Your Microbiome: The “Hidden Organ” Powering Digestion, Immunity, Mood, and Vitality
Let me start with a story that changed the way I think about the microbiome. Although this experiment was carried out in mice, the message is powerful and rings true for human babies too.
The sterile birth experiment that revealed a huge truth
Scientists were curious to see what would happen if they created germ-free mice. A way to do this is to deliver the pups by sterile Caesarean section and raise them in sterile isolators so they’re not exposed to any microbes.
What researchers learned from these germ-free rodent babies is striking: without normal microbial exposure, the baby mice develop major differences in gut and immune physiology, including an underdeveloped immune system and altered gut structure and function. And when challenged (for example, with gut injury/exposure to irritants or inflammation), germ-free and microbiota-depleted mice were far more vulnerable than conventionally raised mice.
They died sooner of horrible disease like cancer and neurodegenerative conditions.
The message is simple but profound: microbes aren’t optional. They help build and regulate the systems that keep us well.
What exactly is the microbiome?
The microbiome is the collection of microbes (bacteria, viruses, fungi and more) that live in and on your body. The majority live in your gut, especially your large intestine.
And, the numbers are mind-blowing. A widely cited scientific estimate suggests a “reference adult” carries roughly 3.8 × 10¹³ (about 38 trillion) bacteria, mostly in the colon. The number of cells you have in your body is numerically fewer than the micro-organisms that live in and on you. So, if we work with numbers alone… you are more microbe than you are human!
A healthy microbiome isn’t about one “magic strain”. It’s about an ecosystem with:
- Diversity (lots of different helpful species)
- Balance (keeping potentially harmful microbes in check)
- Stability (not swinging wildly day to day)
- Resilience (recovering after illness, antibiotics, stress, travel)
What does your microbiome do for you?
1) It supports mood and mental clarity (the gut–brain axis)
Your gut and brain are in constant two-way communication via nerves, immune signals, hormones, and microbial by-products.
A fascinating example: more than 90% of the body’s serotonin is produced in the gut environment (via enterochromaffin cells).
When the gut ecosystem is disrupted, people may notice:
- anxiety or low mood
- insomnia or non-restorative sleep
- brain fog, poor concentration
- low energy and motivation
2) It trains and supports your immune system
The gut is one of the biggest immune “training grounds” in the body. Healthy colonisation helps immune development and appropriate responses, allowing you to heal and recover while protecting you from pathogens and other harmful exposures.
3) It helps digestion, nutrient absorption, and vitamin production
Your microbes help break down what you can’t digest on your own (especially fibre), and they contribute to vitamin and metabolite production that supports many body systems.
4) It helps regulate inflammation
A resilient microbiome helps keep inflammation lower. This is vitally important for metabolic health, heart health, autoimmune tendencies, and healthy ageing.
A quick note on C-sections and childhood allergies
Research consistently shows that babies born by C-section often have different early microbial colonisation patterns compared with babies delivered vaginally. That may contribute to higher observed rates of certain childhood illnesses (like asthma and allergic conditions).
Two important things can be true at once:
- C-sections can be lifesaving for mother and baby.
- Birth mode may influence early microbiome patterns and immune programming.
So if someone had a C-section, the takeaway is not fear or blame, just awareness that early-life gut support matters (breastfeeding if possible, avoiding unnecessary antibiotics, and a fibre-rich, diverse diet as solids are introduced).
You may hear about “vaginal seeding”. Which involves deliberate exposure of babies born by C-section to the microbes in their mothers’ birth canal (using swabs). Currently, there is not enough definitive evidence that this practice should be widely applied, it is still under investigation.
What disrupts the microbiome?
The big disruptors I see most often include:
- Antibiotics (essential when needed, but disruptive because they don’t only target “bad” bacteria)
- Acid-suppressing medications (e.g., PPIs/antacids) which can alter gut environment
- Ultra-processed diets (low fibre, high additives/preservatives)
- Artificial sweeteners (linked with unfavourable microbiome shifts in several lines of research)
- Chronic stress (gut–brain axis works both ways)
- Sedentary living
- Alcohol, smoking/vaping, and broader environmental toxins
- Over-sanitising everything (less exposure to diverse environmental microbes)
A real-life case from practice: when the microbiome–mind link becomes visible
One of the most sobering cases I’ve seen: a gentleman who developed a resistant bacterial sinus infection after sinus surgery. Clearing it required more than 11 courses of antibiotics. Over time, he went on to develop severe depression with psychotic symptoms.
Now, in a real human being, we can’t claim a single cause because mental health is always multifactorial (infection itself, inflammation, sleep disruption, medications, stress, genetics, and more). But clinically, it’s hard to ignore how often prolonged antibiotic exposure coincides with a shift in mood, anxiety, energy and cognition.
The scientific literature supports an association between antibiotic use and increased risk of depression/anxiety, most likely due to microbiome disruption and changes in chemical signals between the gut and central nervous system.
