[Probiotics Episode 2] 90% of serotonin known as the happy hormone is secreted in the intestines | Dr. Esther Lyuh

My second brain, my gut determines my mood

90% of the happy hormones come from the gut.

Surprisingly, our gut also determines our happiness. 90% of serotonin, known as the happy hormone, is secreted in the intestines.  This means that only 10% of serotonin is secreted in the brain. Serotonin is a neurotransmitter that regulates mood. A lack of serotonin causes depression. In the past, scientists thought that serotonin was secreted only in the brain. However, many studies have found that EC cells, a type of nerve cell in the intestine, synthesize large amounts of serotonin.

The gut is the second brain

Professor Michael Gershon, head of the Department of Anatomy at Columbia Medical School in the United States, published a book titled 《The Second Brain》 in 1998. The gut is the second brain. In fact, there are 100 million nerve cells in the gut. These neurons directly control digestion, including intestinal movement and secretion of enzymes. It is the Creator's consideration to ensure that the brain does not care about digestion.

What are the actual research findings?

In 2015, California Institute of Technology researchers announced in the world-renowned scientific journal Cell that serotonin secretion decreased by 60% when all bacteria were removed from the intestines of mice. Injecting the bacteria restored the level of serotonin secretion. This means that gut bacteria, including probiotics, directly affect serotonin secretion.  Serotonin secreted by the intestine is responsible for regulating intestinal movement. It is still unclear whether they act directly on the brain to improve mood and prevent depression. However, serotonin synthesized in large quantities in the intestine is absorbed by platelets in the blood and transported throughout the body through blood vessels. That means it doesn't just affect the large intestine. Recently, a lot of papers are coming out that autism or children's attention deficit disorder are related to the microbes in the gut.

Intuition comes from the gut!

There is a word 'gut feeling' in English. Translated into Korean, it means 'intuition'. It is significant that Westerners expressed intuition as the feeling of the gut. It means that the happiness of the intestines is the happiness of the brain. The intestine is never a trivial organ, but a very important organ involved in immunity and happiness.


Esther Lyuh, Doctor of Preventive Medicine, Seoul National University