Mastication for the mind—The relationship between mastication and cognition in ageing and dementia

Abstract

The goal of this literature review has been to investigate the relationship between mastication and cognition, with a special focus on ageing and dementia, and its possible underlying mechanisms. Since the relationship between mastication and cognition is not yet firmly established, and is investigated in the context of a number of different disciplines, a comprehensive overview will contribute to our knowledge. The results of animal and human experimental studies suggest a causal relationship between mastication and cognition. Furthermore, correlations exist between mastication and activities of daily living and nutritional status. These findings have compelling implications for the development of prevention strategies by which medical and nursing staff may optimize their care for the frail and elderly, suffering from dementia.

Introduction

The world population is ageing (United Nations, 2008). For example, the senior population (persons over the age of 60) in developed regions will increase from 264 million in 2009 to 416 million in 2050 (United Nations, 2008). Given the fact that ageing is one of the risk factors for developing dementia (Fernandez et al., 2008), an increase in patients suffering from dementia is to be anticipated (Wimo et al., 2003). There are several risk factors for developing dementias like Alzheimer's disease (AD), which is one of the most common subtypes of dementia (Kalaria et al., 2008). These risk factors include ageing, illiteracy, a lower level of education, lower socioeconomic status (Kalaria et al., 2008), head trauma (Plassman et al., 2000), genetic factors such as the apolipoprotein E4 (apoE4) allele (Cedazo-Mínguez, 2007), cardiovascular risk factors such as being overweight, smoking, hypertension and diabetes mellitus (Fillit et al., 2008), an inactive lifestyle (Kramer et al., 2003) and, perhaps surprisingly, loss of teeth (Kondo et al., 1994, Gatz et al., 2006). Loss of teeth has also been associated with malnutrition (Nordenram et al., 2001, Kim et al., 2007), mortality and disability (Holm-Pedersen et al., 2008), loss of cognitive function (Bergdahl et al., 2007) and prevalence of dementia (Kim et al., 2007).

This review will focus on the relationship between masticatory and cognitive function in ageing and dementia. The first question we would like to answer is whether the literature supports the existence of such a relationship and whether this relationship is causal, i.e. does a deterioration of the masticatory system impair cognitive functioning in older persons with and without dementia? A related question concerns the mechanisms underlying this relationship. To our knowledge, neither question has been addressed before, which is why a literature search was performed. The literature on the relationship of mastication and cognition that will be addressed in this review will be subdivided into several categories: animal experimental studies, usually with a strong neuroanatomical focus; human experimental studies, with healthy (and typically young) subjects; and clinical studies, either cross-sectional or longitudinal, with a patient population suffering from either age-related or pathological loss of the ability to perform activities of daily living. Both physiological and behavioral changes will be discussed, where applicable.