Shopping Cart

The Second Brain In Your Gut

Shannon Harvey

Some people call it the hangries. Others call it food fury. In my house it’s called the Shan Monster and my husband generally gets a fifteen-minute warning before she starts spitting venom. The only way to fend her off is to feed her. Fast.  

I know I’m not alone when I confess to being grumpy when I’m underfed and this week I’ve found myself looking into the science of how being nutritionally deprived can set off mood changes in my brain. It’s an amazing example of the mind body connection and it’s fascinating.  

If you’ve seen my feature documentary The Connection, you may recall the animated graphic which explains the brain-body connection and how scientists are starting to understand that the communication between the two is far more complex than they’d once realized. If you haven't seen it yet, then here it is.  


  One major break through came with the discovery of the enteric nervous system. But before I continue about the marvel of this discovery, lets brush up on a little biology.   You may have heard of your skeletal nervous system - your brain’s connections to your muscles. This is a conscious system that allows you to pick something up, or feel if something is hot. You may also have heard of autonomic nervous system. This is your unconscious, automatic function in your body, like your heart beating, breathing, or sweating. You may even have heard that the autonomic nervous system is generally thought to have two divisions - the sympathetic nervous system, often considered the ‘fight or flight’ system and the parasympathetic nervous system, often considered the ‘rest and digest’ or ‘feed and breed’ system.  

But a major revelation came when scientists confirmed the existence of the enteric nervous system; a third unconscious system in your body. And it’s found in your gut. They’re calling it the second brain because while it talks to the brain, it also has the ability to act independently and influence behavior.  

It’s estimated that there are between 400 and 600 million neurons in your gut – that’s up to three times as many neurons than in the brain of a rat. You’re not conscious of your gut thinking, but the system produces about 95% of the serotonin and 50% of the dopamine found in your body.   So lets look at the biology of having a bad case of the hangries. Researchers at Monash University have looked at the appetite hormone ghrelin, which is produced in the second brain in your gut and plays a role regulating eating behavior, weight gain and metabolism. It also helps with building muscle mass, reduces anxiety and enhances memory. Receptors for the hormone are found throughout the body, including in the brain in your head. Ghrelin can produce an anxiety response that goes away when you eat. I spoke with a leading researcher in the field, Professor John Furness from the University of Melbourne, who explained that when we think about it from an evolutionary perspective, the role of ghrelin makes sense. If we didn’t have such a strong response, we could easily ignore hunger signals and therefore starve. Ghrelin makes us stronger so we can go and find the food; makes us less anxious so we can face any dangers needed to hunt and gather; and the enhanced memory helps us remember where the food is.  

Research from the University of Cambridge has also identified that serotonin may further strengthen the link between your emotional response and hunger. In one study, low levels of serotonin were induced in people by changing their diet and giving them a chemical mixture that lacked the building block for serotonin. When they studied their brains they noticed that the communication between the part of the brain that sets off feelings of anger (called the amygdala) and the part of the brain that calms down the emotional response (called the prefrontal cortex), was much weaker. The more prone people were to aggression, the more sensitive they were to serotonin depletion.  

The message I’m taking away from all this is rather obvious. What we feed our stomach, also feeds our mind. And this leads me to the microbiome, which you may have noticed is quite the buzz in nutrition circles.  

The microbiome is the term being used to describe the existence of a complex community of bacteria living in your gut. You may not realize this, but it’s estimated that as many as 90% of the cells in your body are bacterial. In fact, bacterial genes outnumber your own human genes at a ratio of a hundred to one. In his TED Talk, one of the researchers at the forefront of studying the microbiome, Professor Robert Knight stated, “The three pounds of microbes that you carry around with you might be more important than every single gene you carry around in your genome.”  

So far researchers know these bacteria harvest energy from the diet, protect against infections, provide nutrition to cells in the gut and can influence our mood and behaviour.  Scientists are still trying to work out the complex interaction between bacteria and the brain, but it seems that bacteria may be able to affect the immune system and produce their own versions of neurotransmitters.  I intend to write more about the microbiome in this blog in the future because in the last few years there has been an explosion of fascinating research indicating that this gut bacteria is linked to heart disease, colon cancer, obesity and irritable bowel syndrome in humans, as well as linked to illnesses like multiple sclerosis, depression and autism in mice. But for now, let me tell you about some remarkable studies done in mice.  

In a study published in the journal Gastroenterology in 2011, researchers from McMaster University gave typically timid mice a mixture of antibiotics, which changed the composition of their gut bacteria. The researchers recorded a striking change in the mice behavior. They became bold and adventurous. In a follow up experiment, the same researchers took bacteria from the guts of the shy mice and swapped it with bacteria from the guts of mice known for their brave, exploratory behavior. They also placed the gut bacteria from the shy mice into the guts of the brave mice. The result was remarkable. The normally nervous mice became more fearless explorers, while the typically venturesome mice grew more timid and shy. The influence of the gut bacteria overcame the genetic predisposition of the mice.  

Findings like this have raised audacious questions about the importance of the second brain and how it may be harnessed to enhance our emotional and physical well being. The day where a patient’s gut bacteria is being analyzed in order to treat their chronic illness may not be far off (I’ll write about experiments being done with ‘Transpoosions’ another time) and while a host of companies have been quick to jump on the bandwagon producing prebiotics and probiotics in various forms in the name of well being, it’s important to note that the research on how gut bacteria may affect our mental and physical well being is still in its infancy.  

If you'd like to read more about the enteric nervous system, I highly recommend the book The Second Brain by Dr. Michael Gershon. He's one of the scientists who identified the second brain and the book is a blend of journey-style story telling of how his discoveries were made as well as covering the fascinating science behind the nerve cells in your gut that act like a brain. His work has lead to a new understanding of gastrointestinal problems like irritable bowel syndrome.   If you'd like to Learn more about the mind-body connection and how the health of your gut can affect your brain, you can watch my film The Connection. Check out the trailer below.  


Older Post Newer Post


I'm dedicated to high-quality health journalism and storytelling. If you value these things too, subscribe to my (mostly) weekly newsletter and be the first to receive my new blogs, podcasts, films and special events. I promise I won’t spam you or share your email address with anyone else.

Join 40,000 others.