Aging-related changes in quantity and quality of saliva: Where do we stand in our understanding?

Saliva is crucial to oral processing of food and consequently is also related to the sensory and textural experience. It is often assumed that the secretion and properties of saliva change with age, which can result in dry mouth conditions, taste aberrations. Such changes may result in reduced nutrient intake and malnutrition besides adversely affecting the quality of life. Based on some recent research findings, this article reviews our current understandings on age-dependent changes on quantity (bulk salivary flow rate) as well as quality of saliva (e.g., composition, viscosity, lubrication) in healthy elderly individuals. The review begins with a short introduction to histological changes of salivary glands upon aging. This is followed by covering different aspects of salivary changes with key articles highlighting decreased flow rate, increased ionic concentration, decreased calcium and mucin content in saliva of elderly subjects consequently affecting the oral coating and flavor perception. We also highlight issues in data associated with respect to variance in saliva collection protocols as well as factors influencing such results other than age, such as health conditions and polypharmacy. Clear gaps in literature have been highlighted with respect to lack of quantitative data in viscoelasticity, rheology, and lubrication properties of saliva in healthy elderly population and the potential impact of changes in these material properties on sensory and textural perception of food and consequently food intake. Such insights will not only have clinical implications for maintaining optimal oral health in elderly population but also serve to optimize food for elderly population. PRACTICAL APPLICATIONS: The population has undergone a fundamental change in its age structure globally, with a rapid increase in elderly population. Innovation of tailored foods is still in its early stage to satisfy the needs of growing aging population. One of the biggest challenges in such food product development is lack of adequate understanding and characterization of endogenous factors, that is, age-related changes in saliva, which may influence oral processing of food and subsequently nutrient intake. Aging affects the salivary glands and alters quantity (flow rate) and quality (e.g., ion and protein composition, rheology, tribology) of saliva. Thus, older adults may suffer from dry mouth, taste aberration, and poor oral hygiene, greatly affecting their quality of life. This review provides insights into how age versus other health conditions influence salivary properties. Understanding of age-dependent changes in salivary rheology and tribology will be of paramount importance to optimize food for elderly population.