Never Underestimate Your Gut - By Pragya Kumar
Abstract
Though the gut-brain relationship had been discovered centuries ago, the grand
importance of this axis has only recently been unveiled. This article can be considered a
comprehensive compilation of recent research regarding the gun-brain axis. This piece first
discusses how the brain and the gut are connected by explaining the details of the enteric
nervous system, the structure of the vagus nerve, and the purpose of the gut microbiome. Then it considers not only the interaction between the gut and mental disorders, which include autism, anxiety, depression, and eating disorders, but also the association between the gut and neurodegenerative diseases, which include Parkinson’s Disease and Alzheimer’s Disease. This article concludes by emphasizing that more research is needed to discover other interconnections with brain health, and how to utilize this link in a therapeutic form.
Introduction
Have you ever had a gut feeling? One that makes you fluttery, queasy, and anxious? One that has the potential to knock you right off of your feet?
That was a combination of up to 100 trillion gut microbes in your body, your brain, and
your second brain working together to produce a cohesive response.
Yes, you read that correctly. Your second brain.
Do you have any idea what it is? Well, what else could it be, other than your
underappreciated enteric nervous system? The enteric nervous system and the gut microbiome are essential to the proper function of the gut, and recently, they have been discovered to have had numerous implications on your brain and mental health. Before we discuss the specific relationship between the gut, composed of the enteric nervous system and the digestive tract’s microbiomes, and the brain, let’s describe how this connection is established first.
How Exactly is the Gut and the Brain Connected?
The enteric nervous system, also called your second brain, involves neurons in the
digestive tract. It first received its glorified name when scientists discovered that this nervous
system could operate autonomously, without direction from the central nervous system, and ever since, it has done nothing but prove its worthiness for the title. In fact, evidence points out how this system might have developed before our actual brain did (Furness and Stebbing)! The enteric nervous system has about 100 million neurons lining the walls of the stomach and the intestines; from the tip of the esophagus to the bottom of the anus (Hadhazy). These neurons allow us to somewhat feel digestion and have their own reflexes. While they are not involved in higher cognitive skills, they can just as much influence our mood and our brain’s health in a variety of different ways.
The Vagus nerve, or formally called the tenth cranial nerve, connects the digestive tract to the brain, along with heart and lung nerves. However, it is imperative to mention that up to ninety percent of this nerve’s fibers are dedicated to sending information from the gut to the brain, not the other way around (Breit et al.). It most importantly sends data to the amygdala (the emotion/fear center of the brain, the thalamus (which is a relay center for sensory information), which can impact how we sense, how we memorize, and how we feel specific stimuli.
In addition, to recite what was stated before, there are up to 100 trillion symbiotic gut
microbial cells in a person (Ursell et al.)! That’s 100 trillion cells not a true part of you. Yet, they are essential to your survival and overall health. The gut microbiota regulate digestion and metabolism, they extract vitamins and necessary nutrients from your food, and they maintain the gut wall, which protects the body from damaging invaders. In fact, they are even connected to the body’s immune system and can attack harmful, rogue pathogens. Working with the enteric nervous system, they can even produce neurochemicals to be sent to the brain, thus emphasizing how the gut can influence the brain.
Now that the basic science is out of the way, let’s specifically take a look at how the gut
can impact mood and lead to mental problems.
The Gut and Mental Disorders
Before we take a look at these disorders specifically, it is crucial that we first assess how
the gut microbiome affects mood in general.
The neurons in the enteric nervous system secrete neurotransmitters similar to the central nervous system. For example, about ninety percent of the body’s serotonin production is induced in the gut, and these neurons depend on gut microbes to produce serotonin (Stoller-Conrad). The similarity in the neurotransmitters produced can explain why therapies intended to stimulate the production neurotransmitters in the brain can affect gut health and have adverse side effects, such as nausea and diarrhea. Furthermore, the neurotransmitter serotonin itself is associated with mood; decreased levels of serotonin can influence whether or not a person suffers from depression. This provides an avenue in which gut microbiomes can impact depressive tendencies in a person, as gut microbiota is also responsible for producing anti-depressants (Evrensel and Ceylan). This can even affect stress and anxiety levels, and other mood-related disorders.
Furthermore, the lack of a substantive gut microbiome can increase anxiety-related behaviors (Crumeyrolle-Arias et al.).
Research regarding autism spectrum disorders shows matching results. Autism may cause about ninety percent of patients to not eat all of what is needed for a healthy diet. They may reject vegetables, fruits, and protein, and suffer from chronic vitamin and mineral deficiencies. This affects their overall gut microbiome, as research suggests, and may increase their risk of more severe symptoms (Mulle et al.). In addition, research regarding eating disorders (Fetissov and Hökfelt) shows even further relationships, and how antibodies produced in the gut may affect the onset of eating disorders.
Without a doubt, more research is needed to clearly define the microbiota responsible for these disorders and to develop effective therapies given this new information.
The Gut and Neurodegenerative Diseases
Even though Parkinson’s and Alzheimer’s Diseases may not involve the deficits of the
same microbiota, there is one thing clear: they both are accompanied by the dysbiosis of the gut microbiome, as were the mental disorders.
Parkinson’s Disease is the progressive neurodegenerative disorder that most commonly
results in impaired movement. This is due to the almost complete, gradual loss of the neurons in the substantia nigra that produce dopamine, a neurotransmitter involved in relaying the information of planning and controlling body movement. Though the cause of this loss is not clearly defined yet, there is consistent research blaming Lewy bodies, which is the unusual aggregation of α-synuclein proteins, for the neuronal destruction. However, what causes even these aggregations? A research study has found that patients with Parkinson’s Disease had a change in the content of 9 genera and 15 species of gut microorganisms (Petrov et al.). They even go as far as to mention that this could trigger the aggregation of Lewy bodies. Though much more research is needed to fully comprehend the extent of the “change in content” and further implications of possible future medications, it is clear that there is a correlation between changes in the gut microbiome and Parkinson’s Disease.
