Gut-Brain Connection: Second Brain or Part of One Amazing System?

If you’re wondering if the gut-brain connection is real, then read this quote from a Harvard post. “The gut-brain connection,” the text reads, “is no joke.”

At a high level, the gut-brain axis consists of two-way messaging that is taking place between the brain and the gut. It occurs through a marvelous biochemical system that is just beginning to be understood. And, when the gut is healthy, and messaging is clear, this can have a profound positive effect on health, both mental and physical. When the microbiota isn’t optimal, disease can occur.

And, although these discoveries are relatively new, in a sense, we’ve long known about the gut-brain connection. After all, how many of these phrases have you ever used? (How many have you used just this week?)

“I have a gut feeling that . . .”
“My gut has been telling me that . . .”
“I should have listened to my gut reaction when . . .”
“When that happened, in the pit of my stomach, I could tell . . .”
“That experience was just gut-wrenching . . .”
“When I realized what was happening, my stomach got full of butterflies and . . .”

People use phrases like these to describe times when they’ve used intuition, rather than relying upon sheer logic and reasoning, and/or to share how their body is responding to a situation. More often than not, these “gut feelings” have picked up on subtleties that our thinking brain didn’t even notice.

So, it is really surprising that researchers are now talking about a “second brain,” a lesser known nervous system located in out gut? According to the Cleveland Clinic, this second brain “communicates with the brain in our head” and this interaction is now being found to play a “key role” in our “overall health.” This connection also plays a crucial part in how we feel, meaning emotionally.

And, before we delve into what this gut-brain connection means for our emotions and our health, we want to share another quote, this one a clarification from Psychology Today: “This is not a thinking brain—it does not reason, write poetry, or solve multi-linear regressions—but mounting evidence suggests that your gut’s health strongly influences your mood.”

Finally, we’ll end our intro with something to consider as you read through this post. Are there really two brains? Or is there one brilliantly integration system, with our gut previously not being given credit for the role that it—and its microorganisms—play?

A Look at the Human Nervous System

In years past, students primarily learned about our body’s central nervous system, which consists of the spinal cord and brain. The outer portion of the brain alone has, according to, between 15 to 33 million neurons (nerve cells), with each one connecting to thousands of other neurons. All told, there are approximately 100 billion neurons in just one person’s brain, with support cells (called glial cells) numbering at about 1,000 billion per person. The brain consists of four different lobes in its two sides, and the brain uses about 20 percent of the total amount of energy expended by our bodies.

Then there is the spinal cord. Thirty-one nerves connect the spine to the brain, making it an extremely powerful communication system for the body as the spine runs down a person’s back. It connects with nerves in the skin, muscles, and joints (the peripheral nervous system), and sends messages to and from the brain. These nerves also connect to every major organ in the body, with nerve cell signals being sent through chemicals called neurotransmitters.

Now, let’s take a look at the gut. There you can find the enteric nervous system that travels along the entire digestive tract, from the esophagus to the anus. Because the enteric nervous system uses the same neurotransmitters that our central nervous system does, growing numbers of medical experts are referring to this system as our “second brain.”

Makes sense, doesn’t it?

Fight or Flight—and the Gut

When something frightening happens, physically or emotionally, it can trigger a response known as fight or flight. When this response kicks in, hormones are released to help your body either stay and fight or flee to where you can be safe. This response was crucial for the safety of early humans, who needed to—as just one example—live alongside large animals with only sticks and stones as protection. This hormone influx gave them an extra rush of energy if they need to combat the animal or run up a tree.
As part of this response, the enteric nervous system slows down, or even stops, digestive processes. After all, they aren’t needed during that lifesaving burst of energy, and this allows more energy to go to parts of the body that would be needed to defend against dangers.

But, in today’s world, stress doesn’t come from wooly mammoths. Instead, it may come from struggling to understand a new tax form or needing to speak in public or realizing you’re short on money for this month’s rent. Your body, though, sees it as a threat and responds with the same fight or flight reaction it has over the millennia. Therefore, your digestive system also responds—and the result can be diarrhea, a churning gut, pain in your abdomen and more. And, problems in the gastrointestinal tract have been found to create anxiety, making these scenarios a double whammy. (More about that later!)

Gut-Brain Axis and Our Physical Health

The human gut is full of microorganisms and researchers are hard at work to determine how each influences our health, with a doctor at Harvard-affiliated Massachusetts General Hospital calling it the “new frontier of medicine, and many are looking at the gut microbiota as an additional organ system . . . It’s most important to the health of our gastrointestinal system but may have even more far-reaching effects on our well-being.”
This microbiota helps us physically in numerous ways, including:

  • Metabolizing food nutrients 
  • Protects us against intestinal infections
  • Creates blood-clotting vitamin K

Communication among the gut, its microbiota and the brain occurs in both directions through the vagus nerve. This nerve can sense information about the microorganisms in the gut and shares it with the central nervous system (largely consisting of the brain and spinal cord). This information becomes part of the body’s network and is used to create a response. Stress can inhibit the actions of the vagus nerve, which can have a negative effect on the gastrointestinal tract.

