The term 'stress' is broadly defined as a reaction to a stimulus or threat that is either real or perceived. Stress is useful in small doses. It stimulates reactions to avoid danger, motivates us to work towards a goal or strive for success. However, when stress becomes chronic and persistent, our physical and mental well-being suffers. Hippocrates, regarded as the founder of medicine, was once quoted for saying ‘all disease begins in the gut.’ Our gut does more than send us on a trip to the bathroom everyday. The gut is largely responsible for the critical functions of the body’s digestive and immune system, and has been directly linked to our behaviour, mood and mental health. The health of our gut is compromised when the body is under chronic stress, wreaking havoc on its ability to function properly. If you suffer from a gut related issues (e.g. irritable bowel syndrome), you’ve probably tried every ‘diet’ under the sun (gluten free, dairy free, the low fodmap diet). But, it may surprise you to learn that chronic stress could be the real cause of your irritated gut.Our 'Second Brain' We have all experienced the phenomenon of the ‘gut feeling’. We get ‘butterflies’ in our stomach when we are nervous, we see something ‘gut-wrenching’ or we ‘go with our gut’ when faced with a difficult decision. It turns out that the ‘gut feeling’ is a real physical phenomenon, not folklore. To understand why our tummy serves as a repository for feelings we commonly associate with the brain, we need to understand how the gut and the brain are intrinsically linked. The gut possesses an unimaginable amount of nerves, so much so that it’s been coined the ‘gut brain’. These nerves help us to ‘feel’ our inner world (together they are referred to as the Enteric Nervous System or 'ENS') and through a series of complex neural pathways relay the information they receive back up to the brain (the Central Nervous System or 'CNS'). The channel of communication between the CNS and ENS is referred to as the ‘Brain-Gut Axis’. Interestingly, the dialogue between these two systems goes both ways - our gut sends signals to the brain and the brain sends signals to the gut. This explains why you stop eating when you are full (the stomach becomes distended and communicates this to the brain), and why your stomach is the repository for all sorts of feelings when you are nervous or overcome with emotion. The Fight or Flight Response Stress is thought to be among the most important stimuli discussed by the brain and the gut. When we come into contact with a stressor, the hypothalamic-pituitary-adrenal (HPA) axis (imagine this as our internal ‘stress control’ center) instigates the production of a chemical called corticotropin-releasing factor (CRF). CRF triggers a cascade of other chemicals which eventually cause the release of cortisol (commonly known as the stress hormone). Cortisol diverts energy away from the gut to our muscles and brains, works to keep blood sugar elevated (to meet glucose demands of the brain) and retention of sodium (to keep blood sugar up), all of which allow us to respond quickly and efficiently to danger. The fight or flight mechanism works best as a temporary response to help with survival. Our ancestors suffered from ‘acute stress’ if they found themselves, for example, fleeing from a sabretooth tiger #cavemanproblems. However, our bodies were not designed to handle long term, chronic stress. In this day and age, our lives are no longer filled with occasional stressors that come and go for short, interspersed periods of time. Rather, with looming deadlines at work, financial burdens, social pressure and the constant inundation of information, it’s safe to say that for the majority of the Western World we are stressed for the most part of a day, most days. In other words, we are in ‘fight or flight’ mode more often than not. As energy is diverted away from the gut during the fight-or-flight response, digestion and immune function are slowed or halted for long periods of time, which can rob us of key nutrients and expose the gut to infection and inflammation.The remainder of this article seeks to provide a broad overview of the physiological changes that occur in our body when we are suffering from chronic stress (or perpetual fight or flight mode), and the impact of chronic stress on the composition of gut bacteria.Physiological Changes: INCREASED GUT PERMEABILITYJust as your skin forms a barrier between your inner body and nasties from the outside world, your gut prevents any nasties entering your body beyond the gut wall. It is the gatekeeper, the nightclub bouncer, the Gandalf of your internal self (You shall not pass!).When we are exposed to chronic stress the lining of the intestine becomes damaged and porous (visualize an umbrella with holes in it). This loss of integrity of the gut lining allows large, undigested food molecules, toxins and waste (you could collectively think of these as drunk gatecrashers) to flow freely into your bloodstream through the porous-like holes in the gut’s lining. The process of increased permeability is often referred to as ‘leaky gut’.Some research suggests that mast cells are to blame for gut permeability in times of stress. Mast cells, found in the gut’s mucosal wall, contain receptors and are responsive to the amount of CRF flowing through the body (remember, CRF is released by the brain during times of stress). Researchers studying rats under water aversion stress found that rats with no mast cells in their intestines didn’t show increased intestinal permeability under stress, whereas rats with mast cells did. What this tells us is that unstable and degranulated mast cells may lead to intestinal permeability. Increased permeability causes an exaggerated immune response, chronic inflammation, and messes with the composition of bacteria residing in the gut. LOWER IMMUNITYThe tissues in and surrounding the gut house about 70% of our body's ENTIRE immune system. This sounds impressive, but it makes perfect sense - on any given day, our gut is exposed to not only the food we eat but foreign pathogens like bacteria, food proteins, parasites, fungi, toxins and