Can Gut Bacteria Cause Anxiety? How SIBO Disrupts Mood from the Inside Out

Struggling with both digestive issues and mood swings? The gut-brain connection may hold the answers.

If you’ve been dealing with anxiety, depression, or fatigue that doesn’t seem to improve with therapy or medication—and you also experience bloating, bowel changes, or digestive discomfort—it might be more connected than you think. Your gut and brain are constantly communicating, and when the gut is imbalanced, that communication can go haywire.

One of the most overlooked contributors to both digestive distress and mood symptoms is something called Small Intestinal Bacterial Overgrowth (SIBO). This refers to an abnormal buildup of bacteria in the small intestine, where they aren’t meant to be in large numbers. It’s also important to note that similar forms of overgrowth exist, including Intestinal Methanogen Overgrowth (IMO) and Intestinal Sulfide Overgrowth (ISO), which involve different microbes and symptoms. To keep things simple, I’ll refer to all of these types of small intestinal overgrowth collectively as “SIBO” throughout this post.

What Is SIBO?

In a healthy small intestine, bacteria are tightly regulated and kept in low numbers. In SIBO, excess bacteria overgrow in the small intestine — a place that’s not designed to house large microbial populations. The small intestine is a space that’s meant to be relatively low in microbial activity.

This bacterial overgrowth can:

·       Ferment carbohydrates and produce gas, leading to bloating, pain, diarrhea, or constipation

·       Disrupt digestion and nutrient absorption

·       Damage the gut lining, contributing to inflammation

·       Trigger immune system activation

These gut changes can affect your body far beyond the digestive tract.

Gut Inflammation Can Affect Your Brain

When the gut becomes inflamed or stressed (as it often does in SIBO), your immune system kicks into gear. It starts releasing inflammatory molecules (cytokines), which can:

·       Cross the blood-brain barrier or signal the brain via the vagus nerve

·       Trigger a body-wide, low-grade inflammatory response

·       Alter mood, cognition, and the way you respond to stress

Research has shown that people with chronic inflammation often experience higher rates of anxiety, depression, and brain fog.

Leaky Gut and Endotoxemia

When SIBO damages the gut lining, it can lead to a condition known as intestinal permeability or “leaky gut.”

In leaky gut:

·       The tight junctions in the intestinal lining become loose

·       Toxins like lipopolysaccharides (LPS) can escape into the bloodstream

·       This condition is called metabolic endotoxemia and it fuels inflammation not just in the gut, but throughout the body and brain

This inflammation can feed into a vicious cycle, worsening both gut symptoms and emotional well-being.

Neuroinflammation and the HPA Axis

Your brain is not immune to this cascade. These inflammatory signals can:

·       Activate microglia, the brain’s immune cells, leading to neuroinflammation

·       Disrupt the production and signaling of neurotransmitters like serotonin and dopamine

·       Interfere with brain regions responsible for emotional regulation, including the amygdala and hippocampus

Chronic inflammation also overstimulates the HPA axis (hypothalamic-pituitary-adrenal axis), the system that controls your stress hormone, cortisol. When this axis is constantly activated:

·       You may feel more anxious or easily overwhelmed

·       Sleep and energy levels suffer

·       Emotional resilience drops

In Simple Terms

SIBO isn’t just a digestive issue—it can significantly impact your mood and mental clarity. Here’s how:

·       Bacterial overgrowth causes inflammation and leaky gut

·       Inflammation and toxins affect brain health

·       Neurotransmitter production and stress regulation become impaired

Some people notice anxiety or depression before they even realize they have digestive symptoms. This is why looking at the gut-brain connection can be a game-changer.

Coming Next Month…

In Part 2, we’ll explore how SIBO affects nutrient absorption, serotonin metabolism, and the vagus nerve’s role in gut-brain signaling. We’ll also dive into tryptophan pathways and how functional nutrition can help restore balance.

*Disclaimer: This blog is for educational purposes only and is not intended as medical advice. Always consult a qualified healthcare provider before making changes to your treatment plan.

References:

·  Foster, J.A., & Neufeld, K.-A.M. (2013). Gut–brain axis: How the microbiome influences anxiety and depression. Trends in Neurosciences, 36(5), 305–312. https://doi.org/10.1016/j.tins.2013.01.005

·  Mayer, E.A., Knight, R., Mazmanian, S.K., Cryan, J.F., & Tillisch, K. (2014). Gut microbes and the brain: Paradigm shift in neuroscience. Journal of Neuroscience, 34(46), 15490–15496. https://doi.org/10.1523/JNEUROSCI.3299-14.2014

·  Kelly, J.R., et al. (2015). Breaking down the barriers: The gut microbiome, intestinal permeability and stress-related psychiatric disorders. Frontiers in Cellular Neuroscience, 9, 392. https://doi.org/10.3389/fncel.2015.00392

·  Dinan, T.G., & Cryan, J.F. (2017). The microbiome–gut–brain axis in health and disease. Gastroenterology Clinics of North America, 46(1), 77–89. https://doi.org/10.1016/j.gtc.2016.09.007

·  Maes, M., Kubera, M., & Leunis, J.C. (2008). The gut–brain barrier in major depression: Intestinal mucosal dysfunction with an increased translocation of LPS from gram-negative enterobacteria (leaky gut) plays a role in the inflammatory pathophysiology of depression. Neuro Endocrinology Letters, 29(1), 117–124.