Most of us can probably guess that stress affects the body negatively – but how does it affect the gut?
The Gut-Brain Axis
Changes in our mental state, like feeling scared or nervous, can lead to problems in the gut. Ever had to do a big presentation or take an important test and experienced heartburn or diarrhea as a result? That’s the brain and the gut in communication.
This goes the other way too. Changes in our gut microbiota and changes in our intestinal permeability can affect our mental state, causing depression and anxiety. (1) These bidirectional signals going from the gut to the brain and vice versa can either keep us healthy or they can cause a great deal of discomfort.
It is vital to deal with any significant health issues that affect either the brain or the gut, but it’s important to note that neither will truly heal if you don’t also focus on the other. If you have a parasite and you treat it without dealing with your chronic stress, you leave yourself open to reinfection or sometimes, your body simply won’t be able to get rid of the parasite because of the chronic stress.
Chronic stress leads to negative changes in the gut, while relaxation promotes gut health. On the flip side, poor gut health exaggerates our stress level, while improvement in gut health lowers stress.
The fact that the gut-brain axis is a two-way street is especially important to remember. No matter how hard you try to boost the well-being of your digestive system, you’ll never fully feel better without dealing with stress; it will put you right back where you started if you don’t address it.
Chronic stress is a risk factor for digestive disease, and those with digestive conditions are also likely to suffer from mood disorders. (2,3) In a country where over 40 million people suffer from psychiatric illness and 70 million are diagnosed with a digestive disorder, addressing the gut-brain connection is essential. (3, 4)
Before we jump into the wild world of neurogastroenterology (that’s a fun one!) and learn how stress affects our gut, let’s get better acquainted with the nervous systems that make up the gut-brain axis: the central nervous system and the enteric nervous system.
The Central Nervous System (CNS)
The central nervous system consists of the spinal cord and the brain. It sends and receives signals to and from the peripheral nervous system and governs nearly everything we do with our body. Given the crucial role it plays, it is commonly referred to as the “control center” of the body.
The CNS communicates with our gut via the vagus nerve, the longest cranial nerve we have, running from near the hypothalamus all the way to our intestines where it reaches the other big player in the gut-brain axis, the enteric nervous system.
The Enteric Nervous System (ENS)
The enteric nervous system is considered to be a part of the autonomic nervous system which is housed under the peripheral nervous system. The ENS is often called the “second brain” which explains why we can sometimes “feel” our emotions in our gut. This “second brain” has many similarities to our true brain – it contains over 100 million neurons (more than the spine, though less than the brain) and produces many of the same neurotransmitters found in the brain such as serotonin, dopamine, and acetylcholine. (5)
Despite the fact that under normal circumstances the ENS is in conversation with the central nervous system, research shows that it is entirely capable of functioning all on its own, even when severed from the vagus nerve that connects it to our brain. (6) No other organ can claim this impressive feat – all require signals from the brain to function. Neat, huh?
Consider this other incredible fact: about 95% of our serotonin (the “feel good” chemical) is found in the gut, not the brain. (7) Normally associated with its anti-depressive properties, this chemical serves many different purposes and is mostly found in the enterochromaffin cells in the gut. Serotonin is released by these cells when food finds its way into the GI tract, signaling contractions to move the food down the intestinal tract.
It’s also responsible for the stomach upset we experience when we eat spoiled food – serotonin is released in high amounts when the gut comes into contact with an irritating food and triggers both diarrhea and vomiting to expel the dangerous food.
Stress: The “Fight or Flight” Reaction
Two other relevant nervous systems to consider are the parasympathetic and sympathetic nervous systems. In times of great stress our sympathetic nervous system is activated, causing the universal experience of a racing heart and rapid breathing.
You may have heard of it referred to as the “fight or flight” reaction, owing to the two courses of action our bodies are preparing us to take in these stressful situations. The parasympathetic nervous system, on the other hand, is often called our “rest and digest” system because when we are in a relaxed state the body focuses on exactly those activities.
In today’s world, we spend far too much time in the “fight or flight” mode and not nearly enough time “resting and digesting”. We’re constantly activating our “fight or flight” response with the many stressors we experience on a daily basis like traffic, a big project at work, financial issues, etc.
