 Illustration by Charlize Nishitoyo Have you ever gotten the late night cravings and found yourself regrettably stumbling around JJ’s place at 2 am? If you have, you might find solace in the fact that your yearning for greasy food isn’t completely dependent on your conscious decisions. In fact, there is actually an intricate communication network between your brain and gut biome that can impact your cognitive functions and emotions. The gut biome is composed of a plethora of microorganisms including but not limited to bacteria, viruses, fungi, and protozoa. At large, this biome is a community of microbes which can either help the body maintain homeostasis or propagate diseases [1]. Although an individual’s gut biome will remain relatively constant throughout their life, a lifestyle change can threaten this carefully balanced environment. Habitual changes, such as consuming more probiotics can positively change the overall gut ecosystem. Probiotics are bacteria found in fermented foods which multiply inside the stomach and can aid in the treatment of diarrhea, inflammation, and even diabetes [2]. Unsurprisingly, there is a lot of scientific evidence supporting the beneficial impact that vegetables can have on your guts. This is because fiber heavy foods can increase prebiotic levels, which in turn, can increase the number of “good'' bacteria. Conversely, certain drugs, such as antibiotics, can inadvertently decrease the number of gut microbes, change metabolic rates, and impact mental health.
Recent studies have found a sophisticated “bidirectional communication [system] between resident microbes of the GI tract and the brain” called the microbiota–gut–brain axis [3]. Different parts of the brain, such as the central, autonomic, and enteric nervous systems, have nerves extending into the gastrointestinal tract. Different parts of the brain, such as the central, autonomic, and enteric nervous system, have nerves extending into the gastrointestinal tract, serving as a communication between the two systems. These nerves impact the absorption of nutrients and excretion of waste by regulating contractions in the GI tract. On the other hand, gut bacteria utilize vitamins and neurotransmitters to communicate with the brain. The exact mechanism by which these molecules interact with the nervous system is unclear due to the presence of the blood brain barrier, which protects the brain from foreign substances. However, scientists hypothesize that these signals target different neurons in the neuroimmune and neuroendocrine pathways to relay their messages. Understanding the intricate relationship between gut bacteria and the nervous system raises a compelling question: how can modifying gut microbial health affect our cognitive abilities? Researchers asked this question after they noticed the correlation between declined cognitive abilities in humans and decreased gut microbial diversity. In order to address these inquiries, scientists hypothesized that increasing the population of microbes in mice could improve the mice’s psychological health [4]. Several studies were conducted by monitoring and experimenting with the cognitive abilities of germ-free mice - mice that were raised in sterile conditions, and consequently, have longer life spans. To test their hypothesis, scientists exposed these germ-free mice to various pathogens. When the pathogen, Lactobacillus, was introduced, the mice’s memory increased. Researchers found that there was an increase in the mice’s hippocampus, a small structure found in the brain that is responsible for the creation of episodic memories. In contrast, exposure of Citrobacter rodentium, a bacteria that infects the intestinal tracts of mice, increased memory deficiencies and was shown to induce anxiety 8 hours after it was introduced into the mice. The most startling discovery was when researchers transferred microbiotas from patients with major depressive disorder into germ-free mice, the mice developed symptoms of depression. Although these experiments prove that there is a definite relationship between the gut and the brain, it doesn’t adequately explain why this relationship exists. Through a parallel experiment on anti-depressants and germ-free mice, researchers discovered the role serotonin played in the development of gut bacteria. For context, serotonin is a neurotransmitter that is largely responsible for controlling emotions and happiness. Scientists found that mice raised in a germ free-environment had 50% less gut serotonin than regular mice. When these germ-free mice drank a mixture of serotonin, the gut bacterias Turicibacter and Clostridia significantly multiplied. There is also evidence that these two bacterias produced molecules that signaled the gut to increase serotonin levels. The connection between gut microbes and serotonin was confirmed when researchers gave the mice selective serotonin reuptake inhibitors, which cut down on the population of Turicibacter and Clostridia. Other studies have shown that an increased ingestion of foods containing large amounts of sugar increases serotonin release, leading to overconsumption of carbohydrate and fat rich foods. Therefore, the health and diversity of the gut microbiome is directly correlated to the brain’s production of serotonin, which in turn can increase one’s craving for junk food. But does this connection between their brains and stomach really explain why you’re craving JJ’s place? In part, the microorganisms in your gut send signals to your brain, influencing what you want to consume. However, as great as onion rings sound for a midnight snack, your gut microbiome can change rapidly in response to your diet and prolonged intake of high cholesterol food can offset the delicate balance of both your gut and neurotransmitters, leading it hard to control cravings as well as overeating. [1] M Hasan Mohajeri, Giorgio La Fata, Robert E Steinert, Peter Weber, Relationship between the gut microbiome and brain function, Nutrition Reviews, Volume 76, Issue 7, July 2018, Pages 481–496, https://doi.org/10.1093/nutrit/nuy009 [2] Leeming, E. R., Johnson, A. J., Spector, T. D., & Le Roy, C. I. (2019). Effect of Diet on the Gut Microbiota: Rethinking Intervention Duration. Nutrients, 11(12), 2862. https://doi.org/10.3390/nu11122862 [3] Carabotti, M., Scirocco, A., Maselli, M. A., & Severi, C. (2015). The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of gastroenterology, 28(2), 203–209. [4] Luczynski, P., McVey Neufeld, K. A., Oriach, C. S., Clarke, G., Dinan, T. G., & Cryan, J. F. (2016). Growing up in a Bubble: Using Germ-Free Animals to Assess the Influence of the Gut Microbiota on Brain and Behavior. The international journal of neuropsychopharmacology, 19(8), pyw020. https://doi.org/10.1093/ijnp/pyw020
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