For me, the clinical lesson is this: protect your microbiome whenever possible—especially when we know we’re going to disrupt it.
How to optimise your microbiome (the practical, effective steps)
1) Eat for diversity (this matters more than “perfection”)
Different microbes prefer different foods. More variety = more diversity.
Simple goal: aim for 20–30 different plant foods per week (vegetables of different colours and types, fruit, legumes/pulses, whole grains, nuts, seeds, herbs, spices).
2) Make fibre your daily non-negotiable
Humans can’t digest fibre for energy, but your gut bacteria thrive on it.
Prebiotic foods (foods that feed beneficial bacteria such as Bifidobacteria) include:
- apples
- berries (especially blueberries and raspberries)
- almonds and pistachios
- artichokes
- whole grains (bulgur, crushed wheat, wild rice, oats)
- lentils, chickpeas and beans
If you’re currently low-fibre: increase gradually + hydrate well to reduce bloating and gas.
3) Enjoy fermented foods regularly
Fermented foods contain beneficial microbes (often including lactobacilli) and can support gut diversity.
Examples:
- natural, unsweetened yoghurt
- sauerkraut
- tempeh
- kombucha
- kefir
- kimchi
If you have IBS or are sensitive: start small (a tablespoon or a few sips) and build up gradually over time.
4) Include polyphenol-rich foods (microbe-friendly plant compounds)
Polyphenols aren’t fully broken down by your digestive enzymes, so your gut microbes get a feast.
Top sources:
- dark chocolate / cocoa
- grapes
- green tea
- broccoli
- almonds
- onions
- blueberries
- (Red wine: if you drink, keep it modest… alcohol can also disrupt the microbiome.)
5) Move daily (your gut likes motion)
More active people tend to have more diverse microbiomes. Walking after meals is particularly beneficial because it supports gut motility and metabolic health.
6) Sleep and routine: underrated microbiome medicine
Regular sleep/wake times and a steady daily rhythm help regulate gut function. Aim for consistency more than perfection.
7) Reduce toxins (especially smoking/vaping and excess alcohol)
Toxins can disrupt the microbial ecosystem and increase inflammation.
8) Get outside (and don’t sterilise your whole life)
Nature exposure is good for immune resilience and microbial diversity. Normal hygiene is great. Constant over-sanitising everything, not good.
9) Try intermittent fasting
Fasting gives your digestive system a break and allows your microbiome to reset and restore. There is clear evidence that intermittent fasting is linked to improved gut health due to positive changes in the microbiome.
Best foods vs worst foods for gut health
Try to include more of the BEST foods to support your gut health, while reducing those foods that could jeopardise your microbiome.
Best foods for your microbiome:
- vegetables + fruit (fibre + hydration + micronutrients)
- legumes (lentils, chickpeas, beans)
- whole grains (unprocessed)
- nuts and seeds
- fermented foods
- green tea, cacao
Worst (most disruptive) foods for your microbiome:
- ultra-processed foods (low fibre, high additives/preservatives)
- sugary drinks and frequent sweets
- very salty preserved foods
- artificial sweeteners
- processed meats
- hydrogenated/trans fats
- alcohol (dose matters)
So… should you take probiotics?
For most people: probiotics are not the foundation. They don’t reliably create long-lasting microbiome changes on their own. Food and lifestyle do.
That said, probiotics can help in specific circumstances:
- after antibiotics (especially if you develop diarrhoea or gut symptoms)
- IBS (some people benefit; the strains of bacteria in the probiotic are important; diet still comes first)
- recovery after gastroenteritis
- certain recurrent issues (like urinary tract infections and diverticulitis), when particular individuals may benefit from taking a regular probiotic
If you do try a probiotic: treat it like a trial, not a lifelong prescription. Review the benefits after 4–8 weeks.
Here’s a gentle 7-day microbiome reset (no extremes, no “cleanses”)
Each day for one week:
- Add one fermented food (yoghurt/kefir/kimchi/sauerkraut)
- Add one legume serving (lentils/chickpeas/beans)
- Add two extra plant colours
- Add one polyphenol source (berries/green tea/onions/cocoa)
- Do 20–30 minutes of movement (a walk counts)
- Keep a consistent sleep window
- Spend time outdoors
Red flags: when gut symptoms need medical review
Please don’t ignore:
- blood in your stool or black stools
- unexplained weight loss
- persistent vomiting or fever with abdominal pain
- persistent diarrhoea >2–3 weeks
- significant change in bowel habit lasting >4–6 weeks (especially >45–50 years)
These symptoms need investigation, and you should not delay seeing a healthcare professional for advice.
Remember:
Your microbiome isn’t just about digestion. It’s intimately linked to immunity, inflammation, hormones, energy, sleep and mental wellbeing.
To optimise your microbiome: eat more diverse plants, prioritise fibre, include fermented foods, move daily, sleep consistently, manage stress, and avoid unnecessary antibiotics.
If you take care of your microbiome, it will take care of you.