The Gut-Brain Axis, Simplified

If you have ever experienced butterflies in your stomach because you were so anxious or nervous, it was likely a message… not from the great beyond, but from your gut. Over the past several years, researchers have begun piecing together an explanation for some of the gut feelings we get, and it may be the gut-brain axis in action. 

What Is The Gut-Brain Axis?

Simply put, it connects the emotional and cognitive centers of our brain with certain intestinal functions. The gut-brain axis (GBA) is like a two-way street between the central nervous system and the enteric nervous system. It carries signals from gut-microbiota to brain and from brain to gut-microbiota through neural, endocrine, immune, and humoral links. Research shows that healthy gut microbiota are integral to these interactions between brain and gut.

The gut-brain axis includes the central nervous system, both brain and spinal cord, the autonomic nervous system, the enteric nervous system, and the hypothalamic pituitary adrenal (HPA) axis.

The HPA axis is part of the limbic system, concerned with memory and emotional responses. It’s the coordinator of our adaptive responses to stressors. Depending on the stressor, cortisol may be released, a stress hormone affecting many organs, including the brain. In this way, neural and hormonal communication can allow the brain to influence intestinal microbiota.

What Does The Gut-Brain Axis Do? 

The brain affects gut health, and it looks like the gut affects certain aspects of brain health too.

Researchers can now see that the GBA not only maintains gastrointestinal homeostasis, but is likely to have effects on our higher cognitive functions, motivation, and more. The role of microbiota in anxiety and depressive-like behaviors is quite clear, and emerging research shows a link between dysbiosis and autism too. Autistic patients have a pattern of specific microbiota alterations, depending on the severity of their disease.

The enteric nervous system which is part of the gut-brain axis, is mainly responsible for controlling digestion. It releases enzymes that break down food, controls nutrient absorption, and even assists in elimination. This is important because for a long time the working theory with patients living with IBS (irritable bowel syndrome) was that anxiety and depression contributed to the issue. But researchers are now seeing evidence that gastrointestinal irritation may result in signals to the central nervous system which trigger mood changes.

“These new findings may explain why a higher-than-normal percentage of people with IBS and functional bowel problems develop depression and anxiety,” says Jay Pasricha, M.D., director of the Johns Hopkins Center for Neurogastroenterology. “That’s important, because 30 to 40 percent of the population has functional bowel problems at some point.” (3)

How Does The Gut-Brain Axis Work? 

Your brain is connected to your gut through millions of nerves, most notably the vagus nerve, which sends signals in both directions. The vagus nerve is not only one of the biggest in our body, but it’s known to have an effect on nervousness and anxiety. You can actually calm anxious thoughts by way of a longer exhale, which whispers to the vagus nerve, asking for help in chilling you out. You can read more about that here.

Your gut and brain are also connected through neurotransmitters, which are chemicals produced in the brain that help control feelings and emotions. Serotonin is a well-known feel-good neurotransmitter you might be familiar with. What’s interesting is that many neurotransmitters also happen to be produced by the trillions of microbes living in your gut. In fact, it turns out much of our friend serotonin is produced right in the gut, not the brain.

Another neurotransmitter that gut microbes manufacture is called gamma-aminobutyric acid, or GABA, which helps calm fear and anxiety. Animal studies in mice have confirmed that some probiotics can increase GABA production, thereby helping to quell anxiety and depression.

Gut microbes utilize the fiber you eat to produce short-chain fatty acids (SCFA) like butyrate, propionate and acetate. These short-chain fatty acids affect brain function in a number of ways.

One study showed that propionate can reduce how much you eat, and what you eat too.

Butyrate, and the microbes that produce it, are key in forming and maintaining the blood-brain barrier.

The Role Of Nutrition In The Gut-Brain Axis 

When you change your diet you can change your microbiome. And when that shift happens, thanks to the gut-brain axis, you can improve brain health too. Whole foods, fermented foods, and probiotics are the tools to help turn the tide.

As we know from Gut Microbiome 101, the food that we put in our bodies has a direct impact on the health of our gut microbes, because it has a direct impact on their environment. A balanced diet involving lots of fish, veggies, cereals, whole fruits, and clean, fresh water is key to a healthy outlook, happy gut flora, and a smooth-sailing gut-brain axis.