This is very different from how our hunter-gatherer ancestors experienced stress. While they may have been chased by predators on occasion, much of their time was spent in the “rest and digest” mode. Their ratio of “fight or flight” time to “rest and digest” time was the exact opposite of ours – and they had better digestive (and overall) health because of it.
I’m going to briefly go over what happens when we come into contact with a stressor so that you can understand the rest of this article, but if you want a more in-depth review, you’ll want to check out our free eBook, Overcoming Adrenal Fatigue, which you’ll receive when you sign up for our newsletter.
The hypothalamic-pituitary-adrenal (HPA) axis is the stress “control center” and begins the activation of the sympathetic nervous system by coordinating the many moving parts in a stressful situation. Upon recognizing a stressor, the HPA axis is stimulated and corticotropin-releasing hormone (CRH) is released from the hypothalamus. CRH travels within the blood to the anterior pituitary gland where it stimulates the production of adrenocorticotropic hormone (ACTH).
ACTH then travels to the adrenal cortex where it ultimately stimulates the release of cortisol. You may have heard of cortisol as the body’s main “stress hormone”. During times of crisis, as in our bear attack, cortisol works to keep blood sugar elevated so we can meet the glucose demands of the brain and helps the body retain sodium to keep blood pressure up. It also moves blood away from the digestive tract and instead toward the muscles and brain. This process is vital to keep us alive during a true “fight of flight” situation as it helps us do exactly that – fight or run away.
Our ancestors led lives that allowed them to have a balanced stress level. Despite running from the occasional predator, as long as they survived the attack they would then have plenty of time to rest, calm down, and turn the stress reaction off afterwards.
This enabled them to produce the right amount of cortisol – not too much, which has been associated with gaining abdominal fat and developing chronic disease, and not too little, which is correlated with exhaustion.
Unfortunately, most of us don’t live like our ancestors did. Instead, we experience chronic minor stressors all day long for our entire lives. Sadly, the HPA axis can’t differentiate much between major stressors like being attacked by a bear and minor ones like being reprimanded by our boss for being late to work.
It’s stimulated just the same either way, going through the same motions and putting us into “fight or flight” mode in either situation.
In the next few sections, we’ll review how chronic stress leads to gut issues by altering intestinal permeability, increasing inflammation and lowering immunity, changing the gut microbiota, and finally, actually increasing the amount of pain we feel.
Stress Opens the Intestinal Gates
We want to prevent our gut lining from becoming permeable – you’ve probably heard about the negative effects of “leaky gut” already. Consider the placement of the gut; from mouth to anus it’s not technically “in” our body, it’s outside. If you think of the body as a donut, the gastrointestinal tract is the donut hole, outside the rest of the donut. Pretty amazing to think that our entire digestive system is technically not even “inside” us.
When we think of the GI tract like this, we realize that it’s exposed to a lot on a daily basis. The gut barrier comes into contact with many different substances, from possibly harmful bacteria to food particles that need to be digested before being allowed into the body. It’s crucial that this barrier functions appropriately to keep the things we don’t want out and only allow the things we need in.
When working normally, the gut lining acts as a sieve, only allowing particles that fit through to get to the other side. When the sieve breaks, things that aren’t supposed to get across now flow through freely. When the gut barrier leaks, we’ve got a big problem.
Researchers have known for a long time that severe physical stress such as trauma or surgery causes the intestinal lining to become “leaky”, but more recent research has started to look at the effect of chronic psychological stress on the gut barrier. (8) To approximate chronic psychological stress in humans, rats are repeatedly subjected to water aversion stress where they are placed on a platform surrounded by water. Researchers have found that this is a mild stressor to the rats, similar to the type of chronic mild stressors we face today.
So what happens to these rats? They develop intestinal permeability that takes several days of no stress to heal. (9) Yes, several days. When’s the last time you went several days without any stress whatsoever?
This research has also shown that mast cells play a large part in the increased intestinal permeability that occurs as a result of stress. You may have heard of mast cells as the cells involved in allergic responses, as they are responsible for releasing histamine when they become unstable or “degranulate”, causing the typical allergic response – runny nose, watery eyes, congestion, etc.
What you might not know about mast cells is that they’re also found along the gut’s mucosal wall and they contain CRH receptors. Remember, CRH is released at the start of the “fight or flight” reaction. Since mast cells have CRH receptors it means that they are responsive to the amount of CRH flowing through the body. When CRH attaches to mast cells, they degranulate and release their many chemicals, including histamine.
