Recent evidence has suggested a potential link between mastication and cognitive function (Lin, 2018; Yokoyama et al., 2017; Onozuka et al., 2003; De Cicco et al., 2016; Fantozzi et al., 2021). This connection may stem from mastication’s ability to influence neuronal metabolism and regional cerebral blood flow across multiple brain areas, including the prefrontal cortex (PFC). For instance, chewing moderately hard food increases cerebral blood flow, while chewing soft and elastic food like gum of varying hardness levels significantly increases brain activity (Suzuki et al., 1994; Onozuka et al., 2002). High levels of blood oxygen have been observed in brain regions with heightened cerebral blood flow, such as the PFC and hippocampus, which are crucial for mastication and cognitive processes like learning and memory (Yokoyama et al., 2017). Clinical studies suggest that individuals experiencing chewing difficulties tend to exhibit lower cognitive function and greater cognitive impairment, whereas activation of cerebral blood appears to positively impact various cognitive functions (Tada and Miura, 2017). However, the precise metabolic changes in the brain associated with cognition following cerebral blood flow increase remain unclear.

Oxidative damage to the brain is believed to play a key role in the decline of cognitive function (Head, 2009; Collin, 2019). Studies on both normal and pathological brains, including those affected by neurodegenerative diseases and aging, have demonstrated higher levels of oxidative damage in aged brains compared to younger ones (Montine et al., 2002). The brain is highly vulnerable to oxidative damage for several reasons. First, human brain consumes approximately 20% of the body’s total oxygen and contains a high amount of polyunsaturated fatty acids, making it susceptible to peroxidation. Second, the accumulation of redox-active metals (such as iron, copper, and zinc) in the brain can catalyze the formation of reactive oxygen species (ROS). Several metabolic processes generate ROS, which can lead to accumulated oxidative damage in human brain (Baierle et al., 2015; Uysal et al., 2018).

To defend against ROS, the antioxidant system consists of several enzymes, such as catalase, superoxide dismutase, glutathione peroxidase, and numerous non-enzymatic endogenous antioxidants such as glutathione (GSH) (Mehta and Gowder, 2015). Among these, GSH is the most crucial antioxidant in the brain for defense against oxidative stress (Meister and Anderson, 1983). In normal adults, GSH concentration typically ranges from 1 to 3 mM (Sanaei Nezhad et al., 2017). Neurons in the brain require high levels of GSH to mitigate oxidative stress, maintain structural integrity, and support cognitive function (Uttara et al., 2009). Magnetic resonance spectroscopy (MRS) has been employed to examine cortical GSH levels. However, measuring GSH with MRS presents challenges due to the low GSH concentration, low signal-to-noise ratio of the brain spectra, and significant spectral overlap between metabolites with varying peak intensities (Lee and Kim, 2019). Consequently, various methods have been proposed to assess GSH concentration in vivo within the human brain (Lagopoulos et al., 2013; Mlynárik et al., 2006; Zhao et al., 2006; Mescher et al., 1998). Among these, the MEscher-GArwood Point RESolved Spectroscopy (MEGA-PRESS) sequence has been widely recommend (Rae and Williams, 2007; Terpstra et al., 2006). This spectral editing MRS method is extensively utilized to distinguish and quantify specific metabolites.

Despite the recognition of the crucial role of GSH, how GSH levels in the brain change with mastication remains unexplored. Additionally, the impact of chewing hardness on the correlation between GSH levels and mastication has not been previously studied. In this study, we utilized MEGA-PRESS to measure brain GSH levels in a healthy cohort and investigate their associations with mastication. The effects of chewing hardness on brain’s GSH levels were then examined using materials with different hardness, including gum and wood blocks. The anterior cingulate cortex (ACC), which is pivotal for cognitive control, was selected as the regions of interest. Subsequently, we explored the relationship between brain GSH levels and cognitive function. Our hypotheses were as follows: (1) brain GSH levels correlate with chewing hardness, and (2) chewing hardness influences the relationship between changes in GSH levels and cognitive function.

In the current study, GSH concentration measured by MRS was significantly higher in the ACC in the wood-chewing group compared to that in the gum-chewing group. Furthermore, the increase in GSH in the wood-chewing group was positively correlated with memory function items in neuropsychological measurements. To the best of our knowledge, this is the first report indicating that mastication can alter the level of antioxidants in the human brain, and that an increase in brain antioxidant levels is associated with cognitive function.

