Part 2: Your microbiome has a mind of its own | The Gut-Brain Axis, the Microbiome & Connection with Mood


Sharon Sun


July 8, 2022

Perhaps if we spiced up the adage, “you are what you eat”, we’d get something like “you and your mind are what you eat”. Sound too remarkable to be true? If there’s anything more remarkable, it’s undoubtedly how the food we eat influences our gut, the gut microbiome and subsequently their interactions with our brain. There’s also a very good reason why we call our gut the second brain! Read on to find out about the connections between the gut and the brain and how their influences our mood and mental wellbeing!

What is the Gut-Brain Axis?

If the highways that takes us from Vancouver, BC to the United States were the epitome of the essential travel routes for North American road trippers, then the same can be said for the connection with gut and the brain.

The gut and brain are closely connected via a communication network called the gut-brain axis. Just as you can take multiple paths or highways to reach your destination, this axis also has several pathways involving our immune systems, our hormones and other neurotransmitters (5,14). One of these main ‘highways’ or largest nerves connecting the gut with the brain is called the vagus nerve (more on this later). The communication along this path goes both ways, connecting the central nervous system with the enteric (gut) nervous system (which is thought of as the ‘second brain’). While communication occurs both ways, about 90% of it goes from the gut to the brain! One essential type of communication involves our stomach’s stretch receptors which detect that our stomachs are expanding during a meal. This stretch directly activates neurons going to the brain to tell us that we are full (15)!

Not only does this axis affect our digestion and appetite but the unique connection also bridges the brain’s emotion and cognition areas with the gastrointestinal tract (6). In fact, our mental health and overall wellbeing are related to what’s happening in our gut. In short, fostering a health gut is paramount for a good mind.

Did you know? The nervous system of the gut (enteric nervous system), has more nerve cells than your spinal cord!

How does the gut communicate with the brain?: The Vagus Nerve, the Microbiome and our Mood

Your gut microbes can influence the Vagus Nerve.

The vagus nerve is one of the largest nerves which connects the gut to brain. It is known to regulate the functions of internal organs while also communicating with the digestive system to send signals to the brain and impacting activities such as digestion. However, the microorganisms that live in our gut (collectively known as the microbiome) can build a home of both good and bad microorganisms which directly affects how the vagus nerve responds.

In particular, building a home of predominantly good gut bacteria (ie. those from probiotics) such as Lactobacillus and Bifidobacterium, can affect mood and anxiety as these microbes can directly stimulate the vagus nerve (8). There is modest evidence that suggests that stimulation of the vagus nerve is beneficial and can be an even be an additional line of treatment for conditions like PTSD, depression and inflammatory bowel disease (13)

Food for thought: How do we build a healthy gut with mainly good bacteria and microorganisms?

How do I know this gut-brain axis thing is real?

If the science is not enough to justify its presence, here are a few real-life interpretations of the gut-brain axis in action:

  • Using the term “Gut Feeling” to describe some form of intuition (7)
  • Feeling “butterflies in the stomach” (ie. from nervousness)
  • Having an urge to use the bathroom after feeling sudden nervousness
  • Feeling physical gut pain when stressed (7)
  • Knowing when you’re full after meal (a response of stretch receptors in the gut that are connected to nerves connected to the brain)

If you have ever found yourself in these situations, your body has been using the gut-brain axis to communicate important information!

What happens in the gut does not just stay in the gut! The power of Serotonin and our mood

If you think that what happens in the gut can’t get any more intriguing, think again! As mentioned, some of these neurotransmitters that transport signals from nerve cells to other cells are actually made in the gut themselves. In fact, 95% of serotonin, also known as one of the ‘happy hormones’ stabilizing our mood and feelings, is made in the gut (5)! It also has an essential role in activating the vagus nerve, stimulating gut muscle contractions (peristalsis) and regulating appetite and sleep (8). Through it’s ability to activate the gut nerves that communicate with other nerves closer to the brainstem, serotonin also has significant a role in treating psychiatric conditions (8). In the case of depression and anxiety for example, the body experiences a chronic over-secretion of inflammatory proteins called cytokines. These signalling molecules are made in the brain and interfere with the actions of neurotransmitters, including serotonin (13). As result, they can be a prime culprit for causing depressive symptoms by reducing our serotonin levels (9,10,11).

The Question: How can we support our gut’s serotonin levels? As gut bacteria are responsible for making hundreds of neurotransmitters that the brain uses, it is important to foster the growth of healthy gut microbiota to support their actions (one of which is through food)!

