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The Connection Between Gut, Mood and Behaviour

Amber Cohen, PsyD, CPsych

Written by: Amber Cohen, PsyD, CPsych

Updated: August 16th, 2023

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Do you ever wonder why you sometimes experience “butterflies” in your stomach when doing something exciting, or have a “gut feeling” about a particular scenario? This is because our gut microbiota, i.e. the community of microorganisms living in the gut, does much more than just aid in digestion, immunity and metabolic functioning – it is a crucial connection to our brain and potentially our mood! Yes, the brain and the gut “talk” to each other – but maybe not in the way you think.

The Gut as the Second Brain

Although the gut tends to be overlooked as a bodily organ that is simply in support of aiding the body’s digestive and metabolic processes, current research suggests that the gut is actually the human body’s “second brain”. In fact, our gut is responsible for creating and harbouring a large percentage of the body’s neurotransmitters. Up to 95% of the body’s serotonin – also known as the “happy” hormone, which is the neurotransmitter responsible for stabilizing our mood, is produced in the gut.1

The Bidirectional Relationship Between the Gut and Brain

The gut and brain communicate with each other in a constant, dynamic and two-way direction through a sophisticated signalling system. This relationship between the gut and the brain is referred to as the “gut-brain axis”. The vagus nerve is an important channel by which information is transmitted between the gut and the brain. This bidirectional communication means that the gut sends signals to, as well as receives signals from the brain. The vast majority (90%) of signals travel upward, keeping the brain constantly informed about gut activity.2

The Microbiota-Gut-Brain Axis

Scientists have known for many years that the gut and the brain are linked. For instance, we know that psychological stress negatively affects gut function and that it is an important factor in the development of gastrointestinal issues like irritable bowel syndrome. More recently, there has been a growing body of evidence indicating that the gut microbiota plays a key role in how the gut and brain communicate. This has led to the concept of “microbiota-gut-brain axis” being established.3

Scientific evidence accumulated in recent years from mice and human studies suggests that the gut microbiota affects some aspects of brain function and behaviour, including emotional behaviour.4

In animal studies, germ-free mice or those with a severely disrupted microbiota had an abnormal response to stress, differing patterns of social interaction and alterations in cognition.5

As for human studies, several have reported that patients with depression or anxiety have an altered gut microbiota.6 Evidence for the influence of gut microbes on the human brain include placebo-controlled trials suggesting that modulation of the gut microbiota may produce changes in mood or behaviour.7

While the underlying mechanisms of the crosstalk between the gut microbiota and brain remain to be fully understood, several pathways have been identified, which include the vagus nerve, the immune system, the endocrine system, and bacterial metabolites and products. Short-chain fatty acids, which are metabolites produced by the gut microbiota, possess neuroactive properties and can initiate gut-brain signalling.3

The gut microbiota also helps preserve gut permeability. Disturbances of microbiota balance, known as dysbiosis, may disrupt these pathways and trigger changes in the blood-brain barrier permeability, leading to not only gastrointestinal issues, but also, possibly neurological ones.8

How Can We Shape Our Gut Microbiota to Help Support Gut-Brain Communication? 

Until more is elucidated on the microbiota-gut-brain axis, we can strive towards healthy gut microbiota, which is important for gut and overall health.

In conclusion, while there is evidence that gut microbiota may influence our mood and behaviour, more research is needed to better understand how this happens. But one thing is for sure, our brain and gut are in constant communication, so we can all benefit from healthy lifestyle strategies to support our gut microbiota.

infographic explaining the connection between the gut, mood and behaviour


  1. Kim DY and Camilleri M. Serotonin: A mediator of the brain-gut connection. Am J Gastroenterol 2000;95(10):2698-709.
  2. Gut Microbiota Research & Practice. Gut microbiota & gut-brain axis: A selection of content from the Gut Microbiota for Health 2016. January 2017.
  3. Cryan JF et al. The microbiota-gut-brain axis. Physiol Rev 2019;99(4):1877-2013.
  4. Mohajeri MH et al. Relationship between the gut microbiome and brain function. Nutr Rev 2018;76(7):481-496.
  5. Luczynski P et al. Growing up in a bubble: Using germ-free animals to assess the influence of the gut microbiota on brain and behavior. Int J Neuropsychopharmacol 2016;19(8):pyw020.
  6. Bastiaanssen TFS et al. Gutted! Unraveling the role of the microbiome in major depressive disorder. Harv Rev Psychiatry 2020;28(1):26-39.
  7. Dinan TG and Cryan JF. Gut-brain axis in 2016: Brain-gut-microbiota axis – mood, metabolism and behaviour. Nat Rev Gastroenterol Hepatol 2017;14(2):69-70.
  8. Sasso JM et al. Gut microbiome-brain alliance: A landscape view into mental and gastrointestinal health and disorders. ACS Chem Neurosci 2023;14(10):1717-1763.
  9. Gut Microbiota For Health. How to eat for a diverse microbiota. 2020.
  10. Smith RP et al. Gut microbiome diversity is associated with sleep physiology in humans. PLoS One 2019;14(10):e0222394.
  11. Canadian Society for Exercise Physiology. Canadian 24-Hour Movement Guidelines. 2021.

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