The Gut-Brain Connection Through the Lens of Traumatic Brain Injury

by Jonathan Chung, DC

It’s no longer a secret the composition and health of your gut has a substantial impact on the health of the brain and nervous system. Research on the role of the gut microbiome has exploded in the last 10 years with blockbuster studies showing your gut bacteria composition can affect mental health conditions like depression and neurological disorders like Parkinson’s disease.

Although chiropractic is generally associated with bad backs and tight muscles, most chiropractors have a deep-seated interest in the connection between the brain, the immune system, and the gut. While no hard studies are available on the topic, some authors are looking at this connection to see if the gut-brain axis may be a link between head injuries and neurodegenerative disease. This specific topic actually ties into all my scientific interests in one shot, so I hope you’ll get a lot from some of the extra content and diagrams I’m going to try to lay out in this article.

A Tale of Two Brains

Almost everyone is aware of the importance of the brain in your head, but a staggering number of neurons also exist in your gut. This bundle of nerves in the gut is collectively known as the enteric nervous system (ENS). An estimated 500 million neurons exist in the gut, exceeding the number of neurons in the spinal cord, making it second only to the brain in terms of neural density. This has led some scientists to affectionately call the ENS the “second brain,” so maybe making “gut” decisions might not be such a terrible thing (that’s a joke).

The number of neurons in the gut might actually be the second most interesting thing about the ENS. The most interesting thing is the ENS can actually function without talking to the brain if it needs to. The gut has its own set of interneurons and integrating centers to let it carry out reflexes and functions without the help of the brain. In normally functioning humans, the brain does talk to the gut through the vagus nerve, but the vagus nerve can be severed, and the gut will continue to work on its own power.

The gut is also a MAJOR producer of the body’s neurotransmitters, which are the chemical currency of the nervous system. The gut produces about 90% of the total serotonin in the body and about 50% of the body’s dopamine, which can have major implications in the function of the brain and mood. We’ll get into the importance of that a little later.

Shields Down: The Gut and the Brain Barrier

The brain and the gut also contain some similarities in both having physiological barriers that have been topics of high interest for neurodegenerative disease.

The gut’s barrier keeps potentially harmful substances from getting IN your bloodstream, while the brain’s barrier  keeps harmful substances in the blood OUT of the brain. The barrier in your brain is called the blood-brain barrier, and disruption of this barrier is associated with a host of neurological disorders

Intestinal permeability, also known as leaky gut, is also well supported in the literature as a driver of systemic inflammation, but has been subject to a lot of abuse from various practitioners overstating its prevalence and significance. While not everything is leaky gut, and not every leaky gut needs an intensive supplement regimen intestinal permeability is a real phenomenon with the potential to create conditions in the gut like celiac, inflammatory bowel disease, and metabolic syndrome.

Losing these barriers is like losing a layer of defense, which can make your body more prone to attack from disease causing agents, or even the cells of your own immune system.

Neuroinflammation – Collateral Damage from your Body’s Defenses

We have these barriers in our gut and our brain that help prevent harmful substances from getting into our blood and our brains. We know when these barriers get disrupted, our body is more susceptible to threats from outside the body. However, the increased permeability of these barriers may be the major driving force in threats from the INSIDE of the body.

Our immune system is made up of several classes of white blood cells and proteins patrolling the body looking for any bacteria, viruses, parasites, fungi, or other organisms that may potentially harm us. While the immune system does a remarkable job protecting us, scenarios can arise when the immune system accidentally does the body harm. This is the case in autoimmune disorders like multiple sclerosis, rheumatoid arthritis, Grave’s Disease, and Crohn’s Disease.

The presence of these autoimmune reactions can be the result of an immune system that isn’t regulated properly or has accidentally built antibodies that inadvertently attack the body’s own tissues. When you have a leaky gut, these immune cells can get primed to attack compounds that don’t normally harm the body (think gluten or food allergies). When there is a leaky blood-brain barrier, these immune reactions can occur in the brain and spinal cord — which normally tries to keep inflammation OUT. When these reactions occur in or around the brain, they can cause neuroinflammation that may gradually deteriorate brain tissue. Some authors suggest post-concussion syndrome may be a form of an inflammatory brain illness, but that hypothesis hasn’t been studied extensively yet.

Autoimmune reactions are  worth paying attention to because many neurodegenerative disorders seem to have a link to the brain being exposed to chronic neuroinflammation, and surely chronic traumatic encephalopathy would fit that bill.

Microglia: When the Brain’s Helper Cells Go Rogue

Your brain is loaded with non-neuronal helper cells called glia. Glia support the neurons in your brain by providing protection, insulation, and repair whenever needed. They take up a huge chunk of brain material and actually outnumber neurons in the brain by a factor of 10.

