Infectious Diseases Topics

The Gut-Brain Axis and Its Relationship to Health

Introduction to the Gut-Brain Axis

Everything is connected. At a large scale, this is true when thinking about our planet: our choices influence our environment, which affects the plants and animals of our ecosystem. Ultimately, the systems come back to influence us once again. These same effects occur locally: the actions and kindnesses we should to friends and family influence their emotions and development, which will impact us on another occasion. These influences are not unidirectional. Instead, they are cyclical, constantly affecting themselves repeatedly. As is valid for the world and community, these bidirectional influences occur in our bodies as well. 

In the medical world, we often view different areas of the body as separate. Our mere vernacular cements this idea. Nonetheless, this mental framework carries great falsehoods that are evident when reflecting upon the relationships between bodily systems. A great example of this is the bidirectional relationship between the gut and the brain, often referred to as the Gut-Brain Axis. 

One may, by default, assume that the relationship between the brain and the gut is simple and boring: the brain chooses the foods we eat, which influences what goes in the gut. However, the mechanisms that associate these systems are far more complex and compelling. When someone supports the two, they may improve their health.

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The Gut’s Influence on the Brain

There is a large body of evidence showing that the gut has a significant effect on the mind. However, this influence is not due to the gut organs (stomach, intestines, etc.). Instead, it is the thousands of colonies of bacterial species along our gastrointestinal walls, known as the microbiome. These probiotics provide health-related benefits and effects throughout the body. In truth, the line between the “self” and other organisms gets hazy when we consider the idea that we humans couldn’t exist in the way we do now without these microbiotas. They are an intrinsic aspect of our body and health.

Of the many influences, these bacteria play a significant role, for example, in developing the brain in early human life (Sharon et al., 2016). Gut health, therefore, is thought to be related to the development of autism spectrum disorder, multiple sclerosis (M.S.), and Parkinson’s Disease (Tran & Mohajeri, 2007). The gut microbiome is also essential for mental health later in life: they influence mood and behavior and may contribute to the development of stress and anxiety (Dash et al., 2015; Rieder et al., 2017). 

If these correlations aren’t convincing enough, consider the relationship between the gut microbiome and serotonin. Serotonin is an essential neurotransmitter. Its functions include temperature regulation, memory, mood, emotions, sleep, wakefulness, appetite, gastrointestinal function, glucose metabolism, regulating blood pressure, platelet aggregation, and more (Jonnakuty & Gragnoli, 2008). A staggering 95% of the body’s source of this neurotransmitter is not produced within the neuronal system but made by the gut microbiome (Bosi et al., 2020). Without the gut microbiome, we would not have the serotonin needed for healthy bodily function. It is no wonder that poor gut microbiome health leads to poor neurological development.

The Greatest Microbial Diversity in Humans is found in the gut and its association to the gut-brain axis
Electron Micrograph of Bacteria. Source: Creative Commons, Janice Carr.

The Brain’s Influence on the Gut

Just as gut health and microbiome content influence the brain, the opposite also occurs. Unsurprisingly, the brain makes decisions on what foods we choose to eat. However, the habits of the brain and the subsequent neuronal patterns also play a role.

Though evidence of this top-down influence is scarce, recent studies reveal something remarkable. A study involving elderly individuals found that alteration in cognitive functions leads to a change in gut microbiome content (Wei et al., 2020). To be more specific, mild cognitive impairment affects the populations of bacteria residing in the gut. These include the species RuminococcusCoprococcusParabacteroidesFusobacteriumEnterobacteriaceaeRuminococcaceae, and Phascolarcto bacterium (Wei et al., 2020).

Even more striking was the observation that meditation, which changes the brain’s functional habits, altered the gut microbiome. The study mentioned above put some of the elderly participants through a mindfulness training program. After the investigation, they found that bacterial diversity differed between the trained and untrained groups (Wei et al., 2020). Additionally, the investigators noted improved memory capabilities in those who underwent the training (Wei et al., 2020)—generating a clear relationship between changes in cognitive abilities and gut microbiome content.

Surprisingly, telomere integrity improved when the participants underwent frequent training (Wei et al., 2020). Telomeres are the ends of chromosomes that slowly deplete in length as cells divide. Once fully depleted, the cell undergoes apoptosis (cell death). Some researchers use this as a marker for aging. Telomere integrity improvement suggests that this mindfulness training even slowed the aging process!

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How Mental Training Could Influence the Gut-Brain Axis

The interrelationship of the gut and brain is surprising enough. The possibility that our mental state and behavior can influence the gut through training is even more astonishing. It is imperative to understand the possible mechanisms to how this could occur. 

