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Effects of Mastication on Cognition

Paper Type: Free Essay Subject: Psychology
Wordcount: 4356 words Published: 8th Feb 2020

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Mastication and Cognition

Dementia is a chronic and/or progressive syndrome that is characterized as a neurocognitive disorder that is associated with memory loss, disorientation, personality changes and loss of motor functions which can lead to the inability to perform activities of daily living resulting in loss of autonomy (Seraj et al., 2017).  Although dementia is commonly seen in older adults, it is not considered a normal part of ageing.  Morphological changes of the brain such as cortical atrophy, and neuronal degeneration can be seen with neuroimaging in dementia patients.  In 2017 there were approximately 46.8 million people worldwide with dementia (Oh et al., 2018).  The World Health Organization predicts the affliction is on the rise and will affect over seventy-five million people by 2030 and triple in number by 2050 (2018). Due to longer life expectancies and the rise in prevalence many researchers are trying to identify modifiable risk factors that could aid in the prevention, and decrease the development, and/or progression of dementia (Oh et al., 2018).

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Mastication is not only an important aspect of oral health and function, but it has also been found to be integral in promoting and maintaining memory. Research has established a link between cognitive health and masticatory function.  (Fukushima-Nakayama et al., 2017).  Epidemiological studies show that masticatory dysfunction related to loss of teeth, inadequate prothesis, and weak muscles of mastication is directly associated with the worsening of dementia (Chen, Iiunuma, Onozuka, & Kubo, 2015).  Dr. David Snowden initiated a 30-year ongoing longitudinal study that related masticatory dysfunction and cognitive decline. This was called the Nun study.  Lin (2018) refers to this pioneering study that first revealed the number of missing teeth is directly related to an increased risk of dementia (Stein, Desrosiers, Donegan, Yepes, & Kryscio, 2007).  Neuroimaging research has determined that mastication not only affects regional brain activity but also activates functional connectivity in associated network.  The brain-stomatognathic axis is a concept that has been developed that defines the communication network between the cortical and subcortical structures in the brain, teeth, and periodontium (Lin, 2018).  Adequate masticatory function offers sensory input from the teeth and oral cavity transmitted through the trigeminal nerve to the hippocampus.  It has been determined that normal activations of the basal nucleus and cerebellum were observed in patients with normal dentition whereas no activations were found in the edentulous patients (Chen, Iiunuma, Onozuka,& Kubo, 2015).  Previous studies have indicated that memory and learning are significantly impaired by masticatory dysfunction associated to tooth loss, periodontal disease, or feeding on a soft diet.  The hippocampus is the area of the brain that controls learning, memory and specifically episodic memory.  Brain derived neurotrophic factor and its receptor encourage proliferation of neuronal cells, differentiation and survival of the neuronal cells as well as synapse formation in the hippocampus.  Masticatory dysfunction due to weakened muscles of mastication and/or tooth loss leads to inadequate stimulation of the central nervous system which results in hippocampal atrophy due to inactivity.  Masticatory dysfunction also causes the downregulation of the brain derived neurotrophic factor pathway which reduces, neurogenesis, neuronal function and synapse development. (Fukushima-Nakayama et al., 2017)).  Other studies reveal that masticatory dysfunction in older adults creates a chronic source of stress and results in elevated levels of circulating glucocorticoids due to activation of the hypothalamic-pituitary adrenocortical axis, also known as the HPA axis. This is a physiological reaction to stress elicited by a hormone response in an effort to maintain homeostasis (Azuma, Zhou, Niwa, & Kubo, 2017).  Chronic elevated levels of glucocorticoids create a stressed condition that leads to a breakdown in both physical and psychological health systems as well as cognitive impairment. It has been determined that chewing is an effective stress coping behavior.  Mastication decreases activity in the hypothalamic pituitary adrenal axis activity and therefore reduces the effects of chronic stress (Chen, Iinuma, Onozuka, & Kubo, 2015).  

Clinical studies demonstrate that the number of teeth, the median occlusion contact area, and that bite force are lower for elderly patients with cognitive impairment compared to those with normal cognitive abilities (Campos, Ribeiro, Costa, & Garcia, 2016).  It has been determined that mastication also affects general health preservation and promotion which influences cerebellar functions and improves hippocampus-dependent cognition (Chen, IInuma, Onozuka, & Kubo, 2015).  Douma et. al (2016) found that moderate chewing intensity increased heart rate and cerebral blood flow .  This study concludes that young dentate persons also have improved decision-making skills and cognition when chewing gum due to stimulation of the pre-frontal cortex.

