Tiffany Ang, Class of 2026

Gut microbes, often referred to as psychobiotics, play a key role in regulating the microbiota-gut-brain axis. This bidirectional pathway facilitates a constant relay of information between the gut and the brain. Fermented foods, such as yogurt, kimchi, and natto, are rich in beneficial gut microbes and microbial metabolites that can be leveraged to target this axis. Their affordability and widespread accessibility across cultures make them an appealing dietary choice. Historically, fermentation was used to extend food shelf life, especially during times of scarcity. 

A team of researchers, including Dr. Balasubramanian from the University College Cork in Ireland has recently compiled a review of the current state of research into the effects of fermented foods on the gut microbiome, nervous system, and immune system. Established in prior research, the hypothalamic-pituitary-adrenal (HPA) axis, a key component of the microbiota-gut-brain axis, is central in regulating mood and behavior. Dysregulation of the HPA axis has been linked to neuropsychiatric disorders such as depression, post-traumatic stress disorder, and borderline personality disorder. Fermented foods containing probiotics help mitigate this dysregulation by reducing glucocorticoid receptor expression, which binds to cortisol, a stress-related hormone. Lowering glucocorticoid receptor expression helps regulate the HPA axis, which works in conjunction with the immune system to alleviate symptoms of neuropsychiatric disorders. Additionally, fermented foods are rich in tryptophan metabolites, which are essential precursors for the biosynthesis of serotonin and melatonin, key hormones involved in mood regulation and the sleep-wake cycle (Balasubramanian et al., 2024). 

Some studies show that these fermented foods, such as fermented cow and goat milk, can reduce ghrelin levels, a hunger hormone, while others show varied effects, emphasizing the individualized responses to fermented foods. Additionally, natto, or fermented soybeans, have been found to upregulate somatostatin, a satiety-related hormone. Fermented foods also influence metabolism and appetite regulation, making them a useful dietary tool for preventing and managing obesity. Fermented foods can also restructure the gut microbiome and increase the abundance of beneficial microbes and microbial metabolites, which further promote gut health (Balasubramanian et al., 2024). Dr. Balasubramanian and others suggest that future research should aim to investigate further how sex influences immunomodulatory responses and modulates the microbiota-gut-brain-immune axis. Previous studies have already established the role of sex as a key factor in the HPA axis, suggesting that sex differences significantly impact gut-brain interactions. Therefore, understanding these differences can help customize fermented food interventions to align with individual needs.

Figure 1: Natto, a traditional Japanese dish made from fermented soybeans, is rich in probiotics that promote gut health. The fermentation process enhances the bioavailability of key nutrients and fosters the growth of beneficial gut bacteria.

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