What Nobody Tells You About the Gut-brain Axis
I have read countless articles on the gut-brain relationship, but very few go into depth about one particular concept. As a Nutrition & Lifestyle Practitioner completing an MSc in Psychology & Neuroscience of the Mind-Body Interface, a few classmates asked me to share my knowledge in this area, so I decided to write about the most important thing I’ve learned in the past several years on the gut-brain axis.
Now, for those of you who don't know much about the gut-brain connection, the best way to understand this beautiful mechanism is to see it as a two-way communication network connecting the mind and various organs, including the digestive system. One of the main nerves connecting the two areas is called the vagus nerve, which contains around 100,000 neurons. Because of this connectivity, I often describe the mind and digestive system as one system.
Now, here is what I've noticed. Many gut-brain articles discuss a bottom-up approach: how your gut can influence the brain and mind. How eating well can improve gut microbiome diversity, influence mood, and optimise cognitive processes, and how certain species of gut bacteria are protective against cognitive decline.
Don't get me wrong, these are great and important findings; but here is what very few authors discuss.
The Importance of Top-down Findings of the Gut-brain Connection
My own personal health journey taught me just how strong this direction can be. So, allow me to discuss how positive life experiences, emotions, and mindsets can support the gut and the rest of the body. I’d also like to discuss how negative experiences, emotions, and mindsets can disrupt digestion and one’s health.
Healthy Function of the Brain & Vagus Nerve
We all love serotonin and dopamine, right? After all, healthy levels of serotonin and dopamine are often associated with good mood states, whereas altered levels of these neurotransmitters have been linked with IBS, depression, and PTSD. Relaxing the mind and body can support parasympathetic activity (the ‘rest and digest’ state), promoting healthy function of the body, including the gut. Additionally, relaxed, healthy states of the mind can support activity in the prefrontal cortex, the area of the brain that helps us make decisions, manage emotions and control impulses, often making healthier lifestyle choices easier.
A Healthy Microbiome
The microbiome is a beautiful ecosystem that, much like our own jungles and forests on this earth, has a variety of species living in various places on our body. The gut microbiome is particularly interesting and has been the most researched. A healthy gut microbiome is often diverse, beaming with all sorts of life.
Each species contributes to the growth and maintenance of its environment, i.e. you as the host. Certain bacteria have been shown not only to synthesise serotonin, but also to consume and metabolise the molecule, encouraging serotonin-consuming species to grow. Other neurotransmitters have similar effects. Oxytocin, released when connecting with others, can also alter and improve the overall composition of your gut microbiome, supporting a healthy and diverse ecosystem.
Healthy Gut Function
Both serotonin and dopamine are important neurotransmitters that regulate gut motility, ensuring the food moves through the digestive tract at a healthy pace so that waste products can be excreted regularly. A more relaxed nervous system helps with the secretion of stomach acid, digestive juices, and enzymes to break down your favourite foods.
Serotonin and dopamine allow for that smooth muscle contraction in the gut, known as the migrating motor complex, which moves digested food along the tract so that waste products can leave the body regularly. Additionally, serotonin can help balance the sensitivity of digestive organs and support gut cells to heal and create more gut cells in injured areas. Both serotonin and oxytocin play a vital role in the maintenance of tight cell junctions in the gut wall, helping ensure nutrients enter the bloodstream and unwanted substances stay out.
Impact of Stress on the Brain
Here is where things can go downhill. Although stress can vary from individual to individual, and small amounts are considered beneficial, large amounts of stress can leave us feeling unable to cope with long-term challenges, potentially threatening our wellbeing or survival. This feeling is not always easily identifiable, especially if we were raised in stressful environments, where high levels of stress become the new normal.
However, when the stress response is activated, we have less activity in the pre-frontal cortex, which is why willpower and self-control can feel much harder to access. It's much more difficult to make healthy lifestyle choices due to this, as the system prioritises dealing with the threat first. These stress hormones often lead to poorer levels of serotonin and dopamine; both vital neurotransmitters needed for maintenance of body health and function.
A Stressed Microbiome
Dysbiosis is a term used to describe an unfavourable change in the microbiome ecosystem, often characterised by a loss of diversity, loss of beneficial species, and increases in potentially harmful bacteria. Stress can contribute to the onset of dysbiosis, as well as larger gaps between the cells that line the gut wall. When these gaps increase, the gut barrier can become weaker, making it easier for unwanted bacteria or their by-products to pass through.
But here are some interesting ways stress can affect the gut microbiome:
Adrenaline & Noradrenaline can elevate certain bacterial levels 10, 000-fold which can crowd out good bacteria.
Stress hormones can travel to the gut and increase bad bacteria to grow, e.g E.coli
In one study, couples in a troubled marriage had more signs of a weakened gut barrier than couples in a more supportive relationship.
Stressed Gut Function
Even low amounts of long-term stress hormones can reduce the body’s “rest and digest” signals and have digestion as a lower priority for the body when it comes to energy distribution.
So that means less stomach acid can be produced, which may contribute to acid reflux, as the stomach needs certain levels of acid for the stomach’s gate to work properly. It also means fewer digestive enzymes can be produced, so the food is not properly broken down and/or absorbed.
Additionally, these stress pathways can directly affect how quickly food moves through the gut by slowing down the intestinal tract, contributing to constipation. That said, adrenaline-mediated stress, which can come from events that make you feel anxious, excited, or fearful, often leads to faster movement through the gut and therefore looser stools.
So, hopefully now you have a better understanding of why your appetite might change as exams approach, or why you feel that horrible pit in your stomach before a romantic date. Whilst optimising your nutrition is important, just remember: real-life situations can significantly influence your biology, either for better or for worse.
Don’t underestimate the power of the placebo effect, or the strength of changing your environment and the people you spend your time with. Supporting your biology also means managing stress in ways that feel realistic: do activities you really enjoy, find support where you can, and make more plans with trusted friends and loved ones. Avoid taking on too many responsibilities at once. These are all crucial to help your body run as optimally as it can.
These small changes can all help your body function at its best.
