We have all experienced stress in different ways and in different phases of life. Stress is normal, unavoidable fact of life. Stress is a feeling we all experience when we are challenged or overwhelmed. But more than just an emotion, stress is a physical response that travels throughout our entire body.
In short-term, stress can be advantageous as it protects us from potential threats. But in case of a chronic stress i.e., when activated too often or for too long, our primitive “fight or flight” response not only changes our brain but also damages many of the other organs and cells throughout our body.
How can stress make us sick?
When we are faced with a stressful situation, our adrenaline glands send a flood of stress hormones like cortisol, epinephrine (adrenaline), and norepinephrine. Through blood circulation these hormones easily reach our blood vessels and heart. Adrenaline causes our heart to beat faster and raises blood pressure, over time leading to hypertension. Cortisol can cause the endothelium (inner lining of blood vessels) to not function normally promoting building up of cholesterol plaque in your arteries. Also, these hormones flying around the body, our breathing and blood glucose levels shoot up. If you are consistently under stress, your hormones and sugar levels will continue to surge increasing your chances of heart diseases, hypertension, headaches, sleep problems, chronic anxiety, and depression.
Apart from this, these prolonged surges in hormones can also affect our digestive system by either increasing or decreasing its activity. For some this can mean slow down or halt in normal digestion, and for others it can result in an urgent need to empty the bowels.
When our brain senses stress it activates the autonomic nervous system which then communicates stress to enteric or intestinal nervous system. This brain-gut connection can disturb the natural rhythmic contractions that move food through our gut. Sometimes this can increase gut sensitivity causing indigestion, heartburn, and IBS.
Gut microbiome – the key player in Gut-Brain axis
Beyond functional changes, stress can also impact the composition and function of the gut microbiome – the community of microorganisms living in our gut. It is now well accepted that gut microbiome can vastly affect the way our brain and nervous system function and are major players in orchestrating the “Gut-Brain Axis”. The Gut-Brain axis is the bidirectional relationship between gut and brain. This connection allows the gut to send and receive signals from the brain. The routes of communication between the microbiota and brain are slowly being unravelled, and include the vagus nerve, gut hormone signalling, the immune system, tryptophan metabolism, and microbial metabolites such as short-chain fatty acids (SCFAs).1
This complex network of gut-brain regulates gastrointestinal homeostasis and connects emotional and cognitive areas of brain with gut functions.2
Scientists using animal models have found that stress can make the intestines more permeable, activating immune and pro-inflammatory responses which triggers additional stress hormones to be released and leading to a continue vicious cycle.3 Scientists have also found that people with stress-associated condition, such as depression and anxiety, have gut microbiomes that are different to those of healthy individuals. Some studies have noted that depressed people had increased levels of bacteria that are associated with poor health, while the beneficial bacteria are decreased compared to healthy subjects. Depressed people were also noted to have lower levels of SCFAs compared to healthy controls.4
Gastrointestinal dysfunctions with alterations in the gut microbiome (or dysbiosis) are frequently accompanied by comorbid mental disorders such as mood disorders, stress, anxiety and even depression5. Irritable Bowel Syndrome (IBS) and Inflammatory Bowel disease (IBD) in humans are two conditions that exemplify the consequences of a faulty gut-brain communication.5,6
The “Superhighway nerve” that facilitate Gut Microbiome & Brain communication
While our gut can communicate with our brain via multiple pathways (neural, hormonal, and immune), the primary neural connection is through the vagus nerve. The vagus nerve, also sometimes referred to as a “wanderer nerve” or “superhighway nerve” runs the length of our body, connecting all our digestive organs such as stomach and intestines to our brain. This superhighway nerve allows the brain and the gut to send signals to each other, however, most of the traffic runs in the direction from gut to the brain.7 Gut is hence, nicknamed as the “second brain”, as what happens in the gut can have significant impact on our overall wellbeing and our mental health.
Serotonin, a neurotransmitter that is often dubbed as the “feel-good hormone” or “happy hormone” is one the many substances that can activate the vagus nerve and send signals to the brain. Serotonin that controls our mood and emotion, also plays a vital role in regulating sleep, appetite and digestion and is hence, an incredibly important metabolite for our overall well-being.5, 7
The majority of serotonin is produced in the gut by enterochromaffin (EC) cells. These cells are stimulated by the presence of microbiota derived SCFAs, butyrate and propionate. SCFAs are incredibly vital metabolites with anti-inflammatory properties that are produced by gut bacteria that feed on prebiotic fibres found in plant foods. Our gut bacteria also generate GABA, another vital neurotransmitter. People with depression are often reported to have lower levels of GABA in their brain.
Studies using animal models have confirmed the benefits of supplementation of prebiotic fibres in achieving better stress tolerance outcomes and reduction in signs of anxiety and depression compared to control animals that did not receive prebiotics.8,9 Prebiotic supplementation has also shown some promising results in humans. For instance, prebiotic supplementation for 4 weeks in a clinical study was found to be effective in reducing anxiety scores in IBS patients.10 In another study, prebiotic intake in healthy adults was shown to lower cortisol response upon waking and alter emotional bias.11 Colonic SCFA delivery has also been shown to attenuate cortisol response to psychological stress in healthy men.12
Therefore, diets high in plant foods that are rich in prebiotic dietary fibres may assist in improving our overall wellbeing and help increase our stress tolerance. Prebiotics deliver these benefits by modulating the composition and function of microbiota to produce SCFAs in the gut.
What this means for strategies to help manage stress
Certain types of diet are associated with improving mental health, while other types, to increase the risk of mental disorders. This could be in part due to the ability of the diet to modulate the gut microbiome. Western diets are known to contribute to the dysbiotic conditions (detrimental alterations to microbiota) that send brain signals to alter diet intake behaviour.13 A diet that continues to have increasing evidence showing benefits for mental health and promotes healthy eating is the Mediterranean diet.14,15
So, to fuel your gut microbiome toward healthy mental state and help manage stress, eat plenty of prebiotic, plant-based foods such as fruits, vegetables, wholegrains, legumes, nuts and seeds.
Happy Gut – Happy You!
By Dr. Tanvi Shinde, PhD