When you were growing up, you might have heard your parents say again and again to chew your food. Their purpose was likely to reduce the chance of your choking, since chewing is the first step in the digestive process. Chewing, with the assistance of enzymes in the saliva, starts to break down the food into smaller bits that can be swallowed and better processed and absorbed by the body. That is typically all people really think about when they consider chewing. However, that is not all chewing does—it also plays a role in cognitive health.
Mastication and the Hippocampus
Mastication, the technical word for chewing, impacts the function of the hippocampus, the center regulator in the brain that plays a role in memory and learning. Therefore, the lack of mastication activity might impact cognitive function through many processes, including reducing neurogenesis as one ages. This effect might be due to the reduction of brain-derived neurotrophic factor (BDNF), which stimulates neurogenesis. In a study, mice consumed either a soft diet or a hard diet, and the researchers reviewed their hippocampus cells to determine the levels of BDNF at 1 month, 3 months, and 6 months. In the 3- and 6-month-old mice, there was a significant reduction in BDNF levels in the hippocampus but not the cerebral cortex in the group that consumed a soft diet. This finding correlates to reduced neurogenesis, which might affect the function of the hippocampus.
There might be further ways lack of chewing negatively impacts the function of the hippocampus. There is a reduction in cell proliferation, or the increase in the number of cells, in rats that consumed a soft-food diet, based on another study that compared the levels of thymidine analog 5-bromo-2′-deoxyuridine, a marker of cell proliferation. The levels of cells with this marker were reduced as the rats aged, especially in those that consumed a soft diet. There might also be a reduction in synaptic formation with the loss of mastication function. All of these negative effects on the hippocampus also affect cognitive ability, especially related to memory and learning.
An Increased Risk of Dementia
Several studies have linked problems with chewing, including loss of teeth and decrease in the force of one’s bite, with dementia. A systematic review of 33 articles found that in 15 out of 17 cross-sectional studies, lower cognitive function was associated with problems with mastication, and 5 out of the 6 prospective studies found that there was an association between a steeper decline and poorer mastication. It was also a significant risk factor for mild memory impairment or dementia in 4 of the 5 cross-sectional studies, and it was a significant risk factor for dementia or mild memory impairment in 4 out of 5 prospective studies, demonstrating the close correlation between the two.
Another study found higher odds of cognitive impairment in those with problems chewing. In a study that looked at 557 people over the age of 77 in Sweden, there was no significant difference in the odds of developing cognitive impairment between those who had their natural teeth and those who did not after adjusting for certain cofactors including age, sex, and education. However, the loss of actual mastication function did lead to significantly higher odds of developing cognitive impairment. This effect demonstrates that it is not the loss of teeth that matter; it is the loss of the ability to actually chew.
In addition to the loss of neurogenesis, cell proliferation, and synaptic ability discussed above, another reason for this connection might be oxidative stress. One study found that rats consuming a soft-food diet, which correlates to a reduction in mastication function, had higher levels of oxidative stress. The nutrient composition of the soft diet was similar to that of the hard diet, demonstrating the main difference was the act of chewing. This oxidative stress impacted the release of dopamine. It might also have further implications, since oxidative stress in the brain is connected with Alzheimer’s disease, Parkinson’s disease, and other neurodegenerative disorders.
Impaired Cognitive Function
Studies have found that reduced chewing or soft diets impact the ability to learn and remember starting from a young age. One study reviewed mice at 3, 6, and 18 months to determine the impact of reduced masticatory function. The mice underwent water maze tests for five consecutive days at each review point in the study, working to learn and remember the position of a platform and escape faster each time. The control group ate a pellet diet while the study group consumed a powder diet. At three months, the two groups had similar performances, including significant reductions in their escape latencies by day three. However, at the six-month test, only the control group could significantly reduce the latency values. At 18 months, both groups were unable to improve their performances and reduce the escape latency during the five days. There was no impact on swimming speed, demonstrating that the impacts had to do with the spatial memory as opposed to a physical or another issue.