Many of the likely causes relate to how the stress state influences both the brain and gut. Stress releases corticotropin-releasing hormone (CRH), disturbing the microbiome (Househam et al., 2017). The absence of stress leads to increased production of short-chain fatty acids (SCFA), which are highly beneficial towards gut health (Househam et al., 2017). The increase in SCFAs also leads to a positive feedback loop of health and strengthens the immune system because SCFAs are anti-inflammatory (Spichak et al., 2021). Moreover, SCFAs affect inflammation in the hippocampus (Li et al., 2019). The hippocampus is the brain area involved in memory, suggesting a precise mechanism of how meditation improved memory in the study by Wei et al. This research suggests that decreased stress via meditation training improves the gut, leading to further health benefits within the brain. 

Lastly, the vagus nerve plays a role in the Gut-Brain Axis. The communication of the gut and brain is thought to be facilitated by, in part, the vagus nerve (Breit et al., 2018; Househam et al., 2017). It is a cranial nerve highly involved in the parasympathetic nervous system (PNS.) Relaxation results from activation of the PNS. . Increases in vagal tone mediate the release of chemicals that repair the gut wall and resolve gut dysbiosis (Househam et al., 2017). There is ample evidence that meditation increases vagal tone (Breit et al., 2018; Heckenberg et al., 2018; Luberto et al., 2018). It may be the single most significant point of explanation on why meditation produces health benefits.

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Mind over Menu: Nurturing the Gut-Brain Axis

Having greater awareness and care of our dietary choices is extremely important. The science of the gut-brain axis is still in its infancy. As it evolves, it will become increasingly evident that our foods have a tremendous impact on all aspects of mood, health, and life. Despite the importance of gut health and dietary awareness, these behavioral changes are not enough. Even if one were to have an excellent diet, a noisy mind full of ruminations and stress would severely hinder its benefits to health.

Our body is one giant ecosystem. One cannot water plants but deprive them of sunlight and expect them to grow. Our body is no different. Every piece of health is essential. Diet is important. Practicing mindful presence is also necessary. Nurturing all aspects that promote health provides the optimal results. 

Everything is connected. No one thing can change by itself.

Paul Hawken.


Bosi, A., Banfi, D., Bistoletti, M., Giaroni, C., & Baj, A. (2020). Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease. International Journal of Tryptophan Research13.

Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus nerve as modulator of the brain-gut axis in psychiatric and inflammatory disorders. In Frontiers in Psychiatry (Vol. 9, Issue MAR, p. 1). Frontiers Media S.A.

Dash, S., Clarke, G., Berk, M., & Jacka, F. N. (2015). The gut microbiome and diet in psychiatry: Focus on depression. Current Opinion in Psychiatry28(1), 1–6.

Heckenberg, R. A., Eddy, P., Kent, S., & Wright, B. J. (2018). Do workplace-based mindfulness meditation programs improve physiological indices of stress? A systematic review and meta-analysis. In Journal of Psychosomatic Research (Vol. 114, pp. 62–71). Elsevier Inc.

Househam, A. M., Christine, Peterson, T., Mills, P. J., & Chopra, D. (2017). The Effects of Stress and Meditation on the Immune System, Human Microbiota, and Epigenetics.

Jonnakuty, C., & Gragnoli, C. (2008). What do we know about serotonin? Journal of Cellular Physiology217(2), 301–306.

Li, J. M., Yu, R., Zhang, L. P., Wen, S. Y., Wang, S. J., Zhang, X. Y., Xu, Q., & Kong, L. D. (2019). Dietary fructose-induced gut dysbiosis promotes mouse hippocampal neuroinflammation: A benefit of short-chain fatty acids. Microbiome7(1), 1–14.

Luberto, C. M., Shinday, N., Song, R., Philpotts, L. L., Park, E. R., Fricchione, G. L., & Yeh, G. Y. (2018). A Systematic Review and Meta-analysis of the Effects of Meditation on Empathy, Compassion, and Prosocial Behaviors. Mindfulness9(3), 708–724.

Rieder, R., Wisniewski, P. J., Alderman, B. L., & Campbell, S. C. (2017). Microbes and mental health: A review. Brain, Behavior, and Immunity66, 9–17.

Sharon, G., Sampson, T. R., Geschwind, D. H., & Mazmanian, S. K. (2016). The Central Nervous System and the Gut Microbiome. Cell167(4), 915–932.

Spichak, S., Bastiaanssen, T. F. S., Berding, K., Vlckova, K., Clarke, G., Dinan, T. G., & Cryan, J. F. (2021). Mining microbes for mental health: Determining the role of microbial metabolic pathways in human brain health and disease. Neuroscience and Biobehavioral Reviews125(February), 698–761.

Tran, S. M.-S., & Mohajeri, M. H. (2007). The Role of Gut Bacterial Metabolites in Brain Development, Aging, and Disease. Genes and Common DiseasesFigure 1, 132–141.

Wei, W., Khine, T., Lian Voong, M., Kheng, T., Ng, S., Feng, L., Rane, G. A., Kumar, A. P., Kua, E. H., Mahendran, R., Mahendran, R., & Lee, Y.-K. (2020). Mental awareness improved mild cognitive impairment and modulated gut microbiome. Aging2020(23), 24371–24393.

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