What is the Gut-Brain Axis?

In order to talk about the gut-brain axis, we have to start with a basic understanding of the gut and of the brain.

(As an aside, I believe health literacy, or understanding what's going on inside your body, is so, so important in understanding when and why things go wrong, making educated decisions on how to manage things on your own, and knowing how and when to seek treatment when it's needed. So try to stick with me if you can!)

In case it's been a minute since biology class, let's start with a brief review of the process of digestion.

When you eat, food moves through your mouth, where the process of digestion starts, down your esophagus, and into your stomach.

While there, acidic digestive juices work on breaking it down chemically, while the rhythmic motions of your stomach work on breaking it down physically. When it's ready, it moves next into the small intestine.

The small intestine is narrow, but it's incredibly long (approximately 22 feet!) and covered in microvilli, small hair-like projections on the surface of the intestinal epithelial cells that make its surface area somewhere in the neighborhood of 2700 square feet. All this surface area is vital to getting as many nutrients as possible absorbed from your food into the  blood, including broken-down carbohydrates (monosaccharides), amino acids, fatty acids, vitamins, and minerals.

What isn't absorbed in the small intestine keeps on moving and ends up in the large intestine - which is a comparatively measly foot-and-a-half in length, but much wider. In the large intestine, the body focuses on absorbing water and a few last nutrients, like electrolytes, biotin, and vitamin K. 

Most of the digested food that enters the large intestine has been reduced to fiber, water, and waste products. Fiber is actually a type of carbohydrate that our bodies cannot digest.

Fortunately, our large intestines are filled with critters that can digest that fiber - our intestinal microbiota. Our bacterial friends ferment (break down) fibers from plant foods that we eat into energy for themselves, and into short-chain fatty acids (SCFAs) for us. These short-chain fatty acids, like butyrate, proprionate, and acetate, are things that we can't produce for ourselves, but that become the main source of energy for the cells lining our large intestines (among other important functions).

By the time the continued rhythmic movements throughout your intestines have brought the waste products to the end of the line, it's been a couple of days - approximately 5 hours in the stomach, 5 hours in the small intestine, and anywhere 10-70 hours in your large intestine. 

While you are focused on chewing and swallowing, your body and brain are hard at work sending and receiving messages. Multiple hormones are released by the stomach, the small intestine, and the large intestine as the food you're eating moves through your digestive system. Other digestive organs, like the pancreas and liver, are activated, and send in enzymes to help with the process of breaking down foods into their nutrient components.

After you eat, the vagus nerve, the longest and most complex of all the cranial nerves, lights up with communication between your brain and your gut, as your digestive system reports back to the brain on the meal you are digesting.

The nutrients absorbed into the body, including vitamins, minerals, amino acids, glucose (sugar), short-chain fatty acids, etc., as well as the hormones produced in the gut, can also themselves send messages to the brain. Some nutrients are able to pass through the blood-brain barrier (the "wall" separating the brain from the rest of the body and protecting it from harmful substances) into the brain, directly impacting brain function (things like increasing or decreasing levels of different neurotransmitters, increasing or decreasing inflammation in the brain, increasing or decreasing different hormones... etc.).

Additionally, problems in the gut (such as irritation and inflammation) are communicated to the brain and can lead to some pretty dramatic shifts in mood, cognition, stress, and anxiety. 

This connection between the gut and the brain is obviously very complex - much more so than we can address in this one blog. But the constant two-way communication happening between these two important body systems is part of why the gut has begun to be called the "second brain" or the "gut brain" - technically known as the enteric nervous system. 

It's clear from the research that we have to stop thinking about mental illness as being "all in the brain." The gut-brain axis is just one example of how interconnected all of our body systems are.

Say it with me:

Mental health is HEALTH. 

Want to learn more? Reach out. I'd love to work with you. And if you're interested in future posts, be sure to sign up for updates or check back soon for more to come on strategies for using our "gut brain" to improve the health of our "main brain."

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Erica Golden, RDN, LD, IFNCP

Gut Health & Mental Health Dietitian
@nourishedmindnutrition