Assessing and Determining Masticatory Function

Batisse et al. (2016) reports that “mastication is considered efficient when an individual can form a correctly reduced cohesive and viscous mixture ready to be swallowed.”  Tarkowska  et al (2016) states that scientific literature regarding mastication revolves around the relevance of masticatory function, the methods used to measure masticatory performance, and the factors that influence masticatory ability.  Mastication involves the maxilla and mandible bones, the periodontium, labial and buccal tissues, along with the muscles of mastication and the tongue.  To assess masticatory function it is necessary to examine the periodontal status, occlusion and number of teeth (Chen, IInuma, Onozuka, & Kubo, 2015).  There are several objective tests used to quantify masticatory performance.  The functional assessments that have been used to evaluate mastication in clinical studies include the complex sieving process, colorimetric measurement with the use of color changing chewing gums, and the use of 2 colors of chewing gum to assess mixture quantification (Tarkowska  et al., 2016). 

According to Stjernfeldt et al. (2017) the most common approach used in mastication performance has been to sieve comminuted natural or artificial food to determine the amount of breakdown that occurred with a predetermined amount of performed chewing cycles.  Tarkowska A et al. (2016) reports that this particular technique is the gold standard used to evaluate masticatory performance.  Campos, Ribeiro, Costa, & Garcia (2016) performed a sieving technique in a study that used Optocal artificial food test putty cubes to assess the relationship between masticatory function and cognitive abilities of patients with mild Alzheimer’s Disease.  Each subject had 17 cubes and were instructed to chew each cube 40 chewing strokes.  The particles were rinsed and allowed to dry at room temperature for one week.  Then each piece was put through a sieving machine and allow to sieve a total of 10 times for 20 minutes using smaller mesh with each sieve.  The mesh ranged from 5.6 mm down to .5mm at the end of the process.  The particles retained on each sieve were weighed on an analytical balance.  Higher weights of particles indicated increased difficulty in comminution of test food material. Cognitive function was assessed using the Mini Mental State Exam administered by a trained examiner.  The results of the tests revealed that mild Alzheimer’s Disease patients had a correlation with decreased masticatory performance.  

The mixing tests involves the use of a chewing gum that consists of two different colors.  Douma et al. (2017) performed a study in which half of the gum was pink and half of the gum was blue.  The subject was asked to chew the gum for 20 seconds.  The gum was then flattened and placed in a sandwich bag.  Both sides of the chewing gum were assessed, photographed, and analyzed using Mathematica 9 software.  Mathematica 9 is a symbolic mathematical computation program commonly used in scientific studies.  In this study it was used to assess the extent of the color mixing that occurred to assign a score of 0-1.  The lower the value the more mixing occurred with mastication whereas the higher the value the less mixing occurred with mastication. 

Color metric measurements have also been used to evaluate masticatory efficiency.  Lin, S. Wu, C. Wu, & Ko (2016) assessed masticatory function using a color-changeable chewing gum.  This test is not a mixing test but relies on a chemical reaction that is produced with mastication.  The reaction is based on the characteristics associated with the color of blue-yellow, and red dyes combined with citric acid.  Prior to chewing the gum it is a yellowish -green color.  During mastication the blue and yellow dye is distributed in the saliva while the citric acid is washed out of the oral cavity.  The red dye is pH sensitive and develops color only under neutral conditions and appears after the removal of the citric acid.  The chewing gum gradually turns into a red color which has varying intensities that are measured and are dependent upon masticatory performance.  In this study subjects were asked to chew the gum for 3 minutes as they normally chew their food.  The gum was then placed between two transparent films and was compressed to 1mm in thickness.  Ten points on Each side of the slide were scanned using a color metric software that quantifies color changes.  The averages of the values are determined and applied to an algebraic formula to assess the degree of overall color change which indicates the masticatory efficiency.


