It's easy to understand how gut-microbiome issues can alter our overall gut health. Similarly, if we were to cut our finger, we probably know what caused it and we probably know how to "fix" it. BUT, let's say that the cut finger somehow gets infected, and then that infection spreads throughout the body. We might not know exactly how the body will be impacted by the infection or how far it could spread. Gut health is kind of like that. It is clearly impacted by the gut-microbiome, but the real key to understanding the gut-brain axis is seeing how gut health alters our overall health, including areas of the body not directly located around the GI tract.
An unhealthy gut directly influences gut-related illnesses like irritable bowel syndrome, inflammatory bowel disease and colorectal cancer. Additionally, it is connected to skin inflammations like psoriasis, mood disorders like anxiety, depression, ADHD, and autism; and, the gut is also associated with coronary heart disease and diabetes. Plus, it has been shown to impact cognition and memory.
You might remember from the previous gut microbiome post, Gut-Microbiome Influences, that there are factors that influence the gut biome such as environment, age, diet, lifestyle and genetics. When our gut is modified by these influences (whether negatively or positively) it impacts numerous facets of our body and mind.
Dysbiosis is basically an alteration of a healthy biome. It can mean a reduction of good bacteria, an increase in bad bacteria, or an overall lack of biome diversity; and a state of dysbiosis impacts the bodies ability to maintain homeostasis.
Digestion
Am I satiated? Should I eat more?
Our bodies regulate our appetite and food intake with satiety hormones - without our conscious participation (excluding rigid "diet" behaviors like under- and over-eating). After ingestion of a meal, the presence of nutrients in the gastrointestinal tract (GIT) initiates neural and hormonal responses which inform the brain of the change in nutritional status. The hormonal responses are initiated by the bacteria in our gut. For example, When bacteria ferments carbohydrates, one result is the synthesis of short chain fatty acids (SCFA, the main source of energy for the cells lining our intestines), which specifically stimulate the hormone secretion regulating appetite and food intake.
Also, when food is broken down into it's macronutrients there is still room for the food particles to be broken down further. The bacteria in our gut help break down these otherwise indigestible particles allowing us access to even more nutrients.
Health
At this point in nutrition research it is difficult to discuss our health without making connections to the gut biome in some way.
The bacteria present in the GI tract are often found at sites enriched with immune cells. These immune cells have an important role to play as they keep a check on the homeostatic relationship between the microbiota and the host (our body). Not only do the bacteria help us access vitamins and amino acids (think protein building blocks) necessary for maintaining health, but they are integral in developing and strengthening our immune system.
The gut microbiota promote production of short chain fatty acids that regulate mucosal immune responses (SCFA do quite a lot). It is the mucosal component of our immune system that largely protects us from infection at our most susceptible sites. Mucosae (or mucous membrane) is found in the inner lining of most organs and body cavities.
The gut biome also increases production of neurotransmitters (chemical messengers in our body) such as serotonin (5-HT) and γ- aminobutyric acid (GABA). Neurotransmitter development is integral to our gut homeostasis and the hypothalamic–pituitary–adrenal (HPA) axis, which plays an important role in stress responses.
Lastly, the gut biome metabolizes xenobiotics (substances, typically chemically made) that are foreign to the body. Our gut bacteria are able to break down these types of substances so they don't hurt us.
Cognition
The gut biome is involved in practically every decision being made within our body, and this is especially true of the gut biome's influence within the nervous system. The nervous system is divided into two systems: the central and peripheral nervous systems. The CNS controls voluntary functions such as walking, laughing and reading; while the PNS is responsible for involuntary actions such as blinking, heartbeat and digestion. It is the gut bacteria that are responsible for regulating numerous CNS and PNS functions (via indirect and direct mechanisms).
Additionally, the biome is directly connected to making serotonin (neurotransmitter), which is known for its impact on cognition, reward, learning and memory.