The researchers also found that the experimental group had a significant reduction of astrocyte numbers at 3 months, although at 6 and 18 months, there was not a significant difference in the number of astrocytes between the two groups. However, there was a difference in the number and distribution of astrocytes in the CA1 hippocampal field at 18 months, identifying a potential reason for the reduction in cognitive function. This study shows that changes in the mastication function at an early age led to cognitive issues later in life.
Beyond Cognitive Function: Chewing and Stress
When you get stressed, do you bite your fingernails? There might be a reason for this: chewing reduces stress. During stressful situations, chewing might increase the expression of glucocorticoid receptors in the hippocampus. Glucocorticoid and mineralocorticoid receptors play key roles in modulating the stress hormones released by the HPA axis. During stressful situations, there is an increase in the expression of mineralocorticoid receptors in the brain. Mineralocorticoid receptors inhibit the HPA axis, and this is modified by the glucocorticoid receptors. Thus, it is important to have a balance of the receptors. In a study on mice, those allowed to chew had an increase in the glucocorticoid receptors while those who were not had an increase in the mineralocorticoid receptors. This activity could impact the way the brain is able to handle stress, especially in terms of special memory.
In another study on rats, those allowed to chew under stress performed better than the group that did not chew. In fact, the group that was able to chew had no significant difference in their performance in the water maze and in their number of glucocorticoid receptors compared to the control group. The researchers postulated that the act of chewing helps to mediate the effects of stress through increasing the expression of glucocorticoid receptors. Another study supports these findings by illustrating that chewing while under stress provides a way to mitigate the suppression of the hippocampal long-term potentiation. In the study, the rats able to chew under stress were able to overcome the stress and get back to homeostasis faster, demonstrating it has a role to play in suppressing excessive endocrine responses during acute stress.
These studies were performed on rats, and chewing on something while stressed as a rat might correlate to humans biting their nails, or it might better correlate to a different stress-relieving activity in humans, such as exercise.
Studies into the effects of chewing gum, a similar action to the chewing by the rats, on stress in humans are mixed, with some demonstrating similar benefits to that in mice. In one study looking at 1,900 healthy volunteers with no dental problems found that chewing flavorless gum for three minutes led to a reduction in salivary chromogranin A(CgA) levels, a marker for stress as a substitute for catecholamine. For this effect to be achieved, the participants had to have high masticatory function. In another study, chewing gum led to an improvement in working memory processing in humans. In the study, subjects chewed plain gum and performed two or three back-to-back tasks that required working memory that was not related solely to stimulation.
Chewing gum also has the potential to reduce anxiety levels during a situation of acute social stress. Further studies have found reductions in work stress, anxiety, and depression in workers that consistently chewed gum. The workers also had a more positive mood compared to when they did not chew gum. There was about a 10 percent change after the participants chewed gum for 14 days. Therefore, when you get stressed or start to feel anxious about a task, you might find chewing gum beneficial.
Chewing and Behavior
The impacts of chewing on the hippocampus and other areas of the brain might also influence behavior patterns, including an increased risk of mental disorders. In another study on rats, the researchers found that consuming a soft diet led to an increase in risk factors for behavior patterns that correlate to schizophrenia and other mental health disorders. There is an association between reduced neurogenesis in the hippocampus and an increased vulnerability for developing a mental health disorder. In this study, the mice were fed a soft diet, a hard diet, or a soft diet followed by a hard diet after weaning. They underwent two behavioral tests. The study found no differences in patterns relating to anxiety or depression, but they did find a significantly lower PPI, which has associations with schizophrenia, bipolar disorder, PTSD, and ADHD.
These studies demonstrate the impact problems with chewing have on the brain, which can occur whether these problems begin at a young age or develop as one ages. Although many of the studies discussed were performed on animals, there are sufficient human studies demonstrating a strong correlation between mastication and brain health, including providing stress relief.
Therefore, taking care of your teeth and jaws is another way you can help to keep your brain healthy as you age. This provides another argument for those who lose teeth to consider implants over dentures, as they are much closer to natural teeth and provided similar mastication functions. It is never too late to mitigate some of the issues, as moving to a hard diet after consuming a soft diet has been shown to mitigate at least some of the problems. Thus, make sure you follow your parents’ advice and chew your food well, not only for your gut’s sake but also for your brain’s.