In a recent review published in the journal Neuroscience and Biobehavioral Reviews, researchers from Ireland investigated a variety of fermented foods, their sources and fermentation techniques while investigating their ability to impact the microbiota-gut-brain axis. In addition, they identified knowledge gaps and challenges in studying the mental health benefits of specific fermented foods and proposed solutions to further understanding their therapeutic potential.
Study: Fermented foods: harnessing their potential to modulate the microbiota-gut-brain axis for mental health. Image credit: Molishka / Shutterstock
Background
The microbes colonizing the gut serve as a vital mediator in the microbiota-gut-brain axis, linking external signals and internal body functions and communicating continuously with the central nervous system. Various factors, including diet, influence the gut microbiota and influence the intestinal, immunological, and neural components of the microbiota-gut-brain axis. Recent advances highlight the potential of dietary interventions targeting the microbiota, such as probiotics, prebiotics, and fermented foods, to improve mental health by modulating the gut-brain axis.
In the past, fermented foods were used to extend shelf life and improve flavor. They are diverse, categorized by substrate, including grains, dairy, meat, fish, vegetables, and legumes, each of which undergoes unique fermentation processes. These foods contain a rich microbial community influenced by substrate type, geographic location, pH, and preparation methods, offering potential probiotics, bioactive peptides, phytochemicals, and vitamins. Researchers continue to study fermented foods for their ability to shape the composition of the gut microbiota, produce beneficial metabolites, and modulate gut-brain pathways, providing a promising avenue for dietary interventions.
The current review explores the mental health benefits of fermented foods by examining their impact on the microbiota-gut-brain axis and discusses challenges in preclinical and human studies.
Fermented food and the microbiota-gut-brain axis
Fermented foods influence the microbiota-gut-brain axis through immune modulation, barrier integrity and neuroendocrine regulation. Microbes and metabolites from fermented foods interact with receptors, influencing immune responses. Fermented foods improve gut and blood-brain barrier integrity, potentially mediated by microbial metabolites. They also affect the hypothalamic-pituitary-adrenal (HPA) axis and serotonin levels. Preclinical and human studies have revealed various immunomodulatory effects of fermented foods, highlighting the need for further investigation of their complex interactions with the microbiota-gut-brain-immune system axis.
Fermented foods have a profound effect on the gut-brain axis by modulating the enteroendocrine system (EES), influencing gut hormones such as serotonin, neuropeptide-Y, glucagon-like peptide 1 (GLP-1), ghrelin, and somatostatin. These hormones regulate motility, appetite, and insulin release, influencing eating behavior. Fermented foods rich in prebiotics and probiotics shape the gut microbiota and enhance the production of gut hormones, especially GLP-1. However, studies on the effects of fermented foods on satiety have been inconclusive, with variations in hormone release observed. Additionally, fermented foods can alter ghrelin and leptin levels, contributing to appetite regulation. Understanding these complex interactions is critical to harnessing the potential health benefits of fermented foods.
Preclinical and clinical landscape
Fermented foods affect gut-brain communication pathways, affect immune responses, gut microbiota, microbial metabolites, and the enteric nervous system. Preclinical models, including Drosophila, zebrafish, mice and pigs, have revealed behavioral improvements and microbiota changes with fermented milk products, soy, sugar-based products and other substrates. Human studies on fermented dairy products have shown mixed cognitive effects, while observational studies have linked fermented food consumption to an altered gut profile and reduced anxiety. Fermented soy products, especially isoflavones, may help cognitive function in women, in contrast to negative associations with tofu consumption. A previous meta-analysis suggested that fermented foods hold promise for cognitive outcomes.
Future trends and challenges
Standardization of fermented foods is challenged by diverse microbial communities influenced by factors such as geography, production scale, and substrate type. Environmental conditions, duration of fermentation, salinity, pH, moisture and ripening time add to the complexity. Regulatory guidelines offer limited insights requiring cultural sensitivity. “Omics” techniques are helping to understand and archive data, but the vast variety of fermented foods requires further study for effective standardization considering both bioactive potential and culture conservation. Designing human studies of fermented food interventions presents a challenge such as accounting for controls, emphasizing the need for non-fermented controls in evaluations. Accurate measurement of fermented food intake is hampered by the variety of foods recommended, requiring detailed methods such as 24-hour food recalls and food diaries. Biomarkers associated with fermented food consumption offer precise measurement using advances such as plant metabarcoding for unique fingerprints. Using appropriate methods to capture microbiomes involves choosing high-resolution techniques such as shotgun metagenomics instead of 16S rRNA (short for ribosomal ribonucleic acid) sequencing.
Conclusion
In summary, the present study reviews the significant health effects of fermented foods using different microbial strains, metabolites, and bioactive compounds to optimize neural and mental health benefits. It highlights the need for further human studies, particularly with non-fermented controls, to fully identify and understand the beneficial effects of fermented foods on the microbiota-gut-brain axis. Despite the challenges, fermented foods are emerging as essential components in the evolution of microbiota-based therapy for mental health.
