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The Gut as the Second Brain

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You are what you eat: The Gut as the “second brain”

    Anyone who has had to give a presentation in front of an audience or take an exam the next day has felt the characteristic “butterflies” in the stomach.  This nervous sensation makes the stomach feel as though it is twisted or constantly shifting, unable to sit quietly in the body.  An interesting theory that explains such phenomenon in the body is that the gut is our “second brain”.  This encompasses the idea that the gut and the brain are connected, each influencing each other to bring about our moods and feelings throughout the course of the day.  If the gut is the “second brain”, then what role does our diet play in its function? Can the foods we eat have a profound effect on our mental health or brain function?

    According to Dr. Michael D. Gershon, who coined the term “second brain”, the connection between the gut and the brain is impossible to miss due to feelings like “butterflies in the stomach”.  Gershon’s explanation for the feeling of “butterflies” is a surge of stress hormones that are produced by the body which affect the stomach as well as other organs in the body.  The brain in the gut is known as the enteric nervous system; it is responsible for the proper digestion of all the food that enters the body (1).  Since the job of a brain is essentially to control behavior, the brain in our head and the one in our gut have similar duties.  Both brains take in input or generate its own and with every input there are a set of proper responses.  The second brain deals primarily with digestion but an interesting find is that ninety-five percent of the body’s serotonin is found in the gut (1).

    Serotonin is a neurotransmitter that is crucial for controlling the feelings of well-being.  It is often the goal of several anti-depressants to raise the levels of serotonin in a patient’s body in hopes of alleviating the symptoms of depression.  Serotonin is also at the root of gut disorders such as irritable bowel syndrome (IBS) (1).  This assumption is based on tests done on IBS patients and since their GI tracts look normal, IBS is assumed to be a psychosomatic disease (1).  Normally, when serotonin is released in the gut, an intestinal reflex follows and SERT (a serotonin transporter) removes the serotonin shortly thereafter.  In IBS patients, there is not enough SERT which causes serotonin to linger in the gut and produce unnecessary intestinal reflexes.  If the majority of serotonin is housed in the gut, does this mean that what we eat can ultimately affect our moods? Would eating healthier foods allow the gut to send “good news” to the brain in the head and help that brain function better as well?

    The connection between the foods we eat and mental health has been neglected for several years due to the assumption that the foods we consume only affect our physical health.  A family friend of mine who had a son with ADHD was a believer of the connection between mental health and food consumption.  She put her son on a strict organic diet with no meat and has claimed she cured him of ADHD without the help of pharmaceuticals.  Based on a study done by the Mental Health Foundation in the UK, they believe that our change in diet over the last 50 years has a direct correlation to the rising number of patients with mental illnesses around the world (3). The diet change which our world has submitted to is one where we consume “…34% less vegetables and two-thirds less fish…” than people did 50 years ago (3).  The lack of emphasis on fresh fruits and vegetables in our diet is what the Mental Health Foundation claims to have caused an increase in illnesses such as ADHD and Alzheimer’s disease. 

    Before jumping to conclusions about whether or not the claims the Mental Health Foundation made are valid, let’s explore the research and reasoning behind their claims.  The brain is made up of neurons, neurotransmitters and several other proteins.  Neurons are made of highly unsaturated fats which are derived from the diet while neurotransmitters are made of several amino acids which are also derived from the foods we eat (2).  Since much of the brain’s structure is derived from the diet, the possibility of diet affecting mental health seems very probable. So in order to function, the brain needs complex carbohydrates, essential fatty acids, amino acids, vitamins/minerals and water (2).  When the brain gets all the nutrients from the food we eat, it can function because all its structural aspects are accounted for. 

    What happens when the brain does not acquire the correct nutrients? There are certain foods that trick the brain and others that damage it.  Foods that trick the brain are ones that make the brain release certain neurotransmitter for a short period of time in order to trick the body into feeling better (2).  Such foods include chocolate which temporarily boosts the levels of noradrenaline which increase feelings of wellbeing and enthusiasm.  By eating these foods that trick the brain, the brain is more prone to slack off periodically. The influx of artificial neurotransmitters causes the brain to stop producing neurotransmitters itself in order to maintain homeostasis.  This is what causes the body to enter a vicious cycle without these foods since the brain stops actively producing the neurotransmitters after a while making the person crave the foods that will temporarily elevate the mood. 

    While there are many factors that influence mood, such as environmental stimuli or certain events, it seems as though the gut and the food we eat could play a role as well.  Based on Gershon’s theory of the second brain, it seems plausible that the food we eat could cause an increase in the levels of serotonin in the body.  Since the second brain has to digest the food, it would be the first to know whether or not that food can be used properly to fuel the rest of the body and the brain in the head.  So then after the brain in the gut releases serotonin into the body, the brain in the head gets a “good” message from the gut if the proper foods have been eaten.  When the Mental Health foundation addresses the “brain” getting the proper nutrients, even though they’re probably not referencing the second brain in the gut, the term “brain” could be used to mean both the brain in the head and the brain in the gut.  Even if the foods we eat could control our moods, the decision to eat healthier foods is still ultimately up to the I-function.  The I-function has to be responsible for by-passing the cravings of the body for foods that only temporarily elevate moods and to make the conscious decision of eating healthier substitutes instead. 

    From a practical standpoint, it would seem that if the brain in the head got all the essential nutrients from one’s diet, it would be able to function properly.  But the brain in the gut is the one digesting the food that the brain in the head would ultimately need to use.  So the combination of the theory of the second brain and food affecting mental health could lead to new treatments for various psychological disorders.  It would be interesting to see if the combination of certain foods could release different neurotransmitters in the body, mimicking the job of pharmaceuticals in the treatment of mental illnesses.


1. Brown, Harriet. “The Other Brain Also Deals with Many Woes.” The New York Times:     August 23, 2005. 4/23/2010
2. Cornah, Deborah. “Feeding Minds” Mental Health Foundation: Jan 2006.¬feeding+minds¬     4/24/2010.
3. “Mental Health link to diet change.” BBC News: Jan 2006.