Probiotics: Yes or No?

Gut bacteria affect brain health, so it follows that changing your gut bacteria may improve your brain health. Adding probiotics to your diet is one way to get there. These are live bacteria that are consumed as a supplement. 

But, contrary to all the advertising out there, probiotics aren’t a cure-all. Certain imbalances require supplementation of the right bacteria, and there are literally a thousand different ones, a thousand possibilities. With that said, if you can land on the right one, probiotics have been shown to help with stress, anxiety and depression. 

You may have heard the term “psychobiotics”; these are probiotics that affect the brain. To some degree I think this is only so much marketing. Here’s why: Since we can’t know which probiotics will help us without trial and error, it seems funny to package and name some of them psychobiotics and others just probiotics, when nearly any probiotic has the potential to be a psychobiotic. 

Prebiotics may also affect brain health. One study found that a three week prebiotic course of galactooligosaccharides significantly reduced cortisol levels in the body. 

Some of the most important foods to support the gut-brain axis are:

  • Omega-3 fats: Found in high quantities in the human brain, and also in oily fish like salmon. Omega-3 fatty acids can increase good bacteria in the gut and reduce risk of brain disorders.
  • Fermented foods: Yogurt, sauerkraut, kefir, and kombucha all contain healthy microbes like lactic acid bacteria. Fermented foods are known to alter brain activity.
  • High-fiber foods: Whole grains, seeds and nuts, fruits and vegetables all contain prebiotic fibers for your gut bacteria to gobble up. 
  • Polyphenol-rich foods: Green tea, olive oil, and cocoa have polyphenols, which increase healthy gut bacteria and are believed to improve cognition.
  • Tryptophan-rich foods: Turkey, eggs and cheese contain tryptophan, an amino acid that is converted into serotonin. 

Why Is The Gut-Brain Axis Important? 

Incredibly, your gut has an estimated surface area of 200 meters squared, about the size of a tennis court. This understanding may help put into perspective the potential for microbial colonization and the impact it can have. It may also shine a light on why diet is so crucial to overall health. It’s a lot of mouths to feed, so to speak. Partly, the GBA is important just because of its sheer size. 

But it also affects our stress levels, and our gastrointestinal health, and can put us in an unhealthy cycle if we’re not careful. Several animal studies have confirmed that stress can inhibit signals sent through the vagus nerve, and stress also causes gastrointestinal issues.

One human study concluded that people with irritable bowel syndrome (IBS) or Crohn’s disease had reduced “vagal tone”, referring to a reduced function of their vagus nerve. 

Researchers studying mice learned that feeding them a probiotic reduced stress hormone in their blood. But interestingly, when the vagus nerve was severed, the probiotic had no effect.

This suggests that the vagus nerve is a key player in stress and the gut-brain axis.

Clear communication between the gut bacteria and the brain is necessary for stable mental health. The gut-brain axis makes this communication possible. Dysbiosis in the gut directly influences this communication, breaking it down, leading to mental health concerns like depression and anxiety.

Autoimmune And Neuroimmune Disorders And The Gut-Brain Axis

There is more and more data coming out all the time to support the role of microbiota in autism, anxiety and depressive behaviors. As we now know, autistic patients present quantifiable microbiota alterations based on the severity of their disease.

A neurodevelopmental disorder, autism is characterized by a high level of difficulty in social interaction, language impairment, and repetitive behaviors. There is a growing body of evidence suggesting that the gut–brain axis contributes to the onset of autism in some way. The way autism presents, and the gastrointestinal problems that tend to accompany it, are associated with modified gut microbiota.

The good news is that data from preclinical and clinical studies suggests a remarkable potential for new treatment options, for gastrointestinal disorders, Parkinson’s disease, autism spectrum disorders, anxiety, depression, and plenty more.

The gut-brain axis is integral to our overall health, and can be affected greatly by the actions we take to support it (or not). So the next time you have a gut feeling about something, take a moment and consider what might be triggering it. And if you’re experiencing trouble with mood or anxiety, dietary changes may be all that’s needed in getting you (and your belly bugs) back to feeling happy again.


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