Oral pain, periodontal disease, and tooth loss are directly related to a compromised masticatory function.  A positive correlation exists between decreased mastication and modified dietary intake.  Dietary modifications can include avoidance of certain foods that are hard to chew such as meats, vegetables and solid fruits and/or physical alterations made to food selections.  Studies report that poor mastication capabilities can induce gastrointestinal disorders in elderly because a proper masticatory process decreases the digestion load placed upon the stomach and aids in nutrient absorption.  They conclude that dietary selections made to ease chewing can cause nutrient intake imbalance and thereby reduce dietary quality.  According to the study subjects with chewing difficulty had diets with Vitamin C and potassium deficiencies, and had less intake of calories, sodium, riboflavin, niacin, calcium, and Vitamin A than subjects with normal masticatory function (Kwon et al., 2017).  According to Seraj et al. (2017) individuals with less teeth are at an increased risk for developing nutritional deficiencies, especially B vitamins which have been found to contribute to cognitive decline and the development of dementia.  Utsugi et al. (2014) performed a study that found the consumption of a soft diet versus hard diet in adult mice was related to cognitive decline.  Mice fed soft diets had less neurogenesis similar to mice with impaired masticatory function due to lack of stimulation to the hippocampus which resulted in a significant reduction in the amount of 5 bromo-2deoxyuridine-immunoreactive cells in the subventricular zone and olfactory bulb in the brain.  Mice that ingested a hard diet had higher Fos-immunoreactivity at the trigeminal nucleus that did mice that consumed a soft diet.  Wada, Kawate, & Mizuma (2017) conducted a cross sectional study to assist individuals or professionals in determining if subjects had the masticatory ability to consume food that does not have to be mechanically altered or pureed.  This study tested masticatory ability using color changing chewing gum.  By using the gum to assess masticatory performance caregivers and or professionals were able to safely evaluate the subjects chewing efficiency.  Participants chewed the gum for 120 seconds and then 5 points were evaluated using the CIELAB color system.  The degree of color change from each of the 5 points was determined along with the mean value.  The same participants were then instructed to chew a mouthful of 5 different foods and spit out the food bolus for evaluation prior to swallowing.  The foods were divided into 5 levels of masticatory difficulty with level one being the easiest and five being the most difficult to masticate.  The weight of the food was measured prior to placing the food in the mouth.  The number of masticatory strokes, mastication time, and the texture of the food bolus after mastication was recorded for each subject and each food.  The food texture properties were then recorded considering hardness, cohesiveness, and adherence and then evaluated with the use of a texture analysis instrument.  The food bolus texture was then categorized as Level zero to three as suitable mastication for swallowing or above level three as poor masticatory ability based on the texture modified food pyramid.  Researchers correlated the masticated food bolus level to the equivalent mastication color value as determined by the color changing chewing gum.  Based on this level of correlation Wada, Kawate, & Mizuma (2017) concluded that this approach could be used to assess if elderly had adequate masticatory abilities and were able to continue a diet with regularly prepared foods safely by using the results from the color changing gum.  This would allow professional caregivers or individuals to decrease the amount of unwarranted preparation of pureed foods for the elderly and have a significant impact on their quality of life. 


Miranda et al conducted a study that concluded patients that wore full dentures or partial dentures had improved mastication which increased brain function.  In this study a majority of the patients that had a decline in cognitive status were not using dentures and were in need of dentures.  Kopplin et al showed that the quality and fit of the denture was an important factor to improve mastication and influence the preservation of cognitive abilities.  Poorly fit dentures can result in severe mandibular bone resorption, mandibular prognathism, residual ridge resorption, mucosal ulcerations, denture stomatitis, hyperplasia, and burning mouth syndrome which have a direct impact on nutrition and proper mastication (Kusdahny et al., 2016).  A study by Geetman demonstrated that patients with implant supported overdentures provided significant greater masticatory performance than patients with conventional dentures (Prithviraj et al.,2014).  The studies each have variable reported results which can make coming to a conclusion difficult on the correct approach and type of prosthesis that should be recommended to a patient.  Marito et al., (2018) reports that not all removeable dentures provide the same masticatory function due to variables such as morphological, physiological, and psychological aspects that affect each patient.  This study concluded that the most important factor that determines masticatory performance with dentures is the height of the residual ridge.  The higher the residual ridge the better the mastication in denture wearing patients.  The residual ridge assisted in masticatory abilities because it allowed for increased stability and retention of the denture..  This data supports previous studies that report patients with advanced ridge resorption had improved stability, retention, and masticatory performance with mandibular implant-retained overdenture versus conventional dentures (Marito et al., 2018).  The residual height and amount of mandibular resorption variance among patients explains the inconsistent results in studies pertaining to the type of denture that provides the most stability, retention, and improved mastication.  

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Kusdhany et al. (2016) reports that patients with prosthetic and pre-prosthetic treatment, such as scaling, restorative dentistry along with extractions of retained root tips improve overall oral health and can positively impact cognitive function.  It has been determined that masticatory ability is not associated with aging and does not decline with age as long as posterior occlusal contact is maintained (Marito et al., 2018).  As hygienist it is our duty to educate the general public and our patients about the significance of maintaining oral health and masticatory ability to preserve normal cognitive functions.  Patients with missing teeth or that are edentulous should be advised to have prosthesis not only for esthetics but to restore masticatory performance to enhance nutritive consumption, assist with stress reduction and affected immune response, activate neurogenesis, enhance hippocampal stimulation, and improve cerebral blood flow which ultimately influence cognitive function and deter the development of dementia.  Everyone should have routine dental visits to evaluate their oral health conditions in which an oral cancer screening is performed, radiographs to assess alveolar bone resorption and/ to visualize implant placement and condition, a chairside masticatory assessment should be completed using the two color chewing gum, and determining what foods are able to be chewed based on those results, and stability and retention of prosthetics evaluated based on residual ridge height.  Prosthetics also need to be examined for quality and fit at each visit to assess need for adjustments and examine and document the condition of soft tissue and/or teeth supporting prosthesis.  The public should be made aware of the oral and overall health correlation and dental actions they can take to help avoid the loss of autonomy, decline of cognitive abilities, decrease the statistics of the projected number of people estimated to develop dementia.  .


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