The Undeniable Link Between the Brain and Gut

 Introduction:

Nausea, heartburn, indigestion, constipation, and stomach pain are all kinds of gastrointestinal problems we have faced before. They are easy to dismiss as merely an upset stomach, but this mentality could build up and ultimately be detrimental to mental and physical health. Improperly caring for the digestive system can lead to extensive intestinal health issues. The microbiome is a collection of all of the microorganisms that thrive in the human digestive system. It is a cohesive network of beneficial, neutral, and negative bacteria, fungi, protozoa, and viruses that help digest foods that are otherwise indigestible by our digestive tract among many other functions. Neglecting intestinal health by not nourishing the microbiome with proper nutrients, abusing medications, or excessive alcohol consumption can lead to a host of different health problems. For example, sleep disturbances, like insomnia, are a common symptom of a struggling gut. They can lead to chronic fatigue because the majority of serotonin, the neurotransmitter that is pivotal in mood and sleep, is manufactured in the gut [1]. A more acute health crisis that can develop from improper care of one’s gut is the formation of peptic ulcers in the stomach or small intestine. Peptic ulcers are caused by a breakdown of the mucus membrane in the digestive tract and result in chest and abdominal pain, weight loss, trouble breathing, and in extreme cases expulsion of blood [2]. Who would have thought that what we eat could affect the development of healthy gut flora, which then can contribute to declining mental and physical health? This article investigates topics regarding the gut’s impact on body and mind, what lifestyle choices cause dysbiosis, and how Parkinson’s Disease can develop beginning in the gut to demonstrate how central the gut is to overall wellness.

Symbiotic Gut Bacteria Elicit Anti-inflammatory Responses

Exploring the intestinal immune system is necessary for understanding how different bacteria affect overall health. For example, bacteria and other microorganisms in your gut work around the clock to ensure that you do not get sick, as maintaining the intestinal region ensures the health of their host, their environment, and themselves. This symbiotic relationship is readily visible via the adaptive immune system. Gut microorganisms act as protection from possible pathogenic invaders and aid in intestinal architecture, the structure that supports the intestinal walls, by clinging to intestinal epithelial cells [3]. A properly functioning microbiota maintains a balance of symbionts that help with intestinal regulation, commensals that serve no benefit or harm, and pathobionts which induce inflammation and harmful pathology. An error in this balancing act can cause dysbiosis, a decrease in the diversity of the commensal bacteria that can result in immunological dysregulation that increases inflammation [4]. The anti-inflammatory response prevents the formation of a host of different autoimmune diseases, such as irritable bowel syndrome, allergies, and cancer [3]. 

What Causes Dysbiosis?

While the gut microbiome is a resilient system that can fight off a lot of invaders, many different foods and lifestyle factors can lead to dysbiosis. Failing to ingest a wide variety of foods starves positive flora of the nutrients that can be used to fight off infection and protect our bodies [5]. Excess sugar has been proven to be very harmful to symbiotic gut bacteria. When sugar is overconsumed, a pro-inflammatory response is triggered, which can cause a die-off of bacteria and decrease diversity of the gut microbiome. A diet high in sugar can result in an increase in Proteobacteria, a bacterium that when elevated causes inflammation, and a decrease in Bacteroidetes, which is an abundant phylum in the gut microbiome that downregulates inflammatory responses [6]. An imaging study has shown that extended sucrose exposure activates the reward centers of the brain much like opioids do. The overconsumption of sugar creates cravings akin to an opioid addiction, which gives light to the intensity of the obesity epidemic and that it may be an addiction problem [7]. A diet change to incorporate more beneficial foods, like whole grains, vegetables, or fruits, can strengthen the gut in as little as a few days [5]. These foods allow gut bacteria to flourish because they contain prebiotic fiber, a sort of fiber that is indigestible by the human digestive system. Prebiotic foods stimulate the growth and action of bacteria. Another very harmful toxin for the gut is alcohol. Excessive alcohol consumption leads to dysbiosis and studies have shown that this differs depending on the type of alcohol consumed. Drinking hard liquor, like vodka and gin, always results in a decrease in positive bacteria. However, red wine has been proven to have the opposite effect because of its high polyphenol levels. Polyphenol is a plant-made compound that is digested by gut bacteria which leads to positive growth and a reduction in blood pressure and cholesterol. Another common factor that can easily lead to dysbiosis is antibiotic use. Antibiotics are prescribed to fight off bacterial infections like strep throat or acne, but they are unable to distinguish between the bad bacteria and the good ones and instead wipe them all out. This results in significant disruption in the delicate balance of the gut flora. The final lifestyle choice that can lead to dysbiosis is the absence of regular exercise. Exercise leads to a decrease in stress levels. This can greatly help the gut microbiome because excess stress can increase gut sensitivity and decrease blood flow [5]. Overall, it is essential to couple a well-rounded diet full of prebiotic foods or supplements, with exercise while being mindful of antibiotic usage. 