Researchers studying rats under water aversion stress found that rats bred to have no mast cells in their intestines didn’t show increased intestinal permeability under stress, unlike their normal mast-cell containing counterparts. (8) This tells us that mast cells play a very important role in the integrity of the gut lining when it comes to stress, and that unstable and degranulated mast cells lead to intestinal permeability. By stabilizing these cells, we can help prevent the breach in our gut barrier.
These rat studies give us a glimpse into what’s going on in our gut while under chronic psychological stress, and it’s not good. Hardly any of us can go a few days without being stressed about something, which never gives our gut barrier time to heal after it’s become permeable thanks to the unstabilized mast cells. Because of this, we’re leaving ourselves open to harmful substances not meant to enter our bodies.
Stress Fuels Inflammation
Glucocorticoids are steroid hormones made by the adrenals in times of stress. We’ve already discussed the most important one in the human body: cortisol. Cortisol plays a significant role in turning off inflammatory reactions. In fact, if you suffer from an inflammatory bowel condition, you may have been prescribed a steroid like prednisone to reduce the inflammation in your gut. When prednisone enters the body it is converted by the liver to prenisolone, a derivative of cortisol, to exert its anti-inflammatory effects.
Let’s say we were running away from the bear chasing us, but we got caught. The bear bit us, but allowed us to survive. The body’s reaction to physical trauma – being bitten – is to rush blood to the wound, which swells the area, turns it red, and makes it hurt a lot. These are all signs of inflammation, which is the body healing itself.
Once inflammation’s job is done, the inflammatory reaction is shut off and cortisol helps this happen. Short-term inflammation like this is a completely normal response and under usual circumstances it helps us. It’s chronic inflammation that gets us in trouble.
One of the recent theories suggests that we can develop chronic inflammation – in the gut and elsewhere – when we’re under prolonged stress. (10) This is thought to occur because chronic stress alters the way our cells respond to cortisol. In effect, when cortisol is high for a significant period of time, our body simply becomes less sensitive to its anti-inflammatory effects.
When we are under stress for even longer and consistently activating the HPA axis, the axis can eventually become overwhelmed and stop producing the hormones that it’s supposed to (like cortisol).
Think about it like the story of the boy who cried wolf. When the body constantly cries out for help with these little stressors and activates the HPA axis over and over again, eventually the HPA axis doesn’t bother answering the cries anymore – or at least not with as much vigor as it did before.
As a result, we produce less cortisol than we’re supposed to. Low cortisol levels also lead to chronic inflammation because we don’t have the ability to fight off bacteria and other unwanted substances that make it through the now permeable gut barrier.
It’s important to remember that we want just enough cortisol: too much, and our tissues become less sensitive to its anti-inflammatory effects; not enough, and we’re open to attack from bacteria and other particles crossing the gut barrier, causing inflammation; just enough, and we’re able to keep inflammation down and respond to stress appropriately.
Are you starting to see the chain reaction developing here? The stress response turns on, our gut gets leaky, cortisol levels soar, and then eventually fall, causing inflammation either way. What’s next?
Stress Lowers Immunity
Did you know that the majority of our immune system is actually housed in the gut? Our gut-associated lymphoid tissue (GALT for short) makes up almost 70% of our immune system by weight. You might be able to imagine why the gut would need to have such a strong immune system – it’s in constant contact with things from the outside world such as food particles, bacteria, and all the other things we inadvertently swallow. That’s a lot to deal with!
As part of the gut’s immune system, our gastrointestinal system secretes something called secretory IgA (sIgA), which is our first line of defense when it comes to all the substances our gut is in contact with. This important antibody is also produced in other parts of the body that are exposed to the outside world – it’s found in saliva, tears, and lung secretions. Chronic stress reduces our production of sIgA and by doing so, leaves us open to colonization by pathogenic bacteria in the gut.
Studies show that students under academic stress have lower levels of sIgA than those under less stress. (11) Up to two weeks after exam stress has dissipated, students still show lower levels of sIgA with no indication of recovery. Relaxation exercises, on the other hand, actively increase sIgA production. (12)
Low sIgA leaves us susceptible not only to infections of the gut but also to infections in the rest of the body. With infections come even more inflammation, thus fueling the inflammatory fire already going when we’re under chronic stress. Low sIgA also gives bad bacteria the chance to take charge, changing our gut microbiota.
