Chen 2015: Chewing and Hippocampal Function
Citation
Chen H, Iinuma M, Onozuka M, Kubo KY. (2015). Chewing Maintains Hippocampus-Dependent Cognitive Function. International Journal of Medical Sciences, 12(6), 502-509. (Chen et al., 2015)
DOI: 10.7150/ijms.11911
Summary
This comprehensive review synthesizes decades of research on how mastication (chewing) affects brain function, particularly in the hippocampus—the brain’s memory center. The evidence strongly suggests that chewing is not just a digestive function but a cognitive-protective activity.
Key Findings
The Masticatory-Hippocampal Connection
The review documents multiple pathways by which chewing affects the hippocampus:
| Pathway | Mechanism | Effect |
|---|---|---|
| Sensory input | Trigeminal nerve activation | Stimulates hippocampal neurons |
| Blood flow | Increased cerebral perfusion | Enhances oxygen/nutrient delivery |
| Stress reduction | HPA axis modulation | Reduces corticosterone damage |
| Neurotrophin release | BDNF upregulation | Promotes neuroplasticity |
Evidence from Animal Studies
Experiments in mice and rats show that:
Tooth loss leads to:
- Reduced hippocampal cell density
- Impaired spatial learning
- Increased stress hormone levels
- Decreased BDNF expression
Restoring chewing function:
- Partially reverses cognitive deficits
- Normalizes stress responses
- Improves maze performance
Human Studies
Epidemiological research in humans confirms:
- Fewer teeth = worse cognitive performance
- Poor masticatory function = accelerated cognitive decline
- Gum chewing can improve attention and memory tasks
The Hippocampus Connection
Why the Hippocampus?
The hippocampus is particularly sensitive to:
- Sensory input deprivation - Loss of stimulation leads to atrophy
- Stress hormones - Glucocorticoids damage hippocampal neurons
- Blood flow changes - Ischemia causes rapid cell death
- BDNF levels - Critical for synaptic maintenance
Chewing affects all four factors, making it a potent modulator of hippocampal health.
Trigeminal Input to Hippocampus
The sensory pathway from teeth to brain:
Periodontal receptors → Trigeminal ganglion → Trigeminal nuclei →
Thalamus → Somatosensory cortex → Hippocampus (via entorhinal cortex)
This means every bite sends signals that ultimately reach memory circuits.
Aging and Mastication
The Critical Period
The review emphasizes that chewing becomes more important with age:
- Aging naturally reduces hippocampal function
- Tooth loss accelerates this decline
- The combination creates a “double hit”
Cognitive Reserve
Maintaining masticatory function may contribute to cognitive reserve—the brain’s resilience against pathological damage. Individuals with better chewing function may:
- Better tolerate early Alzheimer’s pathology
- Maintain function despite brain changes
- Delay clinical symptoms of dementia
Stress and Chewing
The HPA Axis
One surprising finding: chewing reduces stress responses.
| Observation | Interpretation |
|---|---|
| Gum chewing lowers cortisol | Masticatory activity modulates HPA axis |
| Tooth loss increases stress hormones | Loss of oral sensory input dysregulates stress |
| Soft diet mimics tooth loss effects | Even without extraction, reduced chewing is harmful |
Implications
Chronic stress damages the hippocampus via glucocorticoids. By buffering stress responses, chewing may provide neuroprotection independent of its direct sensory effects.
Mechanisms at the Molecular Level
BDNF Pathway
Chewing increases BDNF (Brain-Derived Neurotrophic Factor) in the hippocampus:
- Promotes neurogenesis
- Strengthens synaptic connections
- Supports long-term potentiation (memory formation)
Acetylcholine System
Masticatory activity also affects cholinergic neurons:
- Acetylcholine is critical for memory
- Cholinergic neurons project to hippocampus
- Chewing may enhance this signaling
Clinical Implications
For Patients
- Preserve natural teeth when possible
- Maintain chewing function even with prosthetics
- Consider chewing exercises for brain health
- Avoid exclusively soft diets in elderly
For Clinicians
- Oral health affects cognitive health - communicate this to patients
- Tooth loss matters for the brain - factor this into treatment planning
- Prosthetic design should consider function - not just appearance
- Collaborate with cognitive specialists for at-risk patients
Limitations
The review acknowledges:
- Most mechanistic studies are in animals
- Human studies are largely observational
- Causation vs. correlation remains debated
- Optimal “dose” of chewing is unknown
Conclusions
The authors conclude that:
“Maintaining masticatory function… could be a useful approach for preserving cognitive function.”
This simple intervention—keeping the ability to chew—may be one of the most accessible ways to protect brain health with aging.
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This page summarizes peer-reviewed research for educational purposes.