Have you ever had a ‘gut feeling’ about something you were suspicious about? Or have you ever felt like there were butterflies in your stomach when you looked at someone you love? Has shocking news ever ‘punched you in the gut’? For many of us, that answer is yes.  There is a reason that so many of our emotions can be felt in the pits of our stomachs, and there is mounting scientific evidence to prove it.  Within the human body essentially lies a ‘second brain’.  This brain is not located near our actual brains, and it influences our emotions more than you might think. This second brain is our gut, and data shows that there is a very real, very connected communicative process going on between our brains and our gut microbiota. Either one of these structures can influence or be influenced by the other in similar amounts, and the bidirectional relationship between the gut and brain can explain a little bit more of the relationship between the body and mental health. And in more recent times, the costs for microbiota analyses have significantly decreased, meaning that the amount of research going into the mind-gut connection has soared to new heights.

For example, many of us may know about the ‘happy’ chemical, serotonin.  Serotonin is most commonly known as a neurotransmitter in the brain that modulates happiness, optimism, satisfaction, and a myriad of other positive emotions.  It is reduced in those with depression, and the absence of serotonin can lead to negative mental health outcomes. Because it is a neurotransmitter, one might expect this chemical to be found mostly in the brain. Take serotonin reuptake inhibitors, or SSRIs for example. This medication has been developed with the intention of increasing the amount of serotonin available to brain cells, so wouldn’t it make sense that serotonin is mostly found in the brain? Well, this is not exactly the case. The gut, specifically the plasma within the gut, is actually the home to 95% of our body’s total serotonin! Research has shown that serotonin found in the gut is able to activate nerve endings that are directly connected to the central nervous system, also known as our brain and spinal cord. This means that many of our emotions are tied directly to activity within the gut, and in this blog, we will explore the communication between these body structures and what it can mean for mental health.


Enteric Nervous System

The ‘second’ brain found in the gut is more formally known as the enteric nervous system, or ENS. The ENS is home to a network of over 100 million nerves, neurons, and neurotransmitters that track from the esophagus to the stomach, intestines, and finally, the rectum. This nervous system is dependent on the same neurotransmitters and neurons that are found in the central nervous system, and can explain why there is ‘crosstalk’ between the ENS and the CNS.  The main role of this system is to control digestion; it releases the enzymes that break down our food, controls the blood flow that mediates nutrient absorption, and directs elimination of waste from the body. While these roles may not seem obviously connected to the brain and our mental health, the proof is there. 


How do the brain and gut talk to one another?

Of course, it is one thing to know that the neurons connected to the ENS are the same ones that communicate with the CNS. But how do the brain and gut actually talk to one another? Here are a couple example of how the neurons in both structures talk:

  1. Stress-inducing situations: Have you ever been so nervous about something before that you could feel it in your stomach? You may feel nauseated, a lack of appetite, abdominal pain, diarrhea, or a churning in your stomach that cannot be satiated by anything at all. When you feel these feelings of nervousness, excitement, or stress, your brain actually communicates with your digestive system and ENS to slow down or speed up. Depending on the situation, your brain can tell your digestive system to slow down when your full attention is needed, or it can speed up depending on what your brain needs at that moment. This works bidirectionally, of course. Sometimes, our gastrointestinal system can cause anxiety and nervousness in the brain when it is going through its own set of issues.
  2. Fight or flight response: The brain may directly affect the gut through another mode of communication, and that is during the fight or flight response. When an individual goes through a stressful situation, the brain tells the digestive system to suppress itself so that the body can conserve energy to either fight or flight. The CNS does this by telling the ENS to slow the contractions of the digestive muscles and decrease digestive secretions. On the other hand, when the body is at rest, the brain tells the digestive tract that it is okay to activate itself and the digestive tract is activated again. If this stress response happens on an occasional basis, the body is able to recover quite easily and normal functioning resumes.  However, if the stress response is triggered on a frequent basis, the body cannot return to homeostasis quite as quickly. The flow of digestion can become compromised, and this may lead to a number of gastrointestinal issues such as irritable bowel syndrome. 

