Recent research has investigated the intriguing link between mental health and the activity of the human gut’s microbiome, or the microorganisms that share our body space. These organisms outnumber our own cells by ten to one.
A team led by Eva M. Selhub, M.D., of Harvard Medical School and Massachusetts General Hospital, looked at the influence of fermented food and beverages.
They explain in the Journal of Physiological Anthropology that, “As our knowledge of the human microbiome increases, including its connection to mental health (for example, anxiety and depression), it is becoming increasingly clear that there are untold connections between our resident microbes and many aspects of physiology.”
Our intestine contains about 300 to 500 different species of bacteria that can be roughly divided into health-promoting ones, such as Bifidobacteria and Lactobacilli, and harmful ones such as Clostridia.
The team examined the history and application of fermentation “as a means to provide palatability, nutritional value, preservative, and medicinal properties.” This is an ancient practice that continues to the present day, they state.
In recent years, researchers have discovered many ways in which consuming fermented products affects our intestinal microbiota. For example, fermentation-enriched bioactive peptides, derived from whey milk protein, may have anti-inflammatory effects and reduce high blood pressure.
Selhub and colleagues put forward the argument that fermented food partly explains the link between traditional dietary practices and positive mental health.
The link could manifest itself directly through gut-to-brain communication, they say, or indirectly through beneficial bodily changes such as improved glycemic control antioxidant and anti-inflammatory activity, or reduction of intestinal permeability.
A further mechanism of action may be the influence of fermented food on endotoxins called lipopolysaccharide (LPS), large molecules that are found to be particularly important in depression. Lab tests on rodents and human volunteers show that even small increases in LPS levels can trigger depressive symptoms.
Furthermore, a positive influence on nutritional status from fermented foods could lead to improved neurotransmitter and neuropeptide production in the brain.
The label ‘probiotic’ is often given to beneficial forms of gut bacteria, and one of the foods containing lactobacillus microbes is live-cultured yogurt. Healthy animals given one strain of lactobacillus show a reduction in anxiety and depression-like behaviors under stress.
The animals had alterations in the system of the brain concerning the neurotransmitter gamma-Aminobutyric acid (GABA) that were similar to the effects of antidepressants such as benzodiazepines.
In addition, supplementation with Bifidobacteria, also found in yogurt, “appears to attenuate an exaggerated stress response and maintain adequate levels of the neuropeptide brain-derived neurotrophic factor (BDNF), levels of which are known to be low in depression.”
Even mild chronic inflammation of the gastrointestinal tract can provoke anxiety and diminish BDNF production in animals, the researchers add.
Bifidobacteria supplementation also appears to decrease monoamine oxidase activity in the brain, potentially increasing neurotransmitter levels between synapses.
A combination of Lactobacillus and Bifidobacterium given for a month to healthy adults appeared to lead to significant improvements in depression, anger, and anxiety.
The supplement also lowered levels of the stress hormone cortisol compared with placebo. When given to rodents, the same supplement reduced “behaviors indicative of anxiety.”
A further study of individuals with irritable bowel syndrome suggested that consumption of a ‘prebiotic’ fiber (trans-galactooligosaccharide) significantly reduced anxiety, most likely due to raising bifidobacteria levels in the gut. Prebiotics are defined as non-digestible food ingredients that benefit us by stimulating the growth and activity of beneficial bacteria in the colon.
The authors believe, “Modern research is highlighting the potential value of ancestral dietary practices on mental health, and on resiliency against depression in particular. At the same time, there has been tremendous progress toward better understanding of the role played by the low-grade inflammation and the intestinal microbiome in human health and mental well-being.”
But in their review, the authors caution that not all forms of fermented foods are helpful. For example, some pickled vegetables can grow fungi that increase the production of N-nitroso compounds, which have possible cancer-causing properties.
The treatment of mental health issues is currently taking place in a food environment that includes many foods at odds with our evolutionary past, such as grains and high levels of sugar. These foods “are not only undermining optimal nutritional status, they have untold effects on the microbiome and ultimately the brain,” warn the experts.
They conclude that further research should “continue to illuminate the ways in which the clay fermentation pots of our ancestors might be connected to the emerging discipline of nutritional psychiatry.”
Selhub, E. M. et al. Fermented foods, microbiota, and mental health: ancient practice meets nutritional psychiatry. Journal of Physiological Anthropology, 15 January 2014, doi:10.1186/1880-6805-33-2