Jyoti Verma, St. Xavier's College (Autonomous),
Mumbai, Maharashtra, India
Image credit: https://www.healthline.com/nutrition/gut-brain-connection
It is no surprise that our bodies are the result of millions, or even billions of years of evolution, making it one of the most complex organisms to ever walk on this planet (maybe even the universe). With the advancement of science and technology, we have been fortunately able to discover a lot about the functioning of this diverse, yet fascinating organic structure we call -the human body. One such area worth understanding is the relationship between the gut microbiome and our brain. Sounds weird right? Though these tiny beings help our body in digesting food, process nutrients, help in the production of certain vitamins and immunological molecules, the possibility of far inferior organisms playing an important role in the functioning of one of the most complex organs of the human body is not so easy to digest. But that’s what makes it even more interesting. All organisms, since time immemorial, have known to co-dependently exist with each other. Humans, in this case, are no exception. Our body harbors millions of microbes that impact us, both negatively and positively. One of the major reservoirs of microbes in the gut, which is capable of fostering a large portion of the 3.8 x 1013 microbes present in our body. The group of bacteria, archaea, and eukarya living in the gastrointestinal(GI) tract is collectively known as the ‘gut microbiota’. This complex and mutually beneficial relationship is a result of co-evolution with the human body for thousands of years. The study of these microbes has let us know that their presence and diversity in any individual is as unique as that of fingerprints, and is influenced by environmental factors such as diet, as well as possible host genetics contributions. So, while the connection of the brain and gut seems physically impossible, there is a 24/7 bidirectional communication set up between them called the microbiota-gut-brain (MGB) axis.
Image credit: www.researchgate.net/profile/Christopher_Lowry/publication/318870864_The_Gut_Microbiome_and_Mental_Health_Implications_for_Anxiety-_and_Trauma-Related_Disorders
This is achieved with the help of direct and indirect pathways, which include endocrine(hypothalamic-pituitary-adrenal[HPA]pathway), immune and metabolic pathways, the limbic system, as well as the sympathetic afferent systems. The gut microbiome is known to produce about 90% of the neurotransmitter serotonin, which heavily regulates our emotions. People with severe brain conditions like depression, anxiety, amnesia, bipolar disorder, schizophrenia, etc. show a different composition of gut microbiota than normal individuals.
To show the effects of microbiota on behavior, researchers transplanted microbiota from germ-free (GF) BALB/c mice (high anxiety mouse strain) to GF NIH Swiss mice (low anxiety mouse strain), and vice-versa. The behavioral profile check after the transplant gave some very interesting results. The anxiety levels in the donor and recipient mice became the exact opposite, suggesting a possible linkage of the gut microbiota to the brain. Most of the studies are done by altering the composition of certain strains of microbes in the gut since not all of them give the same cumulative effect to a suffer from a possible mental health issue. Numerous probiotics (live, beneficial organisms) therapies have been carried out on animal models, dealing particularly with the Bifidobacterium and Lactobacillus genera. Chronic treatment of Bifidobacterium infantis showed reduced amounts of depressive behavior, immune alterations, and reduced noradrenaline concentrations in the brainstem of the animal. On the other hand, treating the animal models with Lactobacillus helveticus improved memory dysfunction along with anxious behavior in mice. All the related studies highlight the strain-specific effects of these microbes on the CNS and eventually the behavior of animals. Similarly, long exposures of certain prebiotics(non-digestible substances) to the mice showed changes in the gut microbiota composition possibly increasing the brain-derived neurotrophic factor (BDNF) expression in the brain due to changes in gut hormones. The importance of natural prebiotics like oligosaccharides found in human milk was also studied. It was stated that the mice not fed with this prebiotic were prone to anxiety-like symptoms, reduction of immature neurons in the dentate gyrus, and altered colonic mucosal-associated microbiota when exposed to stressors. The mechanism of probiotics through which they potentially influence our health is through various co-dependent and complex pathways. Some of these include competition with unfriendly bacteria for metabolic interactions, production of growth inhibitory bacteriocin, improvement in the functioning of mucosal barrier function, inhibition of bacterial translocation, regulation of the immune system, etc.
By now, we have an idea that the beneficial alteration of the intestinal microbiota can have long term positive impacts on our mental health. Therefore, healthy changes in diet can promote positive changes in the microbiota that can bring about crucial changes in the cognitive abilities of a person. Hence, it is essential to include gut health-friendly food items like yogurt, kombucha, kimchi, garlic, onions, ginger, peas, sprouts, asparagus, barley, almonds, olive oil, oats, and flax seeds to name a few. The advancing field of Nutri-metagenomics involves the understanding of the interaction between the microbiota, nutrition, and host in the dynamics of associated diseases, and possible therapeutic targets. Although this area of understanding the complex relationship of gut microbiota and mental health is still new, and in the process of further research and development, it has given us enough evidence that the connection between them is real, and is of much more significance than we think it to be.
References:
I Butler, M., Mörkl, S., Sandhu, K. V., Cryan, J. F., & Dinan, T. G. (2019). The Gut Microbiome and Mental Health: What should we tell our patients? National Library of Medicine, 1. https://doi.org/10.1177/0706743719874168
Malan-Muller, S., Valles-Colomer, M., Raes, J., A. Lowry, C., Seedat, S., & M.J. Hemmings, S. (2017). The Gut Microbiome and Mental Health: Implications for anxiety and trauma-related disorders. OMICS A Journal of Integrative Biology, 21, 1. https://doi.org/10.1089/omi.2017.0077
Pennisi, E. (2020, May). Meet the psychobiome. AAAS. https://www.sciencemag.org/news/2020/05/meet-psychobiome-gut-bacteria-may-alter-how-you-think-feel-and-act
Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacterial cells in the body. PLOS Biology, 1. https://doi.org/10.1371/journal.pbio.1002533
Thursby, E., & Juge, N. (2017). Introduction to human gut microbiota. Biochemical Journal, 1. https://doi.org/10.1042/BCJ20160510
Wanucha, G. (2018, October 4). The Gut Microbiome and Brain Health. UW Medicine. http://depts.washington.edu/mbwc/news/article/the-gut-microbiome-and-brain-health#:~:text=Down%20in%20the%20gut%2C%20bacteria,microbiome%20is%20a%20diverse%20microbiome.