Probiotics and brain health
Probiotics are live bacteria that impart health benefits. Research has shown some probiotics can improve the microbiome and benefit the brain, reducing symptoms of anxiety and depression.
A small study showed a probiotic containing Bifidobacterium longum significantly improved symptoms of depression over six weeks. Probiotics such as Bifidobacterium and Lactobacillus, found in yogurt, have been shown to improve mood and reduce anxiety. (33, 34)
However, a large meta-study of over 1500 patients from 14 studies showed that only 3 of trials showed probiotics improved symptoms of anxiety. This was much lower than for non-probiotic interventions, suggesting more work is needed to clarify the benefit of probiotics for anxiety. (35)
Prebiotics and brain health
Unlike probiotics, prebiotics do not contain a live organism. Prebiotics are fibers that are fermented by microbes, and may also influence brain health. One study showed three-weeks treatment with a prebiotic galactooligosaccharides reduced levels of cortisol, a ‘stress’ hormone. Other fermented products such as kefir, kambucha, and kimchi may also improve mental wellbeing, however, further research is needed. (36)
What are psychobiotics?
Psychobiotics is a term for live bacteria that confer benefits to mental health. These are probiotics, most commonly of Bifidobacterium and Lactobacillus families, or prebiotics that protect against diseases such as depression and schizophrenia. Psychobiotics influence the gut-brain axis via the spinal cord, vagus nerve and endocrine (hormonal) systems. (37)
Foods that improve the gut-brain axis
Recently, certain food groups have been shown to improve gut-brain functioning. Many of these contain psychobiotic compounds, and include:
Omega-3 fats: Found in oily fish and flaxseed, have been shown to improve microbiome diversity in adolescence and adulthood. This reduces the risk of brain disorders such as schizophrenia and depression.
Fermented milk products: Suchas various probiotic strains, found in yogurt and cheese. These have been shown to modulate brain activity
Fiber-rich foods: Such as fruits and vegetables, whole grains and nuts. These contain prebiotic fibers that reduce cortisol levels and alter emotional bias.
Cocoa: Found in chocolate products, has been shown to act as a prebiotic and probiotic and increase levels of Bifidobacteria and Lactobacilli microbes. Cocoa is rich in polyphenols, beneficial to the microbiome. (38, 39)
Autism and GI problems
Australian researchers recently found that a single gene causes both autism and gut problems. This helps explains why autism is accompanied by GI issues in 90% of cases. The brain and the GI tract share many of the same neurons, and the newly discovered mutation affects communication in the brain causing autism as well as:
This research further highlights the intimate connection between the brain and the gut. (40)
How gut bacteria can lower stress
Several avenues of research have shown how crucial gut bacteria are to our mental health. In 2004, Japanese scientists showed that gut bacteria influence stress in mice models. Mice that were kept in a ‘germ-free’ environment, without exposure to microbes in their bodies, showed greater fluctuations in the adrenal hormone corticosterone – associated with greater stress. When the researchers then administered the mice with Lactobacillus the mice showed lower levels of stress. This implies gut bacteria are crucial for reducing stress. (41)
What’s more, Chinese scientists showed gut bacteria from depressed patients could be transplanted into ‘germ-free’ mice and induce depressive behaviors. This demonstrates that ‘depressed’ microbiomes may even apply between species. (42)
On a broader scale, population-based studies from as recently as in 2019 have shown differences in the microbiome are linked with mental illnesses such as anxiety and depression. Interestingly, it is the overall ratio of different species rather than individual species that appear to be responsible for this effect. Hence, gut bacteria appear crucial in maintaining our wellbeing. (43)
The Wrap Up
The gut-brain axis joins the digestive system and nervous system and plays an important role in overall health. The brain-gut connection is made up of 'the second brain' - nerve cells of the enteric nervous system - the vagus nerve and gut microbiota. These features are influenced directly by the stress response, which may lead to health problems of the digestive tract such as irritable bowel syndrome (IBS). By contrast, maintaining a healthy gut by consuming foods beneficial to the gut microbiome can improve symptoms of IBS and protect against brain diseases such as Alzheimer's and Parkinson's disease. The gut-brain connection is therefore critical to the health of the gastrointestinal tract and the good health of the mind.
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