Lastly, gut dysbiosis (gut biome alterations) can result in loss of gut barrier integrity and intestinal permeability allowing movement of the gut-bacteria into lymphoid tissues. This movement of bacteria not only leads to increased susceptibility of infection, it can result in the progression and development of various neurological diseases/disorders such as Parkinson's, Alzheimer's, multiple sclerosis and autism.
Mood
Remember how the gut biome influences the programming of the hypothalamic–pituitary– adrenal (HPA) axis, which regulates our stress responses? So much of mental health is connected to our stress responses (and our stress resilience).
Depression is a stress-related mood disorder often associated with a disrupted HPA axis. Individuals with major depression show significant changes in gut microbiota diversity, often having HIGH levels of certain pathogenic bacteria like firmicutes and actinobacteria and LOW levels of "good bacteria" like bifidobacterium and lactobacillus (both of which are largely available as purchasable probiotics).
Anxiety disorders, including generalized anxiety disorder, phobias, panic disorder, post-traumatic stress disorders and obsessive-compulsive disorder, are also associated with a disruption of the HPA axis. Changes in the microbial population in the intestine are linked to brain development and plasticity (the brain's ability to change its response in reaction to stimuli). This can alter motor functions, anxiety symptoms, and social behaviors.
Lastly, research suggests that the gut microbiome exhibits some control over GABA (γ–aminobutyric acid), dopamine and noradrenaline. GABA is a naturally occurring amino acid that works as a neurotransmitter to deliver chemical messages to our brain. [GABA is considered an inhibitory neurotransmitter because it blocks, or inhibits, certain brain signals thereby decreases activity in your nervous system.] Dopamine is another neurotransmitter and it is largely connected to how we feel pleasure. Noradrenaline, a neurotransmitter and hormone, is connected with response and fear.
Read more in the Gut-Microbiome Series here:
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Gut - Biome Series References
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9) Lee, Y., & Kim, Y.-K. (2021). Understanding the connection between the gut–brain axis and stress/anxiety disorders. Current Psychiatry Reports, 23(5). https://doi.org/10.1007/s11920-021-01235-x
10) Monda, V., Villano, I., Messina, A., Valenzano, A., Esposito, T., Moscatelli, F., Viggiano, A., Cibelli, G., Chieffi, S., Monda, M., & Messina, G. (2017). Exercise modifies the gut microbiota with positive health effects. Oxidative Medicine and Cellular Longevity, 2017, 1–8. https://doi.org/10.1155/2017/3831972
11) Rizzello, F., Spisni, E., Giovanardi, E., Imbesi, V., Salice, M., Alvisi, P., Valerii, M. C., & Gionchetti, P. (2019). Implications of the Westernized diet in the onset and progression of IBD. Nutrients, 11(5), 1033. https://doi.org/10.3390/nu11051033 [Diet Pyramids Image]
12) Sandhu, K. V., Sherwin, E., Schellekens, H., Stanton, C., Dinan, T. G., & Cryan, J. F. (2017). Feeding the microbiota-gut-brain axis: Diet, microbiome, and neuropsychiatry. Translational Research, 179, 223–244. https://doi.org/10.1016/j.trsl.2016.10.002
13) Tu, P., Chi, L., Bodnar, W., Zhang, Z., Gao, B., Bian, X., Stewart, J., Fry, R., & Lu, K. (2020). Gut microbiome toxicity: Connecting the environment and gut microbiome-associated diseases. Toxics, 8(1), 19. https://doi.org/10.3390/toxics8010019
14) Wastyk, H. C., Fragiadakis, G. K., Perelman, D., Dahan, D., Merrill, B. D., Yu, F. B., Topf, M., Gonzalez, C. G., Van Treuren, W., Han, S., Robinson, J. L., Elias, J. E., Sonnenburg, E. D., Gardner, C. D., & Sonnenburg, J. L. (2021). Gutmicrobiota-targeted diets modulate human immune status. Cell, 184(16). https://doi.org/10.1016/j.cell.2021.06.01
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