The results from the current study revealed that chewing moderately hard material leads to an increase in the GSH concentration in the brain. GSH is present in almost all cell types and is involved in numerous physiological functions, including detoxication of xenobiotics, intracellular redox homeostasis, cysteine carrier/storage, cell signaling, protein folding and function, gene expression, cell differentiation/proliferation, immune response, and antiviral (Cooper and Kristal, 1997; Pastore et al., 2003). Additionally, GSH acts as a “master antioxidant” across all tissues, particularly in the brain where the GSH plays a crucial role in antioxidant defense (Dringen, 2000). While low levels of ROS are necessary for maintaining normal cellular function, their accumulation leads to oxidative stress, resulting in lipid peroxidation, protein oxidation, and DNA damage that ultimately impair cellular function. The brain is especially vulnerable to oxidative stress. Therefore, the finding that chewing hard materials increases GSH concentration is intriguing and significant because it suggests that mastication may help protect the brain from oxidative stress. Based on these results, we hypothesized that the increase of GSH, brain antioxidant, by chewing hard material may effectively remove the reactive oxygen species in the brain and thus stimulate brain cells to improve the brain cognitive function. Furthermore, this finding may be particularly relevant for elderly individuals, as GSH deficiency contributes to oxidative stress, which is implicated in aging and the pathogenesis of many neurodegenerative diseases (Jenner, 1994; Jenner, 2003; Maher, 2005). An MRS study using MEGA-PRESS sequence provided in vivo evidence that the patients with Alzheimer’s disease exhibit reduced GSH concentrations in the frontal cortex compared to age-matched healthy controls (Mandal et al., 2015).

While the findings in this study strongly suggest that chewing relatively hard material is closely associated with an increase in GSH concentration in the brain, the exact cause of this GSH alteration remains to be elucidated. It is possible that the increase in GSH simply reflects an increase in cerebral blood flow (CBF). Several studies have shown that chewing moderately hard food leads to an increase in CBF, and chewing gum with varying degrees of hardness significantly enhances brain activity (Suzuki et al., 1994; Onozuka et al., 2002). Increased CBF has been linked to improved cognitive functions. According to previous reports, it is tempting to hypothesize that the increase CBF may supply sufficient nutrients to the brain to stimulate GSH synthesis. The correlation observed in the current study between the increase in GSH and cognitive measures found seems to support our hypothesis that cognitive function improves as GSH concentration increases.

The findings of the present study suggest important implications for a therapeutic strategies aimed at increasing neuronal GSH levels in the brain. While elevating brain GSH levels holds promises as a treatment for GSH-related neurodegenerative or neuropsychiatric diseases (Gilgun-Sherki et al., 2004; Pittenger et al., 2011), there are currently no therapeutic drugs available for this purpose. Orally administered GSH is rapidly degraded in the gut, and intravenously administered GSH is quickly oxidized to GSSG in the bloodstream with a half-life of 2–3 min (Ammon et al., 1986; Wendel and Cikryt, 1980). Therefore, the direct administration of GSH does not appear promising for treating GSH-related neurodegenerative or neuropsychiatric diseases. On the other hand, the current study demonstrated that chewing moderately hard materials such as wood blocks can increase brain GSH levels without degradation in the gut or oxidization in the blood. Hence, mastication, especially with moderately hard material, may offer a practical approach to enhancing GSH levels in the brain and is beneficial for maintaining or bolstering antioxidant defense in the brain.

This study has several limitations that should be addressed. Firstly, the participants in the present study were aged 20–30 years and were selected using convenience sampling. Due to this narrow age range, the findings need further validations across broader age groups, including both younger and older individuals. Secondly, the study population was limited in size, necessitating caution when generalizing the findings to a larger populations. Thirdly, our study focused specifically on the ACC, and thus, the results may not be applicable to other brain regions. Future research could explore GSH levels across a broader range of brain regions to gain a more comprehensive understanding of region-specific changes in GSH in response to chewing hardness. Fourthly, while our study limited chewing time to 5 min using relatively soft gum, longer chewing durations are possible with softer gums, potentially affecting brain activity. Moreover, chewing gum and wood can be differed not only in hardness but also in various other characteristics such as texture of the materials. Investigating changes in GSH concentrations that reflect real-world chewing scenarios will be necessary in future research. Fifthly, we were unable to objectively measure chewing hardness. Further research is needed to quantify chewing hardness and determine the optimal chewing force that can enhance cognitive function. These limitations highlight avenues for future studies to further elucidate the effects of chewing on GSH levels in the brain under diverse conditions and across different populations.

In summary, this study yielded two major findings. Firstly, the wood-chewing group experienced stimulated of brain GSH synthesis, leading to increased GSH levels in the brain. Secondly, correlation analysis indicated that the higher GSH levels in the wood-chewing group were associated with improved scores in cognitive measures. Since there are currently no drugs or established practices for boosting brain GSH levels, our findings suggest that chewing moderately hard material could serve as an effective practice for increasing GSH levels in the brain. Based on these results, consuming harder foods might prove more effective in enhancing brain antioxidant defenses through elevated GSH levels.