The gut-brain axis: Is it really a thing?
The gut-brain axis is a trendy topic, that has captured the attention of researchers from all over the world. It is now widely accepted that what happens in our gut affects our mood and behavior. A large body of scientific evidence confirms that the microbiome has an influence on this gut-brain axis when it comes to health and disease.
But what exactly do we know about it? And why do some medical experts call the gut the our "second brain" ?
We often think of our brain as being responsible for our body. But it is not some kind of terrible leader that controls our body. Quite the opposite! The brain and the gut can be seen as more like colleagues instead.
This notion that the brain and our gut talk to each other is not new. In fact, it was widely accepted in ancient Greece. Philosophers such as Hippocrates, Plato, and Aristotle postulated that the brain and the rest of the body were deeply connected.
The gut-brain axis
The gut and brain interact with each other via the immune system, tryptophan metabolism, the vagus nerve, and the enteric nervous system, just as we would use our phones or social media to communicate. And the tools they use to interact are both physical and chemical: nerve cells, microbes, and microbial metabolites (signaling substances) are involved, among others.
So, as we said before, this is not a one-way communication that is controlled by the brain and gives orders to the rest of the body. Today we know that the flow of communication goes in all directions and that all parties influence each other.
So interestingly, the concept of the "second brain" is partly due to the enormous amount of nerve cells located in our gut. But it is also due to its ability to act independently of our brain and make complex decisions, with a huge impact on our overall health.
The connection between the gut and the brain
Let's take a closer look at the physical and chemical compounds.
1. The vagus nerve
The vagus nerve is a large nerve that runs from the brain to the large intestine, physically connecting them. It is the main component of the parasympathetic nervous system, the part that controls bodily functions when we are at rest, and is involved in modulating inflammation, maintaining intestinal balance, and regulating food intake, satiety, and energy balance .
Although it is a relatively new area of research, there is preliminary evidence to suggest that gut bacteria may have beneficial effects on mood and anxiety, in part by influencing the activity of the vagus nerve.
The vagus nerve fibers are not in direct contact with the gut, but they can sense signals through bacterial metabolites. For example, SCFAs (short-chain fatty acids), one of the beneficial groups produced by the gut, activate the vagus nerve. This information goes to the brain and generates a specific response.
Stress is another good example of this connection. Stress inhibits the vagus nerve by causing detrimental effects on the bacterial composition of the gut. This can lead to disorders such as IBS or IBD, which are characterized by dysbiosis and low vagus nerve activity.
Getting to know the composition of your gut bacteria and focusing on boosting the beneficial ones would be important for restoring homeostasis in the microbiota-gut-brain axis.
2. Neurotransmitters
These chemical messengers carry signals between nerve cells and target cells throughout the body. They work every day to regulate digestion and brain function, and influence a wide range of functions such as fear, mood, and pleasure.
The gut microbiome talks to the nervous system and regulates these messengers. Some types of bacteria can actually produce neurotransmitters. These include serotonin and gamma-aminobutyric acid (GABA), both of which are important for our emotional well-being.
Studies show that serotonin is involved in many processes, including digestion, respiration, behavior, and neurological function.
Although serotonin is well known as a neurotransmitter in the brain, It is estimated that 90 percent of the body's serotonin is produced in the gut . In fact, altered levels of this peripheral serotonin are associated with various diseases, such as irritable bowel syndrome, cardiovascular disease, and osteoporosis.
While serotonin is involved in gastrointestinal secretion and peristalsis, it is also used widely throughout the body. In fact, changes in serotonin levels can also send signals to the brain and affect the production of other neurotransmitters there. In the brain, serotonin is really important for controlling both our mood and sleep.
3. The gut-brain connection and the immune system
Another crucial component of the gut-brain axis is our immune system. This network of biological processes protects us from disease and harmful microbes, and is actually modulated, educated, and trained by the gut microbiome. For example, when the gut is exposed to different microbes, there is further diversification of B and T cells. These are the cells primarily responsible for immunity, working together to recognize foreign substances in our bodies.
Under optimal conditions, this immune system-microbiome alliance protects us from pathogens. However, when the gut ecosystem is imbalanced, it can become disrupted.
This phenomenon explains the dramatic increase in autoimmune and inflammatory disorders, as a result of this symbiotic relationship with the microbiota being affected.
And another crucial factor is that a large part of your immune system is actually located in your gut. This again reflects the two-way communication between the systems in our body: Just as your immune system can affect your gut health, your gut microbiome can directly affect your immune system.
Conclusion
- The two-way communication between the brain and gut is a complex system known as the “brain-gut axis.” It includes the immune system, chemical messengers called neurotransmitters, and a physical connection via the vagus nerve.
- The gut microbiome plays a significant role in this communication, regulating gastrointestinal balance, regulating the levels of hormones and neurotransmitters, and also influencing our mood and anxiety by influencing the activity of the vagus nerve.
- This vital connection is yet another reason to explore the unique composition of your gut microbiome and improve your overall health starting in your gut.
Author: Cecilia Clausen (Clinical Dietitian)
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