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Scientists have found a link between fermented foods and improved mental health. Marta Mauri/Stocksy
  • According to a new review, fermented foods may be associated with improved cognitive performance, specifically due to their ability to influence the microbiota-gut-brain axis.
  • Fermented foods keep the intestinal barrier healthy and strong, preventing bacteria and toxins from entering circulation and reducing the chance of leaky gut syndrome.
  • The gut-brain connection is linked with numerous brain functions such as memory, cognition, anxiety, depression, and overall health.

In recent years, numerous studies have focused on gut microbes, specifically looking at how they interact with the brain (the microbiota-gut-brain axis). Since fermented foods, in particular, are known for their gut health benefits, researchers wanted to explore how these foods impact mental health.

A new review published inNeuroscience & Behavioral Reviews looked at the different types of fermented foods, fermentation techniques and their ability to affect the microbiota-gut-brain axis. Researchers also analyzed knowledge gaps and challenges in conducting human studies.

Examples of fermented foods include:

  • kimchi
  • sauerkraut
  • kefir
  • miso
  • tempeh
  • yogurt

The review noted that fermented foods directly impact the enteroendocrine system, which affects hormones such as ghrelin, neuropeptide-Y, glucagon-like peptide 1 (GLP-1), and serotonin. Fermented foods are high in prebiotics and probiotics, increasing the amount of GLP-1. However, further research is needed to understand how fermented foods affect appetite and hunger.

Human studies on fermented dairy have mixed results on cognitive health, whereas observational studies associate fermented food intake with changes in gut health and decreased anxiety.

“We know from previous studies that there is a proven gut-brain axis and that this, therefore, links diet directly to the brain and its behavior based on the health of our microbiota,” said Dr. Nicole Avena, nutrition consultant, assistant professor of neuroscience at Mount Sinai School of Medicine, visiting professor of health psychology at Princeton University, and author of Sugarless.

“I think this review does a good job at showing the history behind fermentation and the physiology behind how it became known as a superfood for gut and brain health,” she said.

Our gut contains hundreds of different strains of bacterium, Avena explained. What makes everyone’s microbiome unique is that an abundance of different factors influences the species and diversity within the gut. These factors span from maternal health prior to birth all the way to the current environment.

“What makes food such an important part of gut health is that it is a tangible method we can use to diversify and strengthen (or weaken) our gut flora,” said Avena. “The gut-brain axis has been linked to the health and diversity of our microbiome – meaning the less diverse the diet, the more mental and brain health can suffer. We know these bacteria help with digestion, absorption, and byproduct of nutrients that can directly affect our mental health.”

The brain and gut are connected by many pathways involving nerves and circulation, Dr. William Li, medical doctor and New York Times bestselling author of Eat to Beat Your Diet: Burn Fat, Heal Your Metabolism, and Live Longer, explained.

“Substances produced in the gut by bacteria can travel or send signals up large nerves, such as the vagus nerve, directly to the brain — triggering different brain activities that can alter mood, behavior, memory, and cognition,” he said.

Li continued: “From the other end, substances from the brain can travel down nerves that begin in the brain and distribute like electrical wires to the gut. These signals can influence the gut bacterial ecosystem.”

From a circulation standpoint, microbes in the gut can produce substances that enter the bloodstream and then circulate those substances directly into the brain. Similarly, chemical signals produced by brain cells can enter the bloodstream from the brain and circulate to the gut, affecting the gut microbiome, Li added.

“While research on exact bacteria-to-brain / brain-to-bacteria effects are still in their infancy, this gut-brain connection is associated with a wide variety of brain functions such as memory, cognition, anxiety, depression, and overall mental health and wellness,” Li stated.

“There are many compelling correlations showing that dysbiosis, or abnormal gut microbiome composition, is associated with depression, anxiety, dementia, Parkinson’s disease, and other neurodegenerative disorders,” he added.

Fermented foods contain bioactives from the original food itself (polyphenols, dietary fiber as examples), and healthy bacteria (probiotics) as well as the metabolites created by these bacteria (post-biotics).

These components of fermented foods contribute to the activity of the gut microbiome of someone who consumes the food, either by feeding healthy gut bacteria — stimulating their action on the gut-brain axis — or by contributing to the gut bacteria or their products directly, Li explained.

“The net effect is to contribute to a healthier gut bacteria ecosystem that activates brain pathways. There are many still unanswered questions about the gut-brain connection, but this is the current view based on research in the lab as well as in human studies,” he said.

The review addresses many of the knowledge gaps and limitations of current research in the gut-brain connection.

For example, “studies involving single bacteria do not capture the full extent that fermented foods play on the gut-brain axis because of the plethora of bacteria, metabolites, and other small molecules present in food that may be playing important roles,” Li said.

Additionally, “clinical studies of fermented foods may not capture sex-specific differences or account for the diversity of diet, lifestyle, behavioral, and genetic factors in their subjects,” he added.

The generalization of findings is also limited by how fermented foods differ across regions by how they are produced, and the environment in which they are stored and consumed. Despite these limitations, however, the review makes a compelling case based on scientific evidence that gut health influences brain health, which influences mood and behavior, Li added.

It’s also important to note this review doesn’t use any original data, as it is a narrative review. While the authors point out where the individual studies that they cite have poor methodology, such as inadequate controls, there is no formal scoring of research quality of individual papers.

“I think one of the only major limitations is the limited number of studies who use human subjects,” said Avena.

“There needs to be more evidence of the exact effects of fermented foods on direct human microbiome and neurotransmitter research.”
— Nicole Avena