The Mechanism by Which the Act of 'Chewing' Activates the Neural Circuits of the Hippocampus Governing Memory and Reduces Dementia Risk. Scientific Evidence on Information Transmission via the Trigeminal Nerve and the Blocking Effect of Stress-Induced Neurotoxicity

In the prevention of dementia and memory impairment accompanying an aging society, the mechanism by which “chewing food thoroughly” directly connects to the maintenance of brain function has been demonstrated by recent neuroscience and brain image analysis. In review papers integrating animal experiments and human clinical studies, it has been shown that sensory input from the periphery due to chewing reaches the “Hippocampus,” which governs memory, via the trigeminal nerve, playing a decisive role in maintaining spatial memory ability and preventing neuron loss caused by stress.

The details of this content can be confirmed in the research paper included in the following PMC (PubMed Central).

Chewing Maintains Hippocampus-Dependent Cognitive Function | PMC https://pmc.ncbi.nlm.nih.gov/articles/PMC4466515/

The “Two Neural Input Pathways” Connecting the Masticatory Organs and the Hippocampus

Numerous studies have suggested that the health status of the oral cavity, particularly the “number of remaining teeth” and the decline in “chewing function,” has a strong causal relationship as a factor in age-related cognitive decline. From a physiological perspective, sensory information triggered by chewing movements in the oral cavity is mainly transmitted directly to the hippocampus through two parallel neural circuits.

• The Pathway via the Cerebral Cortex: Mechanical pressure from chewing is received by the trigeminal sensory nucleus and passes through the cerebral cortex somatosensory cortex and association cortex via the thalamus (ventral posterolateral nucleus). After that, it is projected from the entorhinal cortex to the dentate gyrus (DG), which is the entrance to the hippocampus.
• The Pathway Directly via the Hypothalamus: There is a route that transmits information to the hypothalamus via the brainstem reticular formation, and from there sends signals directly to the hippocampus as opioidergic and histaminergic fibers.

In experimental data on animals, when opportunities for chewing are artificially deprived by the extraction of molars, continuous feeding of a soft diet (soft food), or malocclusion, morphological impairments such as a decrease in the number of neurons and the retraction of dendritic spines in the hippocampus were confirmed. Furthermore, it has been clearly reported that severe deficits occur in hippocampus-dependent “spatial memory ability” in these individuals (e.g., in adult animals, learning ability drops significantly a few months after molar extraction).

The Act of “Chewing” Blocks Neurotoxicity Caused by Chronic Stress

The hippocampus is a region in the brain where “Adult Neurogenesis,” the creation of new nerve cells throughout life, exceptionally takes place. However, under chronic physical and psychological stress, the hypothalamic-pituitary-adrenal (HPA) axis is excessively activated, and high concentrations of glucocorticoids (stress hormones such as cortisol) are secreted. Excessive glucocorticoids are highly toxic to the nerve cells of the hippocampus and remarkably suppress neurogenesis, triggering cognitive decline and depressive symptoms.

As a significant finding of the paper, it has been demonstrated that when a physical task of “chewing on a hard object (chewing behavior such as chewing rats)” is given to individuals under stress conditions, the proliferation of hippocampal nerve cells, which had been suppressed, recovers rapidly.

At the same time, the excessive reaction of the HPA axis itself decreases, and it has been pointed out that the act of chewing itself functions as a “stress buffer” in the central nervous system. In other words, it has been revealed that rhythmic chewing movements endocrinologically suppress the rampage of neurotoxic stress hormones, physically and chemically protecting the hippocampus.

Correlation Between the Number of Chews in Daily Life and Cognitive Function

This series of evidence warns of the danger that modern dietary habits centered on highly processed soft foods (soft diet) bypass the “regular sensory input system to the brain” acquired through evolutionary processes.

It has been reported that a decrease in the number of chews in daily meals not only increases the burden on local digestive organs but also leads to the depletion of neural stimulation to the hippocampus in the long term, potentially becoming a latent accumulation factor for the risk of developing dementia in the future. On the other hand, it concludes that consciously increasing the number of chews at every meal, or continuously applying an appropriate load to the masticatory muscles utilizing means such as gum, is an extremely effective and safe intervention to maintain the structural health of the brain without relying on pharmacotherapy.