viruses which we inadvertently consume. Should any of these nasties make their way through the gut barrier, our gut-associated immune system is triggered and attacks any foreign substance it does not recognize. An inundated immune system causes an exaggerated and prolonged immune response, and this can lead to a myriad of problems such as the development of food sensitivities, systemic inflammatory disorders, irritable bowel syndrome, and autoimmune disease.One theory is that our immune system falters under chronic stress because stress suppresses the production of an antibody called secretory IgA (sIgA). The role of sIgA antibodies, simply put, is to attach themselves to invading nasties, trap them in mucus and stop them from going somewhere they shouldn't. These clever little antibodies then neutralize any damaging toxins given off and help ensure the invaders are shown the door via feces. As we produce less and less sIgA our gut becomes a playground for inflammation and harmful bacteria. CHRONIC INFLAMMATIONInflammation is, generally speaking, the body’s immune response to a stimulus. When foreign substances permeate the gut lining the gut’s immune system is triggered and an inflammatory response is turned on. Cortisol (remember, this is the stress hormone), plays a significant role in turning off inflammatory reactions. For example, if you cut your finger, cortisol is responsible for calming down any inflammation around the wound as the wound starts to heal. Under chronic stress, the HPA axis (which ultimately produces cortisol) becomes dysregulated. Initially, persistent stress will cause our HPA to produce consistently high levels of cortisol. When our cortisol levels have been high for a significant amount of time the body becomes less sensitive to its anti-inflammatory effects. Eventually, the HPA axis can’t keep up with the demand for cortisol, and cortisol levels become low.Too much cortisol causes the body to become desensitized to its effects, leading to increased inflammation. Too little cortisol means that the body can't shut off inflammation effectively. Either way, the gut will be inflamed under chronic and persistent stress, further damaging the integrity of the gut lining by enabling those pesky gatecrashers to invade the gut wall and provoke the immune system.Dysbiosis &amp; Gut Bacteria Our gut houses over 100 trillion bacteria. Gut bacteria fulfill a number of roles which are vital to maintaining our overall health and wellbeing. Our gut bacteria regulate digestion and metabolism, extract vital nutrients from our food, and program the body’s immune system. They assist in maintaining the integrity of the gut wall, defend against pathogens and block harmful microbes from setting up camp in our digestive tract. Gut bacteria also produce hundreds of neurochemicals that the brain uses to regulate basic physiological processes as well as mental processes such as learning, memory, and mood.” When the collection of bacteria in our gut live in harmony or have a mutually beneficial relationship with our body, the good bacteria outweigh the bad, allowing our gut to function properly. This is referred to as a state of symbiosis. When this harmonious relationship is compromised, non-beneficial bacteria outweigh the beneficial bacteria, a state referred to as dysbiosis. There is growing evidence that dysbiosis is associated with the development of numerous disorders ranging from intestinal disorders (such as inflammatory bowel disease, irritable bowel syndrome and coeliac disease) to the likes of allergies, asthma, metabolic syndrome, cardiovascular disease, and obesity. Various studies have shown that stress suppresses beneficial bacteria, and promotes colonization of bad bacteria within the gut. In a study of student’s stools during exam week, it was found that the student’s stools contained fewer lactobacilli (a strain of 'good bacteria') than compared to any other normal day. In another study, characteristically timid mice have put into a cage with more aggressive mice: a "social disruption" stressor. The number of beneficial bacteria residing in the timid mice guts decreased, whilst promoting the overgrowth of harmful bacteria. Stress-induced changes to our gut microbiome may, in turn, affect the brain and behavior. Let’s go back to the timid mice. Researchers found that altering their gut bacteria caused them to become bold and adventurous. When their gut bacteria returned to normal, the mice became timid again. In a follow-up study, mice characteristically courageous and exploratory were colonized with bacteria from characteristically timid mice and vice versa. The typically courageous mice grew hesitant and shy, whilst the shy mice became courageous.We need further testing on the connection between our bacteria and the brain (humans are after all very different from mice). There is also still much uncertainty surrounding the communication channels between our gut bacteria and our brain. Yet, it is easy to see how such results are incredibly tantalizing in the medical arena - we may be facing a future where we are able to use gut bacteria to treat psychological disorders, brain and mood dysfunction. Given that the dialogue between the gut and the brain is bidirectional, manipulating gut bacteria may be able to treat both the physical symptoms of intestinal aggravation as well as the psychological disorders so often present in patients suffering from irritable bowel syndrome. Watch this space!A Holistic Approach If there is one thing you should take away from this article it is this: gut health is affected just as much by the state of your mind as it is by the food you consume. Wellbeing requires a holistic approach to health - we need to take care of our mind, body, and soul. An aggravated mind leads to an aggravated gut. And an aggravated gut will aggravate the mind. If you suffer from a gastrointestinal disorder, consider whether or not your body might be under chronic stress. If so, implement stress-reduction strategies such as meditation, yoga, mindfulness, supplementing with probiotics or diaphragmatic breathing. Your head and your stomach will thank you for it.