It shapes your mood, your immune system, your weight and your risk of chronic disease — and most Australians are quietly wrecking it without realising.
Mia had been exhausted for three years.
Not the sort of tired a good night’s sleep fixes. The sort that’s still sitting behind your eyes when the alarm goes off. The sort that makes an ordinary Tuesday feel like wading through wet sand.
Her GP ran the usual panel.
Thyroid — normal. Iron — normal. Vitamin D — a touch low, so she started a supplement.
She took it for months. Nothing shifted.
Nobody had asked about the one system that was quietly running the show: her gut.
The Organ Your GP Was Never Taught to Check
Gastroenterologists and microbiome researchers have spent the past decade building a mountain of evidence that the gut isn’t just a digestive tube — it’s closer to a command centre for the rest of the body.
Living inside your intestines right now are somewhere around 38 trillion microbial cells — bacteria, fungi and viruses collectively known as your gut microbiome. That figure comes from a 2016 recalculation published in PLOS Biology, which revised the old “100 trillion” myth down to a number roughly equal to the total number of human cells in your body.
Researchers increasingly link this ecosystem to far more than bloating and indigestion: mood regulation, immune function, body weight, and long-term risk of conditions from bowel cancer to depression. To understand why, it helps to look at what the microbiome is actually doing, hour by hour.
What’s Actually Happening Inside Your Gut
It manufactures your mood chemicals
Around 90 percent of the body’s serotonin — the neurotransmitter tied to mood, sleep and appetite — is produced in the gut, not the brain, largely by specialised cells in the intestinal lining interacting with gut bacteria. The gut also plays a role in producing GABA and precursors to dopamine. When the microbiome is disrupted, that production line is disrupted too. This is the biological thread researchers point to when explaining the well-documented gut–brain axis, and why disturbances in gut bacteria are increasingly studied alongside anxiety and depression. It’s a timely connection for Australia: in the 2020–22 National Study of Mental Health and Wellbeing, the Australian Bureau of Statistics found 22 percent of Australians aged 16–85 — around 4.3 million people — had experienced a mental disorder in the previous 12 months, with anxiety disorders the most common at 17 percent. No one is claiming gut health explains all of that, but it’s now considered a genuine contributing factor rather than a fringe theory.
It trains your immune system
Roughly 70 percent of the body’s immune tissue sits in and around the gut wall. The bacteria living there aren’t passive tenants — they actively “educate” immune cells, teaching them what to tolerate and what to attack. Disrupt the microbiome and that training falters. Immune calibration slips, and researchers now consider this a contributing mechanism in chronic inflammation, autoimmune conditions and some cancers. This matters more in Australia than the headlines let on. According to the Australian Institute of Health and Welfare, colorectal cancer rates in people over 50 have fallen roughly a third since 2001, largely thanks to the National Bowel Cancer Screening Program — but rates in people under 50 have moved sharply the other way, more than doubling in raw case numbers since 2000, with the incidence rate climbing from around 7 to roughly 12 cases per 100,000. Bowel cancer diagnoses in Australians in their thirties have risen an estimated 173 percent since 2000. Melbourne researchers studying the DNA of these younger tumours have described the mutation patterns as looking almost like a distinct disease. Genetics hasn’t changed that fast. Something about how we’re living — diet, gut health and metabolic factors among the leading suspects — has.
It helps decide how much energy you extract from food
Different bacterial populations extract different amounts of energy from identical meals. In one of the more striking pieces of research behind this claim, scientists at Washington University transplanted gut bacteria from obese mice into lean, germ-free mice. The lean mice gained significantly more body fat than mice given bacteria from lean donors — without any change to their diet. Same food. Different microbiome. Different outcome. It’s one reason weight management research has shifted so much attention toward the gut over the past fifteen years.
What's Quietly Wrecking It
Ultra-processed food
Emulsifiers — the additives that stop oil and water separating in packaged food — have been shown in laboratory and animal studies to thin the protective mucus layer lining the gut wall, the barrier that normally keeps bacteria out of the bloodstream. When that barrier is compromised, bacterial fragments can slip through, a state researchers call increased intestinal permeability. The immune system responds, and low-grade inflammation can become chronic. Australia doesn’t come out of this well. Research led by Deakin University’s Institute for Physical Activity and Nutrition has found ultra-processed foods now make up close to 40 percent of the average Australian adult’s daily energy intake — among the higher rates recorded internationally. Fibre — the primary fuel source for beneficial gut bacteria — tells a similar story. CSIRO research has found the average Australian adult eats around 23 grams of fibre a day, well short of the 25–30 gram target recommended in national dietary guidelines, and that roughly 83 percent of Australians fail to meet it. Vegetable intake is worse still: 2020–21 National Health Survey data shows only 8.7 percent of Australian adults meet the recommended daily vegetable serves, with men (4.4 percent) faring markedly worse than women (12.8 percent).
Chronic stress
Under stress, blood flow shifts away from the gut toward the brain and muscles, digestion slows, and gut motility becomes irregular. Studies have shown measurable shifts in gut bacteria composition within days of sustained psychological stress. We’re not exactly short of stressors. Constant news cycles, notification-driven work and social media scrolling keep plenty of Australians in a low-grade fight-or-flight state that the gut registers every time.
Antibiotics
A single course of broad-spectrum antibiotics can reduce gut microbiome diversity substantially, and for some people, certain bacterial species never fully return. That’s not an argument against taking antibiotics when you genuinely need them — untreated infections carry their own serious risks — but it is a case for using them appropriately and rebuilding gut diversity afterwards. Australia has one of the highest antibiotic prescribing rates in the developed world. A Safety and Quality watchdog analysis found as many as a third of community antibiotic prescriptions may be unnecessary, and one GP audit found 57 percent of a sample of prescriptions were inappropriate in choice, dose or duration. Every course has a biological cost worth weighing.
