A landmark Swedish study published Tuesday found that the composition of bacteria in your gut can reliably flag your risk of developing type 2 diabetes years before blood sugar levels reach any clinical threshold — and that whether those bacteria protect you or harm you may depend almost entirely on how much fiber you eat
The study, published in Cell Reports Medicine, was led by researchers at Chalmers University of Technology in Gothenburg and tracked 4,685 Swedish adults whose gut microbiomes were profiled through stool samples. Over an average follow-up of five years, 383 of those participants went on to develop type 2 diabetes. Nine gut bacteria were consistently detectable at distinctive levels in those who later became diabetic — and those patterns were visible years before diagnosis.
“Our study was able to show changes in the gut microbiota several years before the disease developed,” said Gaël Toubon, a postdoctoral researcher in food science at Chalmers’ Department of Life Sciences. “This could indicate that the composition of the microbiome plays a role in the development of diabetes, and not the other way around.”
That methodological point matters considerably. Most prior research linking gut bacteria to type 2 diabetes used cross-sectional designs — comparing people who already had the disease against healthy controls. That approach identifies associations, but can’t establish whether the microbiome change preceded the illness or resulted from it. The Chalmers team’s prospective design, which followed healthy adults forward in time, is one of the strongest epidemiological arguments yet that gut microbiome shifts come first.
A Familiar “Healthy” Bacterium Turns Risk Marker
The study’s most striking finding involves Akkermansia muciniphila, a bacterium that has attracted substantial commercial and scientific attention as a potential therapeutic probiotic for metabolic health. Several supplement companies now market products targeting this species as a health intervention. In the Chalmers cohort, however, participants who went on to develop diabetes had elevated levels of it.
The explanation does not mean the bacterium itself is dangerous — it means the dietary context determines everything
“Under favorable conditions, this bacterium feeds on the fiber we get from our diet,” Toubon explained. “But when our fiber intake is too low, it instead starts to break down the gut’s protective mucus layer.”
That is the biochemical switch at the heart of the finding. Akkermansia muciniphila can use two substrates: dietary fiber, when it is available, or mucin — the glycoprotein that forms the protective mucus lining of the intestinal wall. When it is degrading fiber, the bacterium produces short-chain fatty acids including acetate and propionate that help maintain the gut’s epithelial barrier and reduce systemic inflammation. When fiber is scarce and it switches to mucin as a fuel source, the protective lining thins. Other bacteria that would normally be separated from the intestinal lining by that mucus layer come into direct contact with it, triggering inflammatory cytokines. That sustained, low-grade inflammation is a well-established driver of insulin resistance, the metabolic condition that precedes type 2 diabetes.
In other words, elevated Akkermansia muciniphila in a stool sample is not inherently a problem. Elevated Akkermansia muciniphila in someone eating insufficient dietary fiber is a measurable warning
This has direct implications for the probiotic supplement market. Products marketed around this bacterium do not carry a caveat that the intervention may be neutral or even counterproductive in people who do not simultaneously increase their fiber intake. The Chalmers finding suggests that label should exist
Why Low Fiber Turns a Protective Bacterium Into a Risk Factor
The mechanistic pathway the study documents is consistent with a broader body of evidence on short-chain fatty acids (SCFAs) and metabolic disease. When gut bacteria — including Akkermansia muciniphila, among others — ferment dietary fiber through saccharolytic fermentation, they produce SCFAs including butyrate, propionate, and acetate. Butyrate is the primary energy source for colonocytes (gut wall cells), supports the formation of tight junctions between those cells, and has been independently shown to reduce insulin resistance. Diets low in fiber deplete SCFA production — and then simultaneously trigger the mucin-degradation pathway in bacteria equipped to use it.
A second bacterium implicated in the study, Coprococcus catus, belongs to the Lachnospiraceae family and is itself a butyrate producer. It was associated with diabetes risk only when present in very small quantities — below the threshold where meaningful butyrate production would occur. Above that level, the risk association disappeared. This threshold effect is consistent with the SCFA hypothesis: it is not the presence of these bacteria but whether they are present in sufficient abundance to perform their metabolic function.
The study profiled participants’ microbiomes using stool samples and identified bacterial composition through standard metagenomic techniques. Stool sampling was used because it represents the microbial population of the colon — which contains by far the densest microbial community in the human body and accounts for the gut bacteria most relevant to metabolic disease. The nine bacterial species identified as T2D-associated are detectable through existing clinical lab technology, making them plausible candidates for future diagnostic biomarker panels.
Can a Stool Test One Day Screen for Diabetes?
That is the question the Chalmers researchers are building toward, and their language is deliberately cautious about timing
“In future, these bacteria could be used as biomarkers to identify people at risk of developing type 2 diabetes,” Toubon said. “Risk factors such as obesity, heredity and blood glucose levels could be supplemented with a stool sample to better predict the risk of developing the disease and enable preventive measures to be introduced.”
Current clinical screening for type 2 diabetes relies on a combination of fasting blood glucose, HbA1c (a three-month average of blood sugar), body weight, age, and family history. These tests catch metabolic disruption once it is already underway. A stool-based biomarker panel could, in principle, identify elevated risk years earlier — during a window when diet and lifestyle interventions are most likely to prevent the disease from developing at all.
The practical advantage of the microbiome over genetic risk markers is that it is modifiable. Genome sequencing can identify someone at elevated genetic risk for type 2 diabetes, but their DNA cannot be changed. The gut microbiome can be altered through diet, and in theory through probiotic or prebiotic interventions
“If the role of the gut microbiota in the development of diabetes is confirmed, our microbiome could become a target for personalised preventive strategies, as the gut microbiota, unlike our genes, can be modified through lifestyle and diet,” Toubon noted
The researchers conducted the study as part of the EU-funded HealthFerm project, which focuses on fermented foods and the gut microbiome
What the Study Does and Does Not Prove
The Chalmