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The Microbiota-Gut-Brain Axis: Why Our Tummy Is Important for Our Mind

The Microbiota-Gut-Brain Axis: Why Our Tummy Is Important for Our Mind

Contributor Bio

Arianna Ferrini is a postdoctoral research fellow at University College London (UK) and a freelance scientific writer and illustrator. She holds a PhD in Tissue Engineering and Regenerative Medicine from Imperial College London and an MSc in Medical and Pharmaceutical Biotechnology from the University of Florence (Italy).

Would you believe that we humans are mostly microbes — over 100 trillion of them? Microbes outnumber our human cells ten to one. The majority live in our gut, particularly in the large intestine. They are called microbiota. The microbiome is the genetic material of all the microbes — bacteria, fungi, protozoa, and viruses — that live on and inside the human body. The number of genes in all the microbes in one person's microbiome is 200 times the number of genes in the human genome.

The microbiome was not generally recognized to exist until the late 1990s. After that, there has been an exponential growth in our understanding of the human microbiome. Advances in genome sequencing technologies and metagenomic analysis (the genetic study of genomes taken directly from environmental samples) have enabled scientists to study these microbes and their function and to research microbiome-host interactions both in health and disease.

Now we know that the bacteria in the microbiome help digest our food, regulate our immune system, protect against other bacteria that cause disease, and produce vitamins including B vitamins B, thiamine, and riboflavin, and vitamin K, which is needed for blood coagulation. The human microbiome is now recognized as an "organ" on its own, without which the body would not function correctly. Diet, age, stress, and diseases cause increases or decreases in relative abundance and diversity of bacterial species of the gastrointestinal tract and other body sites. Studies in animal models and humans have shown that a persistent imbalance of the gut's microbial community, called dysbiosis, relates to inflammatory bowel diseases (IBD), irritable bowel syndrome (IBS), diabetes, obesity, cancer, and cardiovascular and central nervous system disorders. Let's see how these microbes living in our gut can influence not only our physical health but, interestingly, also our mental health. This involves the gut-brain axis.


The gut-brain axis (GBA) is a bidirectional link between the central nervous system (CNS) and the enteric nervous system (ENS) of the body. It involves direct and indirect pathways between cognitive and emotional centers in the brain with peripheral intestinal functions. The importance of GBA in maintaining homeostasis has long been appreciated. However, the past 15 years have seen the emergence of the microbiota as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinctmicrobiota-gut-brain axis. This axis is gaining ever more attention in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The interaction between microbiota and GBA is bidirectional, namely through signaling from gut-microbiota to brain and from brain to gut-microbiota by neural, endocrine, immune, and humoral links. 1 This bidirectional communication network includes the central nervous system (CNS), both brain and spinal cord, the autonomic nervous system (ANS), the enteric nervous system (ENS), and the hypothalamic-pituitary adrenal (HPA) axis.

Researchers speculate that any disruption to the normal, healthful balance of bacteria in the microbiome can cause the immune system to overreact and contribute to inflammation of the gastrointestinal (GI) tract, in turn leading to the development of symptoms of diseases that occur not only throughout your body but also in your brain. For example, several mood disorders, such as anxiety, depression, and autism spectrum disorders, now have well-established links to functional GI disruptions, whereas GI diseases (e.g., irritable bowel syndrome, irritable bowel disease) often involve psychological comorbidities associated with alteration of the gut microbiome.2

Psychobiotics is a field that investigates the effects of probiotics and mental health.3 Some research shows that certain Lactobacillus species improve stress resilience and anxiety. 4 Some studies even show that taking probiotics can help alleviate symptoms of depression.5,6 Probiotic bacteria provide many health benefits, including for the brain. They naturally reside in the gut but are also found in supplements and fermented foods, like yogurt and kefir.Bifidobacterium, Lactobacillus, and Lactococcus species are all examples of probiotics because they support your whole body and improve mental health too.

The current understanding of the gut-brain axis does add to the growing evidence that a healthy, balanced diet could be an important preventative measure to reduce the risk of developing an illness like depression in the first place. Many of these studies have examined the "Mediterranean diet" — a catch-all term to describe diets rich in vegetables, fruit, nuts, seafood, and unsaturated fats and vegetable oils, and low in refined sugar and red and processed meat. In one study from Spain, people eating the traditional Mediterranean diet were roughly half as likely to be diagnosed with depression over a 4-year period.7 Although there are many potential mechanisms, the Mediterranean diet has been shown to increase the diversity of gut bacteria and reduce the other physiological changes such as chronic inflammation that also seems to accompany depression. 8

But how can the microbiota biochemically alter our mood? Our gut microbes transform food into short-chain fatty acids (SCFA). These SCFAs communicate with neurons, which produce serotonin, a neurotransmitter that regulates your mood, as well as levels of anxiety and happiness. Another important neurotransmitter, gamma-aminobutyric acid (GABA), regulates and improves mood because it helps calm the nervous system and switch off stress reactions. Amazingly, some probiotic gut bacteria can even produce GABA themselves for your body.9 Fundamentally, your diet can help your bacteria protect your mental wellbeing because eating the right foods feeds happy bacteria. And when you have lots of different healthy bacteria, your microbiome is more diverse and produces substances that increase mood-lifting chemicals, like serotonin and GABA.

Another promising way to improve your gut flora might be drinking hydrogen-rich water (HRW). A recent report described that 70% of gastrointestinal microbial species listed in the Human Microbiome Project encode the genetic capacity to metabolize molecular hydrogen, meaning that molecular hydrogen might affect the gut microbial composition.10 Several recent studies in large animal models showed that oral administrations of HRW attenuated oxidative stress in the GI tract and prevented mycotoxin-induced microbiota imbalance.10-13 Moreover, molecular hydrogen dissolved in alkaline electrolyzed water was recently shown to strongly improve the intestinal environment in mice, including the microbial composition in mice.14 Taken together, these recent findings suggest that molecular hydrogen, and especially HRW, might shift the pathways of hydrogen-utilization bacteria and improve the diversity of intestine microbiota towards a "protective" composition.

This might be particularly important for athletes. As more elite athletes suffer from psychological and gastrointestinal conditions that can be linked to the gut, targeting the microbiota therapeutically may need to be incorporated in athletes' diets. What is troubling is that dietary recommendations for elite athletes are primarily based on low consumption of plant polysaccharides, which is associated with reduced microbiota diversity and functionality.15 On the other hand, an enhanced microbiota diversity might be what they need to better handle the stress. For example, a recent study showed that consumption of HRW for 2 months in female juvenile soccer players had positive effects for the antioxidant activity and, most importantly, improved the diversity and the abundance of the gut flora16

To be happy, we need to keep our microbes happy. There are several ways to do that, and attention to the diet, together with consumption of HRW, might be an excellent way to start.