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Follow Your Gut! Exploring the Gut-Brain Axis

You know the term “gut instinct,” but do you know why we get help making decisions from our gut? Our gut has been called our second brain. Our brain and spinal cord compose our central nervous system, and we have another nervous system in our gut, called the enteric nervous system. There are 100 million neurons in our guts, and the system produces approximately 95% of the serotonin and 50% of the dopamine found in our bodies, as well as other neurotransmitters such as GABA, oxytocin, BDNF, and norepinephrine1. These neurotransmitters are pivotal in our mental health. Most psychiatric medications work on these neurotransmitters to treat anxiety, depression, and more. 


Why do you think when you are hungry you’re ‘hangry?’ Ghrelin is a hormone that is released when we are hungry. It tells our brain to be angry by stimulating the amygdala of our brain and makes it harder to control emotions by reducing stimulation to the prefrontal cortex of our brain2. But this causes serotonin depletion, which is associated with depression. It makes sense because if we didn’t get upset when we were hungry, we would be less likely to feed ourselves! There is a lot of research suggesting mental health and our gut reflect each other in countless other ways. For example, children who are born to mothers who experience trauma during pregnancy show different bacteria in the gut than those who had a nontraumatic pregnancy3. The gut also has a relationship with the vagus nerve. The vagus nerve is in charge of stimulating the parasympathetic nervous system which triggers digestion and relaxation. The bacteria in the gut can communicate with the vagus nerve to help reduce anxiety1.  

Butterflies or bacteria?

We’ve all had butterflies in our stomach, but what about bacteria? The microbiome is a term used to describe the complex environment of bacteria living in the gut. As mentioned above, different microbiome compositions are associated with different mental health patterns. Most of the serotonin in our bodies is actually produced by bacteria in our gut4. Some research suggests the composition of our microbiome can change the expression of our genetics and it can even overcome our genetics. While we are unable to change the environment we were exposed to during gestation, we can change the composition of our microbiomes. Some studies have even shown fecal transplants from rats exhibiting adventurous behavior to timid rats resulted in adventurous behavior in the previously shy rats, hypothesized to be due to the transfer of bacteria1. We have also seen a similar behavior pattern observed in fecal transplants in humans, where depression and anxiety were relieved transferring from a mentally healthy donor, and psychiatric illness was induced with transplantation to a mentally healthy donor5. But you don’t need to have a fecal transplant to change your microbiome.


When there is a disruption in our microbiome, it is called dysbiosis. Dysbiosis can be the result of too many ‘bad’ bacteria or not enough ‘good’ bacteria. Antibiotics can reduce the amount of bacteria in the gut, prebiotics can feed the bacteria and result in growth, and probiotics can add new bacteria to the gut1. A generally good recommendation to feed healthy bacteria in the gut can be achieved by adding fermented foods for probiotics, such as yogurt, and fiber as a prebiotic, such as that found in vegetables2. However, very specific bacteria have been associated with mental health outcomes.  

  • Supplementation with Lactobacillus species or the specific probiotic Bifidobacterium longum NCC3001 has been shown to significantly improve depressive symptoms1
  • A double-blind clinical study of Lactobacillus helveticus R0052 and B. longum R0175 resulted in reduction of stress, anxiety, and depression6
  • Lactobacillus reuteri has been shown to increase oxytocin via the vagus nerve, resulting in increased social behaviors in those with an Autism Spectrum Disorder2

These are just a few of many studies of different bacterial strains associated with mental health outcomes.

Happy gut, happy life

There is no question the gut has a large influence on mental health. It provides hope that we are able to create significant changes in our mental health and stress response by adding certain things to our diet. There is a lot more to be discovered about our second brain, but in the meanwhile, taking care of your microbiome is a great way to take care of both your physical and mental health. 

Written by Emily Artz, OMS-III, DO/MPH Candidate

You can check in on your gut health here with our quiz!


  1. Morais, L., Schreiber, H., & Mazmanian, S. (2021, April). The gut microbiota-brain axis in behavior & brain disorders. Nature. 9:241-255.
  2. Cryan, J., et al. (2019, October 1). The Microbiota-Gut-Brain Axis. Physiology Review. 99(4):1877-2013. doi:10.1152/physrev.00018.2018. PMID: 31460832.
  3. Wolkin, J. et al. (2018, June 14). Meet your second brain: The gut. Mindful. Retrieved September 14, 2021, from 
  4. Hrvatin, V. (2020, June 10). Our second brain: More than a gut feeling. Neuroscience. Retrieved September 14, 2021, from 
  5. Meyyappan, A., et al. (2020, June 15). Effect of fecal microbiota transplant on symptoms of psychiatric disorders: a systemic review. BMC Psychiatry 20, 299 (2020).
  6. Messaoudi, M., et al. (2011, March). Assessment of psychotropic-like properties of a Probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects. The British journal of nutrition, 105(5), 755–764.
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