Role of Mastication in Human General Health

A systemic effect of masticatory dysfunction has been suggested as a possible risk factor of senile dementia. In order to concretely evaluate this possibility, using function magnetic resonance imaging (fMRI) in young and aged human brain, we examined the link between masticatory function and senile processes of learning and memory. Chewing resulted in a bilateral increase in blood oxygenation level-dependent (BOLD) signals in the sensorimotor cortex, supplementary motor area, insula, thalamus, and cerebellum in both age groups, but the increase was smaller in the first three regions and higher in the cerebellum in the aged subjects. Interestingly, only the aged subjects showed significant signal increases in the association areas, which receives information from the primary sensorimotor cortex, supplementary area ,or insula, and an increase in the hippocampal BOLD signal and memory acquisition. Similar effects in elderly subjects were seen as a result of more leisurely eating with increased chewing or being the fitting of dentures. Furthermore, in the amygdala whose response is sensitive to stress, chewing attenuated the BOLD signal that was induced by an application of a loud noise (stress), and improved the suppressed recollection in this stressful condition. The results indicate involvement of chewing in the neuronal circuit to the hippocampus and the amygdala in the elderly, suggesting that chewing improves their memories by reducing stress. We strongly point out that chewing plays an important role in preventing the aged-related deterioration in memory processes.