Large study ranks 661 gut bacteria by health impact, linking microbes to disease and diet

For years, the gut microbiome has occupied an increasingly central place in health science—a living ecosystem inside us that researchers believe shapes everything from weight to heart disease risk. What makes it compelling is that unlike your genes, your microbiome can change. You can alter it through diet, through supplements, through deliberate choice. But until now, the science has remained scattered. We knew the microbiome mattered. We didn't know which microbes mattered most, or how to measure that mattering in any systematic way.

A study published in Nature this month begins to change that. Researchers analyzed the gut bacteria, dietary patterns, and health markers of more than 34,500 people across the United States and the United Kingdom, drawing data from the ZOE PREDICT program, a microbiome testing initiative run by the company ZOE. Using machine learning, they linked 661 distinct bacterial species to key health indicators: body mass index, triglyceride levels, blood glucose, HbA1c, and other markers of cardiometabolic risk. The result is the first comprehensive ranking system for gut bacteria—a scorecard that identifies which microbes correlate with health and which correlate with disease.

The ranking works on a scale from zero to one. Bacteria scoring near zero show up more often in healthy people. Those scoring near one cluster in people with obesity, diabetes, and other conditions. The researchers focused particular attention on the fifty most favorable species and the fifty most unfavorable ones, though they ranked all 661. The pattern that emerged was striking: people at a healthy weight carried, on average, 5.2 more of the top-ranked beneficial bacteria than people with obesity. People without disease had significantly higher counts of favorable microbes than those with disease. The correlation held across multiple conditions and multiple populations.

But the study went further. The researchers wanted to know whether changing diet could actually shift the microbiome in a healthier direction. They examined data from two separate dietary intervention trials. In one, participants followed a personalized dietary program designed to improve their microbiome. In the other, participants took a prebiotic supplement. Both groups saw significant changes. Certain bacteria increased: Bifidobacterium animalis, which appears in dairy products and in people who eat more dairy; unknown species of Lachnospiraceae bacteria linked to vegan diets; and another Lachnospiraceae species associated with vegetarian eating patterns. The unfavorable bacteria declined. Diet, in other words, worked. You could measure the effect.

What makes this work particularly valuable is its scale and its honesty about its limits. The researchers analyzed hundreds of thousands of data points from tens of thousands of people. They discovered not just known bacterial species but previously uncharacterized ones—microbes that science had never formally described before. Yet they are careful to note that this is observational research. They can show correlation. They cannot yet prove that changing your microbiome causes better health outcomes, only that the two move together. Future studies will need to establish causation more definitively, and they will need to include more diverse populations than the current dataset represents.

Still, what emerges from this work is a kind of map. For the first time, researchers have a systematic way to think about which bacteria matter and why. They have evidence that diet can shift the microbiome measurably and relatively quickly. They have identified hundreds of specific microbial targets for future intervention. The gut microbiome remains a frontier—there is far more to learn. But this study suggests the frontier is beginning to come into focus.