both cross-sectional and longitudinal (up to 10 years) studies have shown that there is no difference in stimulated parotid secretions in healthy men and women. Unstimulated parotid saliva output also shows no differences in healthy elders in both types of studies, although the longitudinal measurements occurred at a shorter interval (3 years). These general findings with parotid secretion have been reported by different investigators using different study populations and, thus, can be considered as "accepted" fact.
Far fewer studies of submandibular/sublingual salivary secretion have taken place. There is not the uniformity of results seen with parotid studies, i.e., some cross-sectional studies show no change, while others show a decrease, in submandibular/sublingual saliva during "healthy aging". There are only (to my knowledge) two published longitudinal studies of saliva output from these glands and both show no changes in healthy men and women over a 3 year period. At this time, I think a fair conclusion is that while some decrease in submandibular/sublingual salivary output may occur during aging it is likely not of biological or clinical significance. However, more studies are certainly needed. Very little study has been made of minor gland secretions during aging and no conclusions can be drawn now.
There are several studies examining various salivary constituents in different-aged persons (i.e. cross-sectional approach). Most are with stimulated parotid saliva and show little change with age. There are some longitudinal results published also and these are consistent with the cross-sectional findings. For example, over an approximately 10 year period there were no alterations in lactoferrin (an anti-bacterial protein), proline-rich proteins (involved in tooth remineralization), total protein, as well as sodium and potassium ions, in the stimulated parotid saliva of healthy men and women. Thus, it seems that salivary constituents may not be reduced with age, but more study of other components and of other secretions (submandibular/sublingual; minor glands) are needed.
These results generally show that salivary gland function remains intact across the lifespan in healthy persons. However, growing old is associated with many conditions that result in decreased salivary secretion, and that is the source of the common beliefs about dry mouth and aging. The most frequent cause (by far) of reduced salivary output among elders is the use of certain medications. Many prescription drugs taken by older persons (e.g. anti-depressants, anti-hypertensives, anti-histamines, anti-cholinergics) can cause moderate to marked reductions in salivation and the associated oral discomfort and problems. These changes are almost always reversible, i.e. if the patient can be placed on alternative medications, or lower doses of the same medications, the xerostomic effects are alleviated. The other common causes of salivary hypofunction in middle-aged and older persons are autoimmune diseases like SS, and therapeutic radiation to the head and neck (as part of cancer treatment). These changes are (currently) irreversible and have no suitable treatment available. When patients with such causes of reduced salivation are selected out of a population of elderly persons, no meaningful evidence of decreased salivary secretion can be detected. It is important for middle-aged and older persons to recognize that reduced salivary function is not a normal consequence of growing old and is usually a result of systemic disease or treatment. Symptoms or signs like oral dryness (especially during meals), difficulty swallowing, increased dental decay, or the occurrence of candidal infections should signal a need for professional attention.
( Reprinted from the The Moisture Seekers Newsletter, Published by the Sjogren's Syndrome Foundation Inc.)
1-800-4-SJOGREN
4/28/97
Bruce J. Baum, D.M.D., Ph.D.
Gene Transfer Unit
Division of Intramural Research
National Institute of Dental Research
Content From:
National Institute of Dental and Craniofacial Research
National Institutes of Health
Bethesda, MD 20892-2190
http://www.nidcr.nih.gov