Findings to date suggest that these bacteria may be the “key to preventing or treating some diseases.” Research is being done in connection with rheumatoid arthritis, cancer, heart disease, Parkinson’s and Alzheimer diseases, autism, multiple sclerosis, amyotrophic lateral sclerosis, and the overall immune system, among other areas of study. Researchers are also exploring connections with GI conditions, ulcers, constipation and more.

Gut-Brain Axis and Our Mental Health

It has long been assumed that our emotions can have a negative impact on our gut. “Just relax,” someone might advise a friend. “You’re getting yourself upset and THAT is what’s causing your stomach to hurt.”


We now know that microorganisms in your gut can affect mood and your emotional responses to situations. And, we now know that the gut is so full of microorganisms that their number is  “10-fold greater than the number of human cells . . . 150-fold more genes than the human genome.”

And, one of the roles of some of these microorganisms is to produce serotonin, which regulates mood. They include:

  • Candida
  • Streptococcus
  • Escherichia
  • Enterococcus

Still others produce dopamine, which helps to regulate our emotional responses. They include:

  • Bacillus
  • Serratia

Stress, along with diet deficiencies and antibiotic use, can lessen the microbiota diversity. This leads to greater permeability of the intestine, which is known as leaky gut syndrome. Besides the physical problems that it can lead to, it can also play a role in depression, anxiety, schizophrenia and autism.

Numerous studies have shown links between gut health and mental health, including one cited in Clinical Psychopharmacology and Neuroscience that demonstrated connections between bacteria in the gut and depressive behaviors. This study successfully used probiotic therapy to calm rats separated from their mothers, resulting in a “relaxation in neural processes” as the rats better coped with stress. Another study showed how rats showed lower levels of anxiety after taking Lactobacillus helveticus and Bifidobacterium longum for 14 days.

review of studies examined links between mental health and gut bacteria. What was discovered? Digestive system inflammation puts stress on microbiota, which in turn releases cortisol and other stress hormones. One study included in this review focused on human patients who experienced chronic stress. Some were given probiotics; those that were reported feeling, on average, more energetic, composed, elated, clearheaded, confident, and agreeable.

Change in Thinking

A major takeaway from studies, reviews, articles and more that discuss the gut-brain connection is as follows: Yes. Stress, anxiety or depression can have an effect on someone’s gastrointestinal system. But, it’s equally as true that intestinal distress can be what’s causing that anxiety, depression or stress. Communication is two way.

Microbiomes “Catapulted onto the Scene”

That’s a phrase used by the American Psychological Association in 2012 in an in-depth article that starts this way: “If aliens were to swoop in from outer space and squeeze a human down to see what we’re made of, they would come to the conclusion that cell for cell, we’re mostly bacteria. In fact, single-celled organisms—mostly bacteria—outnumber our own cells 10 to one, and most of them make their home in the gut. The gut, in turn, has evolved a stunningly complex neural network capable of leveraging this bacterial ecosystem for the sake of both physical and psychological well-being.”

Research of this type is indicating, according to the article (titled “That Gut Feeling”) that mood and anxiety disorders in the future could be treated with beneficial microbes or drugs that mimic their functions. Chronic gastrointestinal disorders have the potential of being treated that way, too.

Challenges that researchers face when exploring the potential of gut bacteria to improve health include the following:

  • Studies (at least those focused on studying psychological well-being) have been almost exclusively been done on rats, not humans. And, while that research is promising, the impact on human beings is not yet known.
  • These studies are still in early stages and researchers don’t yet understand how the positive effects take place.
  • To use microbiota to treat disease requires a clear understanding of what a healthy gut microbiome really looks like, and scientists are still trying to understand those parameters.

One reasons this avenue of research is so promising is that the human gut is the only organ “to boast its own independent nervous system, an intricate network of 100 million neurons embedded in the gut wall. So sophisticated is this neural network that the gut continues to function even when the primary neural conduit between it and the brain, the vagus nerve, is severed.”

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About Roger & Jane Hardy

Roger and Jane created Hardy Healthy Gut to show you that every problem in life is easier to solve when your mind and body are working properly.
And you don't have to be a health nut or fitness enthusiast to enjoy a mind & body that's firing on all cylinders.

All it takes are simple, daily habits that get easier and easier with practice.

The tips we teach are the same simple daily habits that helped us feel better in our 40's than we did in our 20's.
And these same daily habits have made our 50's and 60's a successful adventure!

Now we look forward to the future and we're enjoying getting older!

Which proves that age is just a number and that number isn't what's slowing you down.
Your age isn't the real cause of those aches, pains, and brain fog.

All of this is possible for you too, my friend!

So thank you for being here. I hope you use these tips to transform your life and enjoy retirement as much as we have!

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