Stress is one of the ways our body naturally reacts to extreme change or imbalance. While everyday mental health has not been treated with the same importance as physical health for many years, public and company attitudes towards mental wellbeing are changing.
Stress can be either positive or negative and is one of the body’s natural defences against real or perceived danger. As one of the ways the body responds to changing stimuli, it is an automated response that can at times be beneficial, and at other times detrimental to health and wellbeing.
Types of stress
Stress can be classified in relation to its duration as acute (such as an exam, job interview) or chronic psychological stress (bereavement, financial problems, unemployment).
Acute Stress tends to be brief and is perhaps the most common. For many, acute is the form we most commonly associate with the term ‘stress’. It is caused by reactive thoughts or apprehension and is often coupled with anxiety1.
Although acute stress can cause significant symptoms in the body and particularly in brain function, it is not thought to have the same lasting damage as chronic. Symptoms of acute stress can include transient emotional distress, tension including headaches and tendon pain, and short-term gut dysbiosis; an imbalance of the gut microbiota.
Chronic stress is thought to be the result of high pressure building up over a longer period of time, and as such is more closely associated with lifestyle and behavioural triggers.
The most important stress response system is the hypothalamic-pituitary-adrenal (HPA) axis, responsible for the release of the major stress hormone, cortisol, which affects the human body, including the brain, muscles and body fat. Permanently increased cortisol due to stress, if left unmanaged, can irreversibly impact physical and mental health2.
Chronic stress has been associated with gastrointestinal disorders such as irritable bowel syndrome (IBS)3. Stress can affect the gut barrier, increasing its permeability4. In addition, some experts think that stress-related conditions, such as depression, can exacerbate colitis5, a type of IBD.
Gut health and stress
The gut has its own role to play in mental health and wellbeing. If you’ve ever wondered why heightened emotions or feelings of anxiety can be accompanied by nausea, or where that ‘butterflies in the stomach’ feeling comes from during episodes of anxiety, the answer lies with channels of communication in the body. The brain and gut send signals to each other in a mechanism known as the ‘gut-brain axis’6.
How does stress affect gut health?
The two-way nature of the gut-brain axis means that the gut is sensitive to heightened emotions signalled by the brain.
Among the stress effects on GI health are changes in intestinal motility, gut barrier permeability and enhanced gut sensitivity. The latter can be expressed for example as abdominal pain7. Stress can also lead to long-term changes in gut microbiome composition and its metabolic activity.
Studies have demonstrated that cortisol and changes to immune system due to stress have direct negative effects on the gut microbiome. As a result, these might impact the gut’s ability to fight pathogens, which may then become more easily attached to the gut wall when the gut microbiome is not balanced (dysbiosis), and may cause sudden diarrhoea, for example.
How does the gut microbiome affect the brain during stress?
The bidirectional nature of the gut brain axis means that signals can be sent both ways; stress can impact the gut microbiome, but the gut can also send signals to the brain on its condition and activity. The vagus nerve is crucially involved in helping these messages pass through the gut-brain axis, as it sends information from the gut to brain regions involved in stress response and feeds back information from the brain back to the gut.
Notably, almost 90% of the neural communication between the gut and the brain originates in the gut, which indicates that the gut could have a powerful influence on brain function, including manipulating the feeling of stress. This field of study is relatively new, and scientists are still developing a deeper understanding of the signalling mechanisms between the gut and the brain. So far, we know that it is not only about these neural routes of communication but it also involves endocrine (such as cortisol or gut hormone signalling) and immune (through modulation of cytokines) pathways.
Importantly, because of the bidirectional communication between the gut and the brain, intestinal distress could potentially be either a cause of heightened stress, or a product of it.
Gut bacteria can profoundly influence brain and behaviour in additional ways too, not only by sending messages using the vagus nerve. Interestingly, the gut microbiome actively produces and helps the host produce (by providing precursors) neurotransmitters and other neuroactive compounds, which affect how the brain functions. For example, approximately 90% of serotonin, a neurotransmitter known for its links to mood, originates in the gut.
In addition, changes in the major neuroinhibitory neurotransmitter, gamma-amino butyric acid (GABA), are linked with behavioural disorders, pain and sleep8, which experts perceive as co-existing with elevated stress. Interestingly, many species of bifidobacteria, a well-known probiotic, are producers of GABA9. There are some ways one can naturally boost good gut bacteria and studies show the promising results that this practice has on stress levels.
Prebiotics (fibre) and stress
Dependent on its nutritional content, the food and drink we consume could help us take care of our gut microbiome, and this may in turn positively impact our mental health, including stress. They say ‘you are what you eat’, and this becomes evident when making dietary changes in pursuit of supporting gut health. An example of this would be taking on a diet rich in prebiotics (such as galactooligosaccharides, GOS), or the consumption of prebiotic supplements. These nutrients are non-digestable, but are instead designed to nourish beneficial gut bacteria, such as bifidobacteria.
Most of the studies looking at dietary interventions with relation to stress have been done using animal studies. There are some notable exceptions. For instance, in a recent study, a 3-week administration of a prebiotic resulted in a decreased cortisol awakening response in a group of healthy volunteers10, while another study using the same prebiotic demonstrated a significant decrease in anxiety scores in a cohort of individuals with IBS11.
It is generally known that anxiety is the most common psychiatric comorbidity in functional gut disorders such as IBS12, which suggests a significant relationship between fibre intake, anxiety and stress.
Although we are just beginning to understand more about the significant role of the gut-brain axis and its wider influence on overall wellness, there is clear evidence of the influence of stress on gut microbiome, and gut microbiome on stress modulation.
The condition of the gut and how we’re feeding our gut microbiome can influence how we cope with stress levels alongside its other known health benefits. This points towards the possibility of modulating stress and anxiety with dietary alterations and adding these to our daily routine.
 Vedhara, K. et al. Acute stress, memory, attention and cortisol. Department of Experimental Psychology, University of Bristol (2000)
 Silk et al. Clinical trial: the effects of a trans-galactooligosaccharide prebiotic on faecal microbiota and symptoms in irritable bowel syndrome (2009) https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2036.2008.03911.x