Before delving into this topic, it’s helpful to revisit what we mean by the gut microbiome and cortisol.
The gut microbiome is a community trillions of tiny living organisms that naturally live in our digestive tract . Together they help us break down food and play an important role in keeping the whole body healthy, including brain health. These microorganisms make substances, such as proteins, that support the health of the gut. Some of these substances can also travel beyond the gut, allowing the microbiome to influence other parts of the body.
Cortisol is our stress hormone released by the adrenal glands in response to stress. Cortisol release is governed by the hypothalamic pituitary adrenal axis (HPA-axis) in the brain. Cortisol is critical for our fight or flight response, our circadian rhythm (sleep cycle) and our overall stress response. High or low levels can impact our health.
Cortisol’s effects on the Microbiome
Research has consistently shown that high levels of cortisol over time in the body can decrease the number of different microbes (diversity), leading to an imbalance in the beneficial and pathogenic bacteria. This can cause inflammation, worsening the stress response and impacting mood, immunity and gut function.
However, this relationship is bidirectional and recent research shows that a healthy and resilient microbiome can help regulate cortisol levels.
Microbiomes’ effect on Stress
Research in University College Cork highlighted how the gut microbiome can regulate the timing of the stress response. The depletion of the gut microbiome results in disruptions in the brain’s circadian rhythm and causes hyperactivation of the stress response. This demonstrates the importance of a healthy microbiome in maintaining synchronisation between circadian and stress systems, allowing us to respond adaptively to stressors across the day.
Diet also plays a significant role. Studies have shown that adherence to a microbiome-supporting diet, such as the Mediterranean diet rich in fibre, is associated with reduced levels of perceived stress. High-fibre foods promote the growth of beneficial gut bacteria, which can reduce systemic inflammation. This reduction in inflammation feeds back to the brain, dampening HPA-axis activity and ultimately reducing cortisol release.
Additionally, specific gut bacteria, such as certain bifidobacteria are capable of producing neurotransmitters such as GABA, the brain’s inhibitory neurotransmitter. GABA-producing bacteria play a role in calming the nervous system and reducing stress. Supporting this idea, one study found that participants who consumed the bifidobacteria loving fibre, galacto-ligosaccharide (GOS) for three weeks showed significantly lower cortisol levels compared to a placebo control group. This suggests that targeted support of beneficial microbes can directly influence physiological stress markers.
Gut microbes are known to modulate our stress responsivity and behaviour. Reductions in microbial diversity or beneficial microbes can trigger a cascade of effects, impacting inflammation, brain signalling, and stress regulation. While advice such as “reduce stress” or “practice mindfulness” is well-intentioned, stress is often unpredictable and unavoidable.
However, by creating a stable and supportive environment in the gut through diet and lifestyle, we can strengthen our internal resilience in the long-term. A healthy microbiome may not eliminate stress, but it can help us manage and adapt to stress more effectively when it arises.