For decades, gut health research has been working toward one deceptively simple question: what does a healthy microbiome actually look like? Not just “more diversity is better” as a general principle, but a concrete, reproducible answer about which bacteria show up in healthy people and which ones don’t.

A landmark study published in Nature in December 2025 took the biggest step yet toward answering it.

The Largest Gut Microbiome Study Ever Conducted

Researchers from ZOE, the University of Trento, and King’s College London analyzed gut microbiome data from over 34,000 people across the US and UK — making it one of the largest microbiome studies in history. Participants provided detailed dietary records alongside clinical health data including BMI, blood glucose, triglycerides, and cardiovascular risk markers (1).

Using machine learning to link gut microbe species to diet and common health risk factors, researchers focused in on 661 non-rare microbial species and identified the 50 most favorably associated with good health and the 50 most unfavorably associated with good health. The result is a system they named the ZOE Microbiome Health Ranking 2025 — essentially a scored list of gut bacteria ranked by how consistently they appear alongside better or worse health outcomes (1). Kara Lydon

The study also identified 22 newly discovered beneficial microbial species — bacteria that hadn’t previously been formally described in science. Nourishnutritionblog

What Did They Find?

The pattern that emerged across all five study cohorts was remarkably consistent. Healthy individuals had more favorable bacteria, while people who were overweight or ill had more unfavorable bacteria — a result that held stable across populations. ScienceDirect

More specifically, individuals with cardiometabolic conditions like type 2 diabetes were more likely to have higher numbers of unfavorably ranked microbes and lower levels of favorably ranked ones than someone without those conditions — and were also more likely to have a higher BMI. ScienceDirect

The researchers then validated the ranking against over 7,800 additional public microbiome samples covering diseases including colorectal cancer, inflammatory bowel disease, type 2 diabetes, and cardiovascular disease. Healthy individuals had, on average, 3.6 more favorable species and 1.6 fewer unfavorable species than patients — results that exceeded those of conventional indicators, demonstrating that the nature of the species present matters more than their quantity. ScienceDirect

Can You Actually Change Your Bacteria?

This is where the research gets practically useful. The team didn’t just look at who had which bacteria — they also analyzed two dietary intervention clinical trials involving 746 participants to see whether eating patterns could shift microbiome composition.

Clinical trials confirmed that healthy eating patterns significantly increased favorable microbial species and decreased unfavorable ones over time. In other words, what you eat doesn’t just affect your health in general — it appears to directly shift the balance of your gut microbiome toward or away from the bacteria associated with better health outcomes (1). ASM Journals

A Note on What This Research Can and Can’t Tell Us

It’s worth being honest about the limits here, because the study’s authors were. The researchers themselves noted that causal inference is not possible without prospective cohort studies and interventional clinical trials. This means the ranking tells us that certain bacteria are consistently found alongside good health — not definitively that those bacteria cause it. The direction of the relationship is still being worked out. MDPI

Some outside researchers have also noted that labeling bacteria as simply “good” or “bad” may not capture the full complexity of how microbial communities interact — context matters, and a bacterium that is unfavorable in one scenario might behave differently in another.

That said, the scale and consistency of the findings across wildly different populations is significant, and the fact that dietary changes produced measurable shifts in the ranking is a meaningful signal.

What This Means for How You Think About Gut Health

For most people, the practical takeaway from this research isn’t about chasing a specific bacterium. It’s that your gut microbiome is responsive — to what you eat, how consistently you eat it, and the overall quality of your diet over time. The bacteria associated with better health aren’t mysterious or inaccessible; they’re the ones that tend to thrive on fiber-rich, varied, minimally processed diets.

Supporting a balanced gut environment — reducing populations of disruptive bacteria while creating conditions where beneficial ones can flourish — is exactly the direction this kind of research points. IgY-based supplements work along those lines, targeting dysbiotic bacterial populations to help restore the kind of microbial balance this research now has a much clearer name for.

The Bottom Line

Science just got measurably closer to defining what a healthy gut microbiome actually looks like. The ZOE Microbiome Health Ranking 2025 isn’t the final word, but it’s the most rigorous attempt yet — and the signal it sends is clear: the quality and diversity of what you eat shapes which bacteria call your gut home, and that matters for your health in ways that go well beyond digestion.

Learn more about supporting your gut at igynutrition.com.

References

  1. https://www.nature.com/articles/s41586-025-09854-7
  2. https://medicalxpress.com/news/2025-12-large-scale-gut-microbiome-microbes.html
  3. https://www.medscape.com/viewarticle/diet-modulated-gut-species-predict-bmi-and-cardiometabolic-2026a1000dlt
  4. https://zoe.com/learn/new-microbiome-breakthrough
  5. https://www.nutritioninsight.com/news/zoe-gut-microbiome-health-ranking-bacteria.html