A special type of glia exists in the brain called microglia. Microglia are macrophages that help clean dead or unnecessary debris hanging out in the brain. They play a role in protecting the brain from infections, but they also do really cool things like prune unused synapses, or get rid of andclean dead brain cells after injury.

Like most immune cells, their default setting is turned to the off switch. You don’t want overly active immune cells, otherwise they create excessive inflammation. When infections or injuries arise, these cells become primed and active to help initiate the cleaning and repair inside the brain.

This means they start eating away at dead cells and recruiting other immune cells to create inflammation. Short-term inflammation is essential to healing, so we need these cells to generate inflammation for short periods of time while tissues heal. But sometimes, when a cell gets turned on, the off switch gets broken and it stays on, leading to chronic inflammation.

Chronic activation of microglia has been implicated in multiple neurological diseases with autism, MS, and Alzheimer’s disease chief among them.

The Vagus Nerve – The Bridge for the Gut-Brain Connection

The vagus nerve is a specialized nerve that comes off the brainstem and is connected to many of the body’s vital organs. It has a particularly important role in the gut-brain axis because it is a primary conduit for the brain in your gut to talk to the brain in your head.

This becomes really important when we consider the brain acts as a biological thermostat for multiple functions in the body, including regulation of the immune system. Changes in your gut bacteria can dictate inflammation in the brain and brain damage can influence gut permeability. Many scientists suspect the vagus nerve is a central player in these phenomenon.

How important is this bridge? Some evidence suggests the vagus nerve may be a conduit for how rogue proteins in Parkinson’s disease can spread into the brain.

Concussions: Disrupting the Barriers and Stirring the Pot of Inflammation

So now it’s time to put it all together. How does something like a concussion affect this entire system? Two recent review papers have gone into this concept with some detail, but here are the big ideas:

• Traumatic brain injury can cause dysautonomia resulting in poor functionality of the vagus nerve and poor motility of the gut.
• Animal models have shown experimentally- induced brain injury can lead to more porous gut permeability within three hours of TBI.
• TBI disrupts the blood-brain barrier.
• TBI will lead to priming of microglia and neuroinflammation. Structural signs of brain injury are correlated to the amount of microglia primed in the brain.
• A disrupted gut lining after TBI is more susceptible to rogue bacteria infiltrating the bloodstream and creating systemic inflammation. Systemic inflammation can further impact the brain’s microglia promoting more neuroinflammation long after TBI.

In a worst-case scenario, the disruption of the gut barrier and the brain barrier allow for a persistent cycle of systemic inflammation and constant activation of brain microglia.

Do we know if this happens in humans yet? Truthfully the answer is no. So far, no experiments have looked at this relationship, so it’s too early to say if this is a real phenomenon that can tie together brain injury and neurodegeneration.

So what probiotic should I take after a concussion?

The natural question after reading this is “What type of treatment do I need after a concussion? “

When we talk about guts, the usual line of thinking leads to probiotics, but that probably won’t lead us to the answers people with brain injuries really need.

Remember that big cast of characters we talked about before we addressed the topic of concussion? Here’s a refresher:

• The brain
• The “brain” in your gut (enteric nervous system)
• Intestinal barrier
• Blood-brain barrier
• Microglia
• Vagus nerve

Brain injury is a multi-faceted injury with wide effects on numerous parts of the body. No magic potion will specifically hit everything in a positive way. Here are some ways we’ve seen patients improve with problems in the gut-brain axis:

• Cervical, vestibular, ocular rehabilitation with graded exercise is becoming the gold standard in concussion recovery
• Cardiovascular exercise to improve hippocampal and global neuroplasticity
• Correction at the craniocervical junction to improve cerebrospinal fluid dynamics, decrease stress on the blood-brain barrier, and improve circulation of neuroinflammatory compounds
• Vagus nerve stimulation to improve neuroplasticity, decrease systemic inflammation, and increase gut repair
• Neurofeedback for plasticity and to improve parasympathetic tone
• Pre- and probiotics to repair gut permeability
• Ketogenic/fasting type diets to decrease neuroinflammation and alter gut biome
• Reduction of common dietary gut irritants

We could add a lot more to this list, but these are some of the most common things we see that can help some of the more challenging patient presentations.

Will these therapies stop or prevent neurodegenerative diseases? We can’t say for sure, but they all tend to improve the lives of people with early signs of neurological deterioration. Time will tell if this can impact the brain injury population as a whole.

Jonathan Chung, DC, is the founder and upper cervical chiropractor at Keystone Chiropractic and Neuroplasticity in Wellington, Florida. Learn more about their cervical vestibular rehabilitation program at www.chiropractickeystone.com