A narrow diet
Research from the ZOE/British Gut Project, led by King’s College London epidemiologist Professor Tim Spector, found that people eating 30 or more different plant foods a week had substantially more diverse gut microbiomes than those eating fewer than 10 — and diversity is strongly linked to gut resilience. Most people on a typical Western diet, Australia included, eat somewhere around 8 to 10 different plant foods a week. Every bacterial species does a slightly different job. When a species disappears from your gut, its function disappears with it — quietly, invisibly, often over years.
A Practical 5-Day Reset
None of this requires a juice cleanse or a cupboard full of supplements. Gut bacteria respond to dietary change within days, not months — a fact borne out repeatedly in microbiome research. Here’s a simple five-day structure clinicians commonly recommend as a starting point.
Day 1 — Cut the disruptors. Ultra-processed food, added sugar and alcohol, out for five days. Not forever — just long enough to stop active damage and let the gut lining start to repair.
Day 2 — Add one fermented food daily. Live-culture yoghurt, kefir, kimchi, sauerkraut or kombucha — one serve, every day from here on. A widely cited 2021 Stanford study published in Cell found that a diet high in fermented foods reduced markers of inflammation and increased microbiome diversity over ten weeks, an effect a high-fibre diet alone didn’t replicate over the same period.
Day 3 — Eat a plant first at every meal. Any vegetable or salad, before anything else on the plate. The fibre slows the absorption of what follows, blunts blood glucose spikes, and feeds beneficial bacteria first.
Day 4 — Add a prebiotic food to every meal. Garlic, onion, leek, asparagus, oats and banana all feed the bacteria already living in your gut. One of these at every meal, then keep it going as a habit.
Day 5 — Introduce one new plant food. Just one — a vegetable, legume, herb or spice you don’t normally eat. Different plants feed different bacterial species. The long-term goal is something like 30 different plant foods a week; most Australians are sitting closer to 8–10. You don’t need to get there this week. You need to add one plant. Then another next week.
The Common Thread
Mood, immune resilience, body weight, inflammation, energy — each one has a documented connection back to the gut. That’s not a reason to treat the microbiome as a cure-all, but it is a reason to stop treating gut health as a niche wellness topic and start treating it as basic preventive health, on the same footing as sleep, movement and blood pressure. The next time you’re deciding what’s for dinner, it’s worth remembering: you’re not just feeding yourself. You’re feeding roughly 38 trillion tenants who have a say in how you feel tomorrow.
Ready to Take Your Gut Health Seriously?
If you’ve been dealing with unexplained fatigue, low mood, digestive symptoms or a body that just doesn’t feel right, your gut is worth investigating properly rather than guessing at another supplement. A conversation is the easiest place to start.
References
- Sender, R., Fuchs, S., & Milo, R. (2016). Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002533
- Australian Bureau of Statistics (2022). National Study of Mental Health and Wellbeing, 2020–2022. https://www.abs.gov.au/statistics/health/mental-health/national-study-mental-health-and-wellbeing/latest-release
- Australian Institute of Health and Welfare. Cancer data in Australia — An overview of colorectal cancer in Australia. https://www.aihw.gov.au/reports/cancer/cancer-data-in-australia/contents/cancer-data-commentaries/an-overview-of-colorectal-cancer-in-australia
- ABC News (2025). Cancer rates in Australians under 50 are rising at a pace that’s alarming researchers. https://www.abc.net.au/news/2025-07-07/cancer-diagnosis-rates-under-50s-rising-causes-four-corners/105495620
- University of Sydney (2026). Bowel cancer rates are rising among young people. https://www.sydney.edu.au/news-opinion/news/2026/03/02/bowel-cancer-rates-are-rising-among-young-people.html
- Turnbaugh, P.J., Ley, R.E., Mahowald, M.A., et al. (2006). An obesity-associated gut microbiome with increased capacity for energy harvest. Nature, 444, 1027–1031.
- Deakin University, Institute for Physical Activity and Nutrition. Australia is one of the world’s biggest prescribers of antibiotics, but often they’re unnecessary and related IPAN research on ultra-processed food intake in Australian adults. https://www.deakin.edu.au/research/research-news-and-publications/articles/australia-is-one-of-the-worlds-biggest-prescribers-of-antibiotics-but-often-theyre-unnecessary
- CSIRO (2019). Gut health and weight loss — Health & Biosecurity report. https://www.csiro.au/~/media/News-releases/2018/gut-health-to-tackle-obesity/1800623HBGutHealthWeightLossReportJan19.html
- Australian Bureau of Statistics / Cancer Australia, National Cancer Control Indicators (2020–21 National Health Survey data). Vegetable consumption. https://ncci.canceraustralia.gov.au/prevention/diet/vegetable-consumption
- Australian Commission on Safety and Quality in Health Care (2024). Antimicrobial use in the community: 2023. https://www.safetyandquality.gov.au/sites/default/files/2024-08/antimicrobial_use_in_the_community_2023.pdf
- Leeming, E.R., Johnson, A.J., Spector, T.D., Le Roy, C.I. (2019). Effect of Diet on the Gut Microbiota: Rethinking Intervention Duration. Nutrients, 11(12), 2862. (ZOE/British Gut Project plant-diversity findings, Prof. Tim Spector)
- Wastyk, H.C., Fragiadakis, G.K., Perelman, D., et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137–4153. (Stanford fermented foods study)
This article is general information, not medical advice. If you’re experiencing persistent fatigue, digestive symptoms or mood changes, speak with your GP.
