It is not rare to have the feeling of butterflies in our stomach when we are nervous. A time of high stress also leads to stomach ache sometimes. The link between the gut and the brain has always fascinated scientists. The gut is the largest collection point of nerves after our brain and is often referred to as the second brain.

There have been various studies that have proved that our brain activity can affect the functioning of our gut and vice versa. Serotonin, also known as the happy hormone is one of the keystones in the puzzle of depression. There is a very close connection between our digestive system, our cognition (thinking abilities) and serotonin.

There have been studies that looked into how the collection of various bacteria in our gut can have an effect on the mental well being of people as well.

Serotonin and Digestive System

95% of the serotonin in our body is found in the Gastro-intestinal tract. One would think that the hormone so essential to our mental well-being would be mostly found in the brain but one would be wrong. Our brain has only 5% of the serotonin in our whole body. The rest lies in the cells and neural connection of the digestive tract.

It also plays a role in the blood clotting mechanism and is found in our platelets. This could explain why self harm causes a temporary relief. The platelets in our blood are broken and release serotonin and other blood clotting factors.

Gut Microbiota and Depression

The biome of microbes found in our digestive tract is known as the gut microbiota. It is the group of various micro-organisms that stay in our intestines. They help us digest various substances that our body cannot digest naturally.

It has been observed that the type of gut microbes you gain while in a foetal stage may make you more or less susceptible to neuropsychiatric disorders like schizophrenia or major depression. It was tested on mice and found that a particular bacterium, if infused in the gut of a mouse, can have an anti-depressant effect on the mouse over a period of 4 weeks.


The research into the gut biome and its effect on our brain development and depression is a relatively new field of research. The review I am quoting was published in 2015 which gives an idea on how recent all these findings are. Existence of serotonin in our gut in such a high volume gives more hints about the link between out digestive system and our mental health. The field promises to have a bright future. Slowly, we learn more about the communication between our first and second brains and how it can change how we look at life.


Cryan, J. F., & O’mahony, S. M. (2011). The microbiome‐gut‐brain axis: from bowel to behavior. Neurogastroenterology & Motility23(3), 187-192.

Kim, D. Y., & Camilleri, M. (2000). Serotonin: a mediator of the brain–gut connection. The American journal of gastroenterology95(10), 2698.


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