The relationship between Alzheimer’s Disease and the gut is very similar. This disease is
the progressive neurodegenerative disorder that most commonly results in worsening dementia. It involves the degeneration of N-methyl-d-aspartate glutamate receptors in the hippocampus. However, there are still disputes regarding what causes this issue: is it the unusual aggregation of amyloid-beta plaque proteins, or the irregular clustering of Tau proteins? There are even more hypotheses proposed, hypotheses that even combine each other. However, new research has begun to present how imbalances in the gut microbiome may contribute to the onset of this disease. In fact, the researchers go as far as to mention, “all the results suggest that AD [Alzheimer’s Disease] may begin in the gut” (Hu et al.).
Conclusion
Well, there you have it. A substantially complete view of how the gut-brain axis affects
mental health; at least, most of those, of what has been researched to date. There are implications of the gut microbiome on neuropsychiatric disorders, such as schizophrenia (Wei and Hemmings). Unsurprisingly, however, gut health also influences other health conditions. For example, research has found that diabetes can be impacted by an unhealthy microbiome. All in all, though much more research must be conducted to fully understand this axis, one of the best ways you can protect yourself from some of these disorders is to have a healthy diet. Yes, it does means you have to eat your veggies and get your daily amount of protein in! It may be a nuisance to you, but it could mean the difference between whether or not you display signs of mental illnesses. So remember: a happy gut means a happy brain!
Works Cited
Breit, Sigrid, et al. “Vagus Nerve as Modulator of the Brain–Gut Axis in Psychiatric and
Inflammatory Disorders.” PubMed Central (PMC), Frontiers in psychiatry vol. 9 44. ,
Mar. 2018, doi:10.3389/fpsyt.2018.00044.
Crumeyrolle-Arias, et. al., “Absence of the gut microbiota enhances anxiety-like behavior
and neuroendocrine response to acute stress in rats”, Psychoneuroendocrinology,
Volume 42, Pages 207-217, ISSN 0306-4530, 2014, doi: 10.1016/j.psyneuen.2014.01.014.
Evrensel, Alper, and M. E. Ceylan. “The Gut-Brain Axis: The Missing Link in Depression.”
PubMed Central (PMC), Clinical psychopharmacology and neuroscience : the official
scientific journal of the Korean College of Neuropsychopharmacology, Dec. 2015,
doi:10.9758/cpn.2015.13.3.239.
Fetissov, Sergueï, and Tomas Hökfelt. “On the Origin of Eating Disorders: Altered Signaling
between Gut Microbiota, Adaptive Immunity and the Brain Melanocortin System
Regulating Feeding Behavior - ScienceDirect.” ScienceDirect.Com | Science, Health and
Medical Journals, Full Text Articles and Books., Current Opinion in Pharmacology
Volume 48, Pages 82-91, Oct. 2019, doi:10.1016/j.coph.2019.07.004.
Furness, J. B., and M. J. Stebbing. “The First Brain: Species Comparisons and Evolutionary
Implications for the Enteric and Central Nervous Systems - PubMed.” PubMed,
Neurogastroenterology and motility : the official journal of the European
Gastrointestinal
Motility Society, 2018, doi:10.1111/nmo.13234.
Hadhazy, Adam. “Think Twice: How the Gut’s ‘Second Brain’ Influences Mood and Well-
Being - Scientific American.” Scientific American, Scientific American, 12 Feb. 2010,
https://www.scientificamerican.com/article/gut-second-brain/.
Hu, Xu, et al. “Alzheimer’s Disease and Gut Microbiota.” Science China Life Sciences 59.10,
2016, doi:1006-1023.
Mulle, Jennifer, et al. “The Gut Microbiome: A New Frontier in Autism Research.” PubMed
Central (PMC), Current psychiatry reports vol. 15,2 , Feb. 2014,
doi:10.1007/s11920-012-0337-0.
Petrov, V. A., et al. “Analysis of Gut Microbiota in Patients with Parkinson’s Disease |
SpringerLink.” Bulletin of Experimental Biology and Medicine, Bull Exp Biol Med 162,
734–737 , Apr. 2017, doi:10.1007/s10517-017-3700-7.
Stoller-Conrad, Jessica. “Microbes Help Produce Serotonin in Gut | Www.Caltech.Edu.”
California Institute of Technology, Caltech, 9 Apr. 2015,
https://www.caltech.edu/about/news/microbes-help-produce-serotonin-gut-
46495#:~:text
=Although%20serotonin%20is%20well%20known,%2C%20cardiovascular%20disease%
2C%20and%20osteoporosis.
Ursell, Luke K., et al. “Defining the Human Microbiome.” PubMed Central (PMC), Nutrition
reviews vol. 70 Suppl 1,Suppl 1 (2012), 2012, doi:10.1111/j.1753-4887.2012.00493.x.
Wei, J., and Gwynneth Hemmings. “Gene, Gut and Schizophrenia: The Meeting Point for the
Gene–Environment Interaction in Developing Schizophrenia - ScienceDirect.”
ScienceDirect.Com | Science, Health and Medical Journals, Full Text Articles and
Books., Medical Hypotheses Volume 64, Issue 3, Pages 547-552, 2005,
doi:10.1016/j.mehy.2004.08.011.