Stress and the Microbiota-Gut-Brain Axis

Stress is a part of life; it is a natural, biological response to a perceived threat. The truth is that we all experience stress throughout our lives, and it is not always harmful. Think about that job interview or big presentation that you gave. Although stressful, you can thank your body’s stress response for increasing your productivity to get you through it! According to the American Institute of Stress(1), stress has been defined as a “physical, mental, or emotional strain or tension”. When we think of stress, we often focus on the negative feelings and emotions that accompany it. So, if stress is unavoidable and helps us respond to challenging situations, then what’s the problem?

According to the World Health Organization “stress is the health epidemic of the 21st century”. Prolonged elevated stress levels or daily stress can have a serious negative impact on our mental and physical well-being and health. Cast your mind back to that interview. Within the nervous system, stress triggers the production of neurotransmitters and hormones that lead to physiological changes (fight or flight response). How we respond to stress is regulated by the hypothalamic pituitary adrenal (HPA) axis which releases cortisol – the main stress hormone in humans.

Chronic, daily stress keeps the HPA axis constantly activated, and we are in a constant state of fight or flight. Over time, this has a serious negative impact on our health, increasing the risk of developing mood disorders such as depression and anxiety, behavioral problems such as irritability and loss of humor, physical problems such as poor sleep, chest pains and headache, and cognitive problems such as an inability to focus.(2)

Targeting the Gut for Better Brain Health

We’ve long known of the connection between the gut and the brain – we’ve been saying it all our lives! Have you ever had ‘butterflies in the stomach’, or had a ‘gut instinct’? Stop and think about that for a second. The gut-brain axis is a bidirectional communication pathway between the gut and the brain involving the central nervous system, enteric nervous system (our ‘second brain’ which controls the function of our gastrointestinal tract), as well as endocrine and immune systems, and various other metabolic pathways in the body.(3) We now know that the microbes that live in our gut (collectively known as our gut microbiota) can communicate with the brain along the gut-brain axis through several direct and indirect signaling pathways and through the production of metabolites.(4) Microbes within the gut can produce neurotransmitters and neuroactive molecules that can influence the brain. The scientific evidence shows that our gut microbes can influence how our brain functions, how we respond to stress, our mood, and even how we behave. The microbiota-gut-brain axis therefore plays an essential role in our overall health, including mental wellness and cognitive health.

The Microbiota-Gut-Brain Axis:?

Stress impacts the physiology of the entire body, including the gut and the microbes that live there. Not only can the environment of the gut be physically altered by stress hormones, but studies have shown that stress exposure throughout life can change the composition of the gut microbiota.(5) HPA axis programming and responses to stress also depends on the microbes within the gut.(6) When stressed, mice lacking an intact gut microbiome experience an exaggerated stress response and an increased production of stress hormones. Interestingly, the stress response in these mice could be normalized once they became colonized with healthy bacteria such as Bifidobacterium infantis.(7) Now that we have established that stress affects our gut microbes and that our gut microbes can influence how we respond to stress, can we target the gut microbiota to influence stress and mood? The answer is yes! Clinical evidence has demonstrated that probiotics can influence both how we respond to stress and how we experience stress every day.(4) Such findings have led to the concept of psychobiotics,(8) which are any exogenous influence whose positive effect on mental health and well-being are bacterially mediated, probiotics for example. As the effects of probiotics are strain specific, it is important to choose one that has been investigated in humans and which has demonstrated a positive effect on mental health and well-being.?With high stress levels now increasingly prevalent in our modern 24-hour society, we need new solutions to support us and prevent the harmful effects of chronic stress disrupting our mental and physical health. Understanding that the microbiota-gut-brain axis and stress are completely connected, future research and innovation can focus on targeting the gut as an exciting new frontier for stress management!

1. https://www.stress.org The American Institute of Stress. 2. https://www.mind.org.uk How to Manage Stress. 3. Cryan et al. The Microbiota-Gut-Brain Axis. Physiological Reviews (2019). 4. Foster, Rinaman, Cryan. Stress & the gut-brain axis: Regulation by the Microbiome. Neurobiology of Stress (2017). 5. Morais, Schreiber, Mazmanian. The Gut Microbiota-Brain Axis in Behavior and Brain Disorder. Nature Reviews, Microbiology (2021). 6. Clarke, G. et al. Gut Reactions: Breaking Down Xenobiotic–Microbiome Interactions. Pharmacological Reviews. (2019). 7. Sudo et al. Postnatal Microbial Colonization Programs the Hypothalamic-Pituitary-Adrenal System for Stress Response in Mice. The Journal of Physiology (2004). 8. Dinan, Stanton, Cryan. Psychobiotics: a novel class of psychotropic. Biological Psychiatry (2013)