how the gut brain axis works

By Laura Leite, FDN, KResser Institute Provider

In this Gut Health series Part I we will look at :

- How ancient are intestinal cells compared to other human cells.

- What is Gut-Brain axis, and why the gut called second brain.

- The correlation in between gut health and psychiatric illnesses.

- Gut flora’s impact on obesity and metabolic dysfunction.

- The impact of gluten in the brain, as related to decreased memory, depression, and digestive symptoms.

- How the gut is closely linked to our immunity.

- The importance of regular elimination.

 Basics on Gut Structure and Functi

Above in pink we see the small intestine. The first stop for nutrients after they go through the stomach.

The sophisticated internal walls of the small intestine are similar to our tongue, with micro folds or microvilli, which increase the surface that absorbs nutrients. If we stretched it out, it would cover the area of an entire tennis court. in this way, nature concentrated as much contact surface for nutrient absorption in as little space as possible. 

The small intestine walls filter beneficial nutrients allowing them to flow into the blood stream.

Next, comes the area in grey: the large intestine. The colonies of bacteria in the large intestine ferment and digest nutrients that have not been digested until that point. Mostly carbohydrates. Water is then removed from the nutrients, and the large intestinal musculature eliminates feces.

How important is it?

About 75% of the immune system is located in the gut, because the lymphatic tissue is, like a web, interlaced within our intestinal walls.

When elimination is not occurring, a serious consequence can be less acidic pH in the intestinal environment. Lack of acidity can weaken the immune system and make the whole body vulnerable to pathogens and free radicals. Malfunction in the large intestine will also be caused by:

- Unbalanced flora due to antibiotics.

- A diet poor in insoluble fibers.

- Decreased quantity and variety of carbohydrates in general.

All the above contributes to decreased fermentation, leaving the environment vulnerable to the attack of harmful bacteria.

Unbalanced gut flora, or microbiota, as we explain later, has immediate effects over the brain, the immune system and hormones.

Our gut microbiota has been primarily given to us by contact with our mother during birth and breastfeeding. There is evidence that the flora acquired during childhood will determine several aspects of our health, including protection against infections, susceptibility to obesity, and psycho-emotional conditions.

Many studies closely correlate an unbalanced flora to diabetes, and inflammatory brain processes such as autism, Alzheimers, Parkinson's and auto-immune diseases.

Besides that, around 80% of our serotonin is produced in the intestine. Serotonin is the neurotransmitter important for our mood, social behavior, appetite, digestion, sleep, memory and sexual desire. 

Our Second Brain? Really?

Think of a web of interlaced threads that permeates the digestive tract, from the mouth to the anus. These threads are neurons and the web is called Enteric Nervous System. The ENS existed way before the origin of human race. Our species is about 2 million years old, while the Enteric circuits existed 500 million years ago, even within invertebrates! The ENS is way more ancient than the brain and is extremely sophisticated! The ENS communicates to the Central Nervous System through the vagus nerve, in yellow below:

vagus nerve.png

The vagus nerve, which begins at the back of our heads, controls involuntary movements of our vital organs, such as heart, lungs, liver, and intestines. Around 80% to 90% of the energy produced by the brain is sent to where the vagus nerve originates.

Recently, the Enteric Nervous System has been referred to as "second brain", due to the discovery that about 90% of the signals passing through the vagus nerve are coming from the intestine and going to the brain, and not the other way around! The sophistication and intelligence of the gut has become a new paradigm for scientists and an alert to healthcare professionals. This article 1 and this study 2 illustrate that.

How the Gut-Brain connection happens? In simple terms:

  1. All nutrients that come from the stomach are selected in the highly sophisticated gut wall, go into the blood stream.

  2. These nutrients modulate the immune system, which is within the intestinal mucosa.

  3. This process, and the filtered nutrients send signals to the brain via vagus nerve.

  4. Hormones are regulated.

  5. Nutrients intelligently filtered by the gut pass through the blood-brain barrier.

  6. The brain receives nutritious or inflammatory molecules (inflammatory cytokines) that were filtered in the gut wall.

  7. The brain also receives nervous impulses, coming from the vagus nerve, that can bring vitality, homeostasis and harmony, or stress, agitation and inflammation.

In the case of inflammation, the brain and gut interact creating a vicious cycle:

  • A low grade inflammation originating in the intestine is transmitted via inflammatory cytokines to the brain.

  • The inflammatory state is retransmitted by the vagus nerve back to the intestines, and then again projected to the brain.

This processes explain why psychiatric conditions occur simultaneously with digestive disorders and vice-versa!

Science explaining gut-brain-modern disease connection :

•  This study 3 documents that between 50 and 90% of the patients with Irritable Bowel Syndrome also suffer from psychiatric disorders.

•  This study 4 observes the efficacy of "anti-depressant" drugs in the improvement of the Irritable Bowel Syndrome.

• In this study 5 individuals were monitored for up to 12 years. It shows that psychological stress can predict the start of a gastrointestinal disorder  and vice-versa. 

• This study 6 concludes that unbalanced intestinal microbiota in children is key factor to predict if they will be obese when adults.

•    This study 7 observed that the appendix is not just an organ kept as evolutionary trace. It is a confluence of several lymphatic tissues, closely related to the immune system. The appendix offers protection against pathologic contaminants found in the faecal stream.

•    This study 8 shows how small amounts of gluten aggravate symptoms such as lack of memory, mental confusion, depression and abdominal distention. It monitored a group of people who did not have celiac disease, or allergy to gluten. They were just like most of us!

•    This study 9 elaborates on the impact of less and more acidic microbiome. It is an alert for those who consume insufficient amounts of carbohydrates, or insufficient insoluble fibers to feed the microbiome. Less fermentation, or less acidity, leaves us vulnerable to the harmful bacteria. This is relevant for the importance of having carbohydrates daily to assist in the formation of the fecal  mass, and the importance of regular elimination.

Did this article ring a bell for you? Please share your thoughts or experiences with us. We would love to have your input! 


Laura Leite is a Certified Functional Diagnostic Nutrition Practitioner and KRESSER INSTITUTE Provider, Personal Trainer and Pilates instructor. She and her partner live in Berkeley, California. Instagram

* Laura Leite published a longer version of this text in the awesome site by Flora Refosco  "What is in for today" You can visit Flora's site here.