Does stress play a role here? A Case of a ‘Leaky Gut’

Absolutely! The presence of stress can bring a myriad of body and mind-related responses which may be the start of an undesired recipe of unwellness. The two-way impact of stress on our gut and vice versa, is unquestionable.

Stress can increase how accessible our gut barrier is. Under ideal conditions, our gut barrier is a protective barrier to allow only necessary metabolites into our circulation. However, chronic stress can weaken our gut barriers and create an imbalance of gut bacteria (dysbiosis), resulting in a ‘leaky gut’ that allows bacteria to enter into our body’s circulation and cause inflammation (16). At the same time, stress has a strong influence on the composition of our gut bacteria, leading to an imbalance (dysbiosis) and promoting the growth of the bad bacteria (16) which compete in the gut and lower the number of good bacteria like Lactobacillus (12). Some of these mood-related effects can present as or worsen depression and other mood disorders. If that’s not enough, even mild cases of stress can change our microbiome and metabolism; promoting food cravings and encouraging unhealthy eating habits (16).

The brief: Stress weakens our gut barrier, promotes the growth of unhealthy bacteria and can cause inflammation.

Is there a recipe for a healthy gut?

There’s no denying that our gut has a mind of its own and this can serve as a powerful influence when it comes to its daily communication with the brain. A disrupted or imbalanced microbiome can lead to inflammation and poor mental wellbeing but a well-nourished gut through a balanced diet is one way to foster a home of good bacteria in no time. In fact, a gut microbiome change can happen in as fast a day! Find out some of the best foods to include for a happy and healthy gut!


  1. Dabke, K., Hendrick, G., & Devkota, S. (2019). The gut microbiome and metabolic syndrome. The Journal of clinical investigation, 129(10), 4050-4057.
  2. Kolodziejczyk, A. A., Zheng, D., & Elinav, E. (2019). Diet–microbiota interactions and personalized nutrition. Nature Reviews Microbiology, 17(12), 742-753.
  3. Singh, R. K., Chang, H. W., Yan, D. I., Lee, K. M., Ucmak, D., Wong, K., ... & Liao, W. (2017). Influence of diet on the gut microbiome and implications for human health. Journal of translational medicine, 15(1), 1-17.
  4. Ormsbee III, H. S., & Fondacaro, J. D. (1985). Action of serotonin on the gastrointestinal tract. Proceedings of the Society for Experimental Biology and Medicine, 178(3), 333-338.
  6. Carabotti M, Scirocco A, Maselli MA, Severia C. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Ann Gastroenterol. 2015;28(2):203-209.
  7. Forsythe P, Kunze WA. Voices from within: gut microbes and the CNS. Cell Mol Life Sci. 2013;70(1):55-69.
  8. Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus nerve as modulator of the brain–gut axis in psychiatric and inflammatory disorders. Frontiers in psychiatry, 9, 44.
  9. Homan, P., Neumeister, A., Nugent, A. C., Charney, D. S., Drevets, W. C., & Hasler, G. (2015). Serotonin versus catecholamine deficiency: behavioral and neural effects of experimental depletion in remitted depression. Translational psychiatry, 5(3), e532-e532.
  10. Jeon, S. W., & Kim, Y. K. (2016). Neuroinflammation and cytokine abnormality in major depression: cause or consequence in that illness?. World journal of psychiatry, 6(3), 283.
  11. Schiepers, O. J., Wichers, M. C., & Maes, M. (2005). Cytokines and major depression. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 29(2), 201-217.
  12. Foster, J. A., Rinaman, L., & Cryan, J. F. (2017). Stress & the gut-brain axis: regulation by the microbiome. Neurobiology of stress, 7, 124-136.
  13. Mörkl, S., Wagner-Skacel, J., Lahousen, T., Lackner, S., Holasek, S. J., Bengesser, S. A., ... & Reininghaus, E. (2020). The role of nutrition and the gut-brain axis in psychiatry: a review of the literature. Neuropsychobiology, 79(1-2), 80-88.
  14. Mayer, E. A., Tillisch, K., & Gupta, A. (2015). Gut/brain axis and the microbiota. The Journal of clinical investigation, 125(3), 926-938.
  15. Wilde, P. J. (2009). Eating for life: designing foods for appetite control. Journal of diabetes science and technology, 3(2), 366-370.
  16. Madison, A., & Kiecolt-Glaser, J. K. (2019). Stress, depression, diet, and the gut microbiota: human–bacteria interactions at the core of psychoneuroimmunology and nutrition. Current opinion in behavioral sciences, 28, 105-110.