If you’ve spent any time in the wellness space over the past five years, you’ve heard the phrase “gut health” approximately ten thousand times. Probiotics, fermented foods, the gut-brain axis — it’s been the wellness topic of the decade, and frankly, a lot of the conversation has outpaced the evidence.

Which is why I was genuinely excited to see the results of the CSIRO’s Australian Gut Health Project, published last month. It’s one of the most comprehensive microbiome studies ever conducted in this country — 12,000 participants, tracked over two years, with detailed dietary records, stool samples, mental health assessments, and lifestyle data.

Here’s what they found. Some of it will confirm what you’ve heard. Some of it might surprise you.

Dietary Diversity Matters More Than Any Single Food

The most consistent finding across the entire study was that microbial diversity — having a wide range of different bacterial species in your gut — was most strongly associated with dietary diversity. Not any specific superfood. Not a particular supplement. Just eating a wide variety of plant foods.

Participants who consumed 30 or more different plant foods per week had significantly more diverse gut microbiomes than those who ate fewer than 10. And microbial diversity was consistently linked to better mental health scores, lower inflammation markers, and healthier body weight.

This isn’t a new idea — the American Gut Project found similar patterns — but the scale of this Australian data makes it compelling. The takeaway is simple: eat more different plants. Vegetables, fruits, legumes, whole grains, nuts, seeds, herbs, spices. Variety is the thing.

Probiotics Were a Mixed Bag

The study found that regular probiotic supplement use was not consistently associated with improved gut diversity. In some cases, participants taking high-dose probiotic supplements actually had less microbial diversity than those who didn’t supplement at all. The finding aligns with growing research suggesting the probiotic supplement market has significantly overpromised.

Fermented foods, on the other hand, showed a much clearer positive association. Participants who regularly consumed yoghurt, kimchi, sauerkraut, kefir, or miso had greater microbial diversity. The hypothesis is that whole fermented foods deliver beneficial bacteria alongside the substrates they need to survive — something a capsule can’t replicate.

The Gut-Brain Connection Is Real, But Complicated

The study confirmed a significant relationship between gut microbiome composition and mental health. Participants with greater microbial diversity reported lower rates of anxiety and depression, even after controlling for exercise, sleep, and socioeconomic factors.

However — and this is important — the researchers couldn’t establish clear directionality. Does a healthy gut lead to better mental health? Or does better mental health lead to healthier eating habits, which lead to a healthier gut? Probably both, in a feedback loop that researchers are still mapping.

What the study did find was that chronic stress appeared to reduce microbial diversity independently of diet. Participants who reported high stress levels had less diverse microbiomes even when their diets were varied. This suggests that stress management isn’t just good for your head — it’s good for your gut.

Australians Aren’t Eating Enough Fibre (Still)

This finding was depressing but unsurprising. The average fibre intake among study participants was 22 grams per day. The recommended intake is 25-30 grams. And the participants who showed the best gut health markers were consuming closer to 35-40 grams daily.

Fibre is the primary fuel source for beneficial gut bacteria. Without adequate fibre, those bacteria decline, and less beneficial species can take over. The CSIRO researchers specifically called out the low intake of legumes in Australian diets as a missed opportunity — lentils, chickpeas, and beans are among the best fibre sources available and they’re cheap.

If you do nothing else after reading this article, add an extra serve of legumes to your week. Your gut will notice.

What to Actually Do With This Information

I know research summaries can feel abstract, so here’s my practical translation:

Aim for 30 different plant foods per week. This sounds like a lot, but it includes herbs and spices. If you cook a stir-fry with garlic, ginger, broccoli, capsicum, mushrooms, cashews, and brown rice, that’s seven right there.

Eat fermented foods regularly. A dollop of yoghurt at breakfast, kimchi with dinner, miso soup a couple of times a week. Small, consistent amounts seem to matter more than large occasional doses.

Prioritise fibre, especially legumes. Lentil soup, chickpea curry, black beans in tacos. These are some of the most affordable, accessible health foods available.

Manage your stress. Not just for your mind, but for your microbiome. Movement, sleep, social connection, and yes, meditation — they all contribute.

Be sceptical of expensive supplements. The evidence for probiotic capsules remains underwhelming. Spend that money on a wider variety of groceries instead.

The gut health conversation has been dominated by product marketing for too long. This CSIRO study is a reminder that the fundamentals — diverse food, enough fibre, less stress — are still the most powerful tools we have.

Jess Morley is a Brisbane-based wellness writer and yoga instructor. She writes about evidence-based wellbeing at SoulShine.