Gut Microbiome 101: Your Internal Ecosystem

Medically reviewed by Tracy McCarthy, MD — Board-Certified Psychiatrist, IFM-Certified Functional Medicine Practitioner

You Are Not Alone in Your Body

Inside your gut live trillions of bacteria, fungi, viruses, and other microorganisms collectively called the microbiome. The vast majority live in your large intestine, and the vast majority of them are working for you — helping digest food, regulate immunity, produce neurotransmitters, metabolize hormones, and communicate constantly with your brain.

When that ecosystem is diverse and functioning well, the downstream effects are felt throughout the entire body and brain. When it's disrupted — a state called dysbiosis — the consequences can be equally widespread.

What the Microbiome Actually Does

The microbiome is involved in far more than digestion. A comprehensive review published in Nutrients (https://pmc.ncbi.nlm.nih.gov/articles/PMC10146621/) outlines several key functions relevant to mental and physical health:

• Producing neurotransmitters, including GABA and serotonin precursors, which travel through the vagus nerve and bloodstream to influence brain function

• Regulating immune activation — approximately 70% of the immune system resides in the gut, and the microbiome plays a direct role in calibrating inflammatory responses

• Metabolizing estrogen through a system of bacteria sometimes called the estrobolome, which influences how much estrogen is recirculated versus excreted

• Producing short-chain fatty acids like butyrate, which support gut barrier integrity, reduce inflammation, and have measurable effects on mood

Disruptions in microbial diversity are associated with distinct microbial signatures in individuals with anxiety and depression compared to healthy controls. People with major depressive disorder have also been found to have lower levels of beneficial short-chain fatty acids produced by the microbiome.

The Gut–Brain Connection

The gut and brain communicate through multiple channels — the vagus nerve, the immune system, the enteric nervous system, and the bloodstream. This is a two-way relationship. A review published in Current Opinion in Psychiatry (https://pubmed.ncbi.nlm.nih.gov/25415497/) confirms that the microbiome-gut-brain axis is of substantial relevance to mood and behaviour — and that diet influences microbiota composition in ways that meaningfully affect mental health outcomes.

This is why chronic stress disrupts digestion, and why gut imbalances can produce mood symptoms. The two systems are not separate — they are in constant dialogue.

What Affects Microbiome Health

• Diet — particularly fiber intake, fermented food consumption, and degree of food processing

• Antibiotic use, which disrupts microbial balance and may have lasting effects

• Stress, which alters gut motility, intestinal permeability, and microbial composition

• Sleep quality and consistency — the microbiome has its own circadian rhythm

• Environmental exposures, including pesticides and toxins that affect microbial populations

Small Steps to Consider

• Increase dietary diversity — more varied plant foods support more varied bacterial populations

• Consider adding fermented foods (yogurt, kefir, sauerkraut, kimchi) if tolerated

• Discuss whether comprehensive stool testing might be appropriate given your symptom pattern

• Reduce ultra-processed food intake, consistently associated with lower microbiome diversity

• Address sleep consistency — the microbiome is regulated partly by circadian cues

A Final Thought

Your gut is not just digesting your food. It is influencing your mood, your hormones, your immune system, and your brain. That relationship deserves the same attention as any other aspect of your health.

My free symptom assessment (https://health-report.scoreapp.com/) helps you identify patterns across gut health, mood, energy, and hormones — through a functional medicine lens.