The psychological stresses associated with mental health issues can influence a number of physical conditions, including:

  • Bloating
  • Cramps
  • Gas
  • Heartburn
  • Gastritis
  • Indigestion
  • Inflammation
  • Nausea
  • Infections
  • Stomach pain
  • Acid reflux
  • Constipation
  • GERD


Gut Microbiome and Anxiety

Various studies have evaluated the relationship between the gut microbiota and anxiety disorders.  Two clinical trials studied the effects of how taking probiotics would affect the overall health of young, healthy adult participants. Participants were instructed to take a probiotic known as Lactobacillus gasseri, and researchers aimed to measure changes in the mental health, sleep quality, and gut microbiome of these individuals when they underwent stressful conditions. Over the course of this 4-week trial, researchers found that individuals who took this probiotic had improvements across all conditions when they underwent stress-inducing events when compared to a placebo group. Specifically, participants who took the probiotic were actually able to weaken the responses of two different stress-induced bacteria known as Bifidobacterium spp. and Streptococcus spp

Studies have also shown that the diversity of the gut microbiome differs between healthy controls and those with mental illness. For example, case studies have shown that patients who have generalized anxiety disorder have lower fecal bacterial diversity, reduced amounts of Firmicutes and Tenericutes bacteria, and decreased short chain fatty acid-producing bacteria. On the other hand, these individuals also had an overgrowth of Escherichia-Shigella, Fusobacterium, and Ruminococcus gnavus when compared to healthy controls. Those with even higher levels of anxiety were proven to have less amounts of Eubacterium coprostanoligenes, Ruminococcaceae UCG-014, and Prevotella 9. 

Additionally, research has also been done to investigate the mental health of pregnant women and how the number of adverse childhood events (ACEs) may affect the outcome of gut microbiomes of their newborn babies. Researchers found significant results. Pregnant women who experienced two or more ACEs over the course of their lifetime had elevated levels of fecal Prevotella when compared to women who had zero to one ACEs.  Studies also showed that mothers who experienced heightened levels of generalized anxiety during their pregnancy had less diverse microbiota communities in their offspring. In specific, mothers with higher anxiety levels and psychological stress had a less diverse meconium microbiota with less abundance of the Enterococcaceae bacteria in their children. This demonstrates supports the idea of mental health influencing the composition of the gut microbiota.


Probiotics and Depression

A number of studies have been conducted to study the effects of probiotics on individuals with depression. During one meta-analysis of 10 clinical trials, 1,349 patients with pre-existing depressive mood symptoms were given probiotics to take over an 8-week trial. After the trial was over, those individuals who took the probiotics had a significant mood improvement when compared to placebo controls. One of the studies also demonstrated that there was a significant decrease in the number of depressive symptoms for patients with mild to moderate depressive disorder (MDD) when they were prescribed Lactobacillus helveticus and B. longum probiotics versus patients who were receiving prebiotics.

Another study looked at the effects of probiotics on the moods of patients with depression. One study discovered that patients who were treated with the Lactobacillus plantarum 299v probiotic in conjunction with a SSRI had improved cognitive performance when compared to a group of MDD individuals who were taking a SSRI with a placebo. Additionally, those with MDD who were taking a supplemental probiotic were noted to have a 70% treatment responses and 35% remission rate when they were taking their antidepressants in conjunction with a probiotic compared to a placebo group who were only receiving the antidepressant and no probiotic.

With the body of research only growing larger, researchers have found that there is a strong correlation between the brain, mental health and the gut microbiome. Because of the interconnectedness of these bodily structures, therapies that treat one can affect both.  If you or someone you know may be struggling with GI or mental health issues, prioritizing the reduction of stress, anxiety, and physical symptoms may prove to be very beneficial for the journey to overall health.

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