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The Second Brain: The Gut and Serotonin

Posted By Administration, Thursday, May 6, 2010
Updated: Friday, April 18, 2014



by Zina Kroner, DO

This is not the “shock and awe” study of the year, but it elucidates an excellent medical phenomenon.  A recent study in the American Journal of Gastroenterology showed that St. John’s Wort, an herb used to treat mild depression, was less effective than placebo at treating irritable bowel syndrome (IBS).  Improvements in quality of life were similar in both placebo and the herb treated groups.  Gastroenterologists have been treating IBS with anti-depressants for quite some time.   I would like to bring to light the theory behind it, as this may have clinical implications beyond the use of medication.

The gut, often referred to as the second brain due to the powerful enteric nervous system that it houses, is tightly connected to serotonin, the feel good neurotransmitter.   Dr. Michael D. Gershon, the chairman of the department of anatomy and cell biology at Columbia has brought this to light in his book entitled “The Second Brain.” 

We have all felt a twinge in the stomach prior to a major exam or a public speaking event.  It is quite common for my patients with a psychiatric disorder to have a concomitant gastrointestinal issue. 

There are several reasons why stress or anxiety can cause irritable bowel syndrome.  First, with any fight or flight response, cortisol, a stress hormone, is released. Cortisol fires up the sympathetic nervous system and makes the parasympathetic nervous system less efficient.  It is the parasympathetic nervous system that we need in order to maintain bodily homeostasis, such as breathing, digestion,etc.  Therefore, with a cortisol surge, digestion becomes ineffective and irritable bowel syndrome can kick in.

Second, it is important to note that serotonin has a profound effect on gastrointestinal function, being that 95% of the body’s serotonin is cradled in the gut.  At the start of digestion, it is the enterochromaffin cells that release serotonin into the gastrointestinal tract, which houses many serotonin receptors.  The receptors then initiate a process via nerve cells that starts the flow of digestive enzymes. 

Serotonin then relays messages up to the brain, letting it know what is happening.  Therefore, certain foods may elicit a feeling of nausea, etc.  Once serotonin is released in the gastrointestinal tract and the process of digestion is stimulated, normally, it is cleared out of the way by SERT, a serotonin transporter. 

These transporters are found in the gut walls.   Often, those with IBS, may not have an appropriate level of functioning SERTs, and they are therefore unable to clear out the serotonin cells.  This can stimulate diarrhea. Once the serotonin receptors are supersaturated, the effect is constipation, thus the infamous Irritable Bowel Syndrome.  Therefore, medications as well as supplements such as St. Johns Wort which manipulate the serotonin may potentially help IBS symptoms. It is no wonder that in this recent study, placebo was quite beneficial in IBS.  This shows how much mind and body are connected.  Focusing on stress management is key as well. 

A third contributor to IBS as it related to the enteric nervous system is allergens.  Often, the barrier of the gut becomes damaged and certain allergens, etc, may enter the bloodstream, triggering the brain to send a message to the gut to increase the production of histamines and other inflammatory cells in order to try to get rid of the allergens.  This inflammatory process may trigger the neurons in the enteric nervous system (in the gut) to become hyperactive and therefore contribute to diarrhea. 

The challenge now is to optimize the efficiency of the gastrointestinal tract by preventing unnecessary cortisol surges so not to disrupt the sympathetic and parasympathetic nervous system harmony, to maintain a healthy serotonin and SERT level, and to prevent unnecessary allergens from entering the GI tract so not to trigger the inflammatory process involved in diarrhea.  The gut really is the Second Brain! 

Citation:  Saito YA et al. A randomized, double-blind, placebo-controlled trial of St John's wort for treating irritable bowel syndrome. Am J Gastroenterol 2010 Jan; 105:170.

Tags:  gut health  inflammatory bowel disease 

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