Connecting the Dots

There is a substantial link between the gut and a broad range of neurological and psychiatric disorders. The gut-brain link is regulated by microbial metabolism of short-chain fatty acids, neurotransmitters (specifically, serotonin and GABA), hormones like cortisol, and a wide variety of immune system modulators [8]. A study was recently conducted using a cohort of 1,070 individuals to sequence metagenomes of human gut microbiota to find correlations between the gut and mental health. The researchers were trying to find evidence that an increase or decrease in specific bacteria can be detrimental to a healthy mind and body. Faecalibacterium and Coprococcus bacteria are known to produce butyrate, a metabolite necessary for proper gut flora development and function by breaking down indigestible short-chain fatty acids. These bacteria were consistently associated with increased quality of life indicators that were identified through self-reported survey results. Also, low levels of Dialister and Coprococcus spp. bacteria are linked to depression, even after antidepressants were administered. Since antidepressants were not effective, it can be concluded that antidepressants do not affect gut bacteria associated with depression, but function solely as neurotransmitter modulators. Quality of life was measured using the RAND-36 health-related quality of life survey. The survey covers eight different topics: role limitations caused by emotional health problems, social functioning, emotional well-being, overall energy levels, physical functioning, physical health, body pain, and general health perception. The results of the RAND-36 survey were cross-checked with profiles derived from the participant’s gut metagenomes, and the results showed evidence that there is a link between the gut microbiome and mental health. Participants with increased levels of Faecalibacterium and Coprococcus bacteria had an increased quality of life value assigned from the survey, and those with low levels of Dialister and Coprococcus spp. bacteria had revealed a reduction in quality of life [8]. One drawback of this study is that there is not a substantial database created which can be referenced to draw more conclusions based on the connection between gut microbes and neurological disorders. With increasing focus on this topic, there will be more research conducted to solidify the findings.

Development of Parkinson’s Disease Begins in the Gut

Have you ever experienced ‘runner’s high’ or had goosebumps radiate down your body when listening to your favorite song? If so, then you know what it feels like to have a rush of dopamine release in the brain. Dopamine is a neurotransmitter that controls reward, pleasure, motor function, specific control of movement, mood, and decision making [9]. Parkinson’s is a neurodegenerative disease that progressively kills neurons associated with dopamine production and signaling. A recent study conducted at the Johns Hopkins University School of Medicine mapped the progression of Parkinson’s Disease from the gut to the brain via the vagus nerve. The vagus nerve is a cranial nerve that is responsible for supplying the heart, lungs, digestive tract, and other organs in the chest and abdomen with neuronal signals from the brain. The brain-gut axis creates a direct link between the brain and the upper digestive tract because the muscles that surround the gut are rich in connections to the vagus nerve, which travels directly through the brain stem into the cortex [10].  The researchers were able to get mice to develop early and late-stage Parkinson’s symptoms by injecting synthetic forms of alpha-synuclein, the protein that coagulates and creates toxic clumps in the brains of Parkinson’s patients, into regions of gut muscle high in vagus nerve connections. After around one month, the injection had reached the bottom of the brainstem, and after two months clumps of alpha-synuclein were recorded in the substantia nigra pars compacta, amygdala, hypothalamus, and prefrontal cortex. Within seven months, the protein had taken over harder brain regions to penetrate: the hippocampus, striatum, and olfactory bulb. Parkinson’s Disease may take decades to progress to this stage which reveals the shocking pace that it was able to progress in this rodent model. The coagulation of protein is what causes a die-off of brain cells in all of these regions. A loss of dopaminergic neurons in the substantia nigra pars compacta and striatum leads to the symptoms seen in Parkinson’s Disease [11]. These findings reveal how crucial the vagus nerve is in Parkinson’s Disease and could allow future researchers to examine ways to prevent or stop Parkinson’s at all stages of development. Theoretically, this means that Parkinson’s can be combated before someone is showing symptoms and even when it is fully developed. Future studies on this topic will look into the gut-brain axis. Specifically, these studies will investigate if removing the vagus nerve can reduce the development of Parkinson’s Disease, how to stop the misfolding and coagulation of alpha-synuclein, and how to stop its spread to the brain [11].

Conclusion

Maintaining a healthy gut and understanding the link between the gut and brain is vastly important for keeping an eye on one’s overall health. If gut health goes neglected without proper nutrients or destruction via alcohol or medications, a decrease in commensal symbionts in the gut can lead to dysbiosis. This change can develop into a wide range of bodily and psychiatric disorders that extend far past the gut. Further research into the role of the gut in Parkinson’s Disease, among many others, could shed light on possible treatments. Fortunately improving gastrointestinal health can be accomplished at home, and in as little as a few days positive changes can be noted. This can be done by incorporating diet changes, exercising more regularly, or eating a serving of probiotic yogurt a day. The gut is vastly overlooked, but incorporating this knowledge into everyday life can lead to comprehensive bodily health.

References:

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