The Gut Microbiome and Anxiety

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Introduction to the Gut-Brain Axis

By Laura Grosh

When people think of organs, they would likely not list the gut microbiome. Increasing research, however, is starting to change this assumption. The gut microbiome is a dynamic collection of microbes that live in our intestinal tract, and we are beginning to see that this microbial community is as integral to our health as the organs that may initially come to mind such as the heart or lungs. The gut microbiome is huge; recent estimates state that in a 70 kg person, 0.2 kg are bacteria, many of which reside in the gut. One way that the gut microbiome is vital to us in through their connection to our nervous system. Microbes are directly responsible for making neurotransmitters and neuropeptides, integrating them into the nervous system—but the connection between these seemingly separated systems extends beyond this. The vagus nerve connects the brain and the gut, a direct representation of the gut-brain axis. Autonomic, immune, and endocrine responses further complicate and interact with this connection between the gut microbiome and our nervous system. Research has addressed this connection by performing metagenomic analyses on the gut microbiome, studying model organism lacking commensal bacteria, altering the gut microbiome through probiotics or antibiotics, activating or deactivating the vagus nerve, among many other molecular and genomic manipulations Studying this connection, as complicated as it is, is beginning to uncover the implications of the gut microbiome on mental health. The gut microbiome is increasingly believed to facilitate relationships between stress and anxiety, both in direct and indirect ways. This has direct implications on public health and the field of mental health, and could even increase treatment options for anxiety in the future.

Stress, Inflammation, and the HPA axis

One way that the gut microbiome may influence anxiety is through the relationship between the gut microbiome and the HPA axis, stress and inflammation. The HPA axis is a system that modulates stress responses. When a physical or psychological stressor appears, the hypothalamus sends signals to the pituitary gland, which releases adrenocorticotropic hormone. This hormone acts on the adrenal glands to release cortisol, which is the bodies main stress hormone. In an ideal world, cortisone provides negative feedback for both the hypothalamus and pituitary gland. Of course, in the less than perfect body’s we inhabit and in an increasingly stressful world, this can go sometimes go wrong. Our stress response is not the same thing as clinical anxiety, but there are many correlations that make the HPA axis an interesting thing to study when asking questions about anxiety. Disregulation of the stress response and HPA axis, behaviorally observed as chronic stress, is highly comorbid with anxiety. To further support the connection between HPA axis and anxiety, patients with anxiety disorders show an increased cortisol response, suggesting an increase in HPA axis activity. Understanding how the gut microbiome influences the HPA axis is the first step in understanding how the gut microbiome affects anxiety.