Your Microbiome Explained: The Gut's Second Brain

Trillions of microbes live inside you. They control more than digestion – they influence your mood, immunity, brain function, and weight.

The gut microbiome hosts ~38 trillion microbial cells – roughly equal to the number of human cells in the body.

What is the Gut Microbiome?

The gut microbiome hosts ~38 trillion microbial cells – bacteria, fungi, viruses, and archaea that actively shape your health.

The gut microbiome refers to the vast community of microorganisms – bacteria, fungi, viruses, and archaea – that reside in your gastrointestinal tract, primarily in the large intestine. A healthy adult carries approximately 38 trillion microbial cells, which roughly equals the number of human cells in the body.

These organisms are not passengers – they are active participants in virtually every aspect of human physiology. They digest fibre, synthesise vitamins (B12, K2, folate), regulate the immune system, produce neurotransmitters, and protect against pathogens.

Why Microbiome Diversity Matters

Diversity is the cornerstone of a healthy microbiome. The more species of bacteria you host, the more metabolic functions your microbiome can perform. Research from the American Gut Project – the largest citizen science microbiome study – found that people who eat 30+ different plant species per week have significantly more diverse microbiomes than those who eat 10 or fewer.

Low diversity (also called low alpha diversity) is consistently associated with:

• Irritable Bowel Syndrome (IBS)
• Inflammatory bowel disease (Crohn's, ulcerative colitis)
• Type 2 diabetes and obesity
• Depression and anxiety disorders
• Allergies and autoimmune conditions

Impact on Mood & Immunity

Gut bacteria produce serotonin, GABA, and short-chain fatty acids that directly influence brain function and immune regulation.

The gut microbiome influences the brain in at least three major ways:

1. Neurotransmitter production: Gut bacteria produce or regulate serotonin, GABA, and dopamine – the brain chemicals governing mood, focus, and pleasure.
2. Immune training: Approximately 70% of the immune system is gut-associated. Gut bacteria train immune cells to distinguish friend from foe, preventing both infections and autoimmunity.
3. SCFA signalling: Bacterial fermentation of dietary fibre produces short-chain fatty acids (butyrate, propionate, acetate) that feed gut lining cells, reduce systemic inflammation, and directly signal the brain via the vagus nerve.

What is Dysbiosis?

Dysbiosis is an imbalance in the gut microbiome – too many harmful species and too few beneficial ones. Common causes include:

• Antibiotic overuse (wipes out bacteria indiscriminately)
• Ultra-processed food diets (low in fibre, high in emulsifiers)
• Chronic stress (cortisol reduces Lactobacillus populations)
• Sedentary lifestyle
• Poor sleep

How to Improve Your Microbiome

Eating 30+ different plant foods per week is the single most powerful way to increase microbiome diversity.

The good news: your microbiome can change within 24-48 hours of dietary shifts. Here are evidence-backed strategies:

Probiotics & Prebiotics

Probiotics (live beneficial bacteria) can replenish gut flora, particularly after antibiotics. Prebiotics (fibre that feeds existing bacteria) are arguably even more important for long-term diversity.

Indian diet naturally offers rich probiotic sources: dahi (yogurt), idli, dosa batter, kanji, and buttermilk. Adding psyllium husk (isabgol), garlic, onion, and green bananas provides prebiotic fibre.