Fermented foods and brain health: Gut–brain axis mechanisms and clinical insights

The gut-brain axis represents a complex bidirectional communication network connecting the central nervous and gastrointestinal systems. Fermented foods and their phenolic compounds, which increase their bioavailability due to microbial transformation in their contents, have the potential to affect the gut microbiota and therefore the gut-brain axis positively. Fermented foods such as kefir, yogurt, miso, natto, tempeh, kombucha, and their polyphenols have an effect on the gut microbiota and on the provision of neurological activities through neuroactive components that affect the nervous system. Phenolic compounds appear to have direct or indirect effects on brain tissue through various mechanisms such as reducing neuronal oxidative stress, suppressing microglial activation, supporting synaptic plasticity, and slowing down neurodegenerative processes such as Alzheimer's and Parkinson's. In addition, the polyphenol content enriched in fermented foods has been shown to exhibit psychobiotic effects in depression and anxiety models; it has been shown in clinical studies that it improves systemic inflammation and hypothalamic–pituitary–adrenal (HPA) axis dysfunction. Current data support the inclusion of fermented, polyphenol-rich foods as a noninvasive strategy to enhance neuroprotection and mental health. However, enhanced clinical studies are needed where heterogeneity in the fermentation process and dosage adjustment are standardized. This article reviews the current literature on the effects of fermented foods and polyphenols on brain health via the microbiota and gut-brain axis.