Stress Unbalances Gut Bacteria
The gut microbiota is absolutely vital to our digestive health (not to mention the health of our whole body!), but stress changes its composition in our gut, shifting it in a less favorable manner. Under chronic psychosocial stress, mice develop a condition called dysbiosis, the relative overabundance of bad bacteria coupled with low amounts of good bacteria in the gut. (13) This imbalance is associated with digestive problems like IBS and Crohn’s disease, and even conditions like fatty liver disease and acne.
Not only does stress alter the balance of our bacteria, but it also reduces our gut’s microbial diversity (how many different types of gut bacteria we have). Interestingly, the less diverse our gut bacteria, the more likely we are to be overweight and have allergic diseases.
Using germ-free mice, researchers have been able to prove that the intestinal microbiota also play a vital role in the development of the HPA axis (remember that’s our stress control center). Germ-free mice are often used in studies because they allow us to monitor what happens when the body exists without gut bacteria, providing us the opportunity to see exactly how the microbiota affects physiology. When these adult mice are exposed to stress, they produce higher levels of ACTH and cortisol than mice with normal gut microbiota. (14)
To further elucidate this relationship, researchers then colonized the gut of the germ-free mice with bacteria from the normal mice which partially reversed the exaggerated stress response. The effect was fully reversed when the mice were colonized with a specific strain of probiotics (good bacteria).
It is clear that stress alters our gut microbiota, and that the opposite is true too – our gut bacteria affect how we respond to stress. When our microbiota is negatively altered as a result of stress, it then sends signals back to the brain which manifest as even further stress. But wait, there’s more!
Stress Increases Pain
Did you know that most patients with IBS and other gut disorders often show enhanced perception of pain? (The fun science term for that one is visceral hypersensitivity.) While the normal response to stress is to increase the pain threshold, patients with digestive diseases unfortunately experience the exact opposite.
Think about it like this: if you were being chased by a bear and you stepped on a sharp twig, you’d probably continue running and barely even notice that you’re hurt. That’s an exaggerated example of the normal pain experience in response to stress – we can handle more of it.
However, in studies looking at the pain response to gastric distention (gas or air in the gut causing bloating), those with gut disorders have a lower pain threshold – meaning they experience more pain – than those without digestive problems. (15)
Research further shows that patients with gut conditions exhibit even more digestive symptoms like gas and pain when they’re under mental stress and feeling anxious. These symptoms decrease during periods of relaxation, further proof of the tight link between the brain and the gut. Here’s the unfortunate fact: if you’re someone with a digestive disorder, stress literally makes your stomach hurt.
Let’s go back to how the stress response starts for a moment: CRH is released from the hypothalamus, which begins the cascade of hormones eventually resulting in the production of cortisol. In rats, CRH administration causes mast cell degranulation in the colon.
Remember the mast cells? These are the cells that, when not present in the intestines of rats, resulted in the rats no longer developing intestinal permeability as a result of stress. Research has shown that when those with IBS are given a mast cell stabilizer they are less sensitive to pain. (16) This tells us that having stabilized mast cells is vital to having an intact gut barrier and having appropriate pain sensation.
It’s theorized that stress management techniques such as yoga can inhibit mast cell activation, thus having a positive effect on all types of conditions where mast cell activation is problematic. (17)
This brings us to the end of the stress train-wreck. When we’re stressed, our gut becomes permeable and inflamed, our immune system is compromised, and we’re subject to an altered balance of the bacteria living in our gut. As if this weren’t enough, being stressed out makes us feel worse by increasing the amount of pain we experience!
Overcoming A Stressful Life
I don’t want all of this to overwhelm you or make you feel like you’re destroying your gut health by being stressed out. I wanted to take you through this so that you can recognize the many negative effects chronic stress can have on the digestive system.
So what should you do if you’re always stressed out?
Laura and I have outlined some specific steps to take in our free eBook, Overcoming Adrenal Fatigue that you can get by signing up for our newsletter. You’ll learn exactly what you need to do to help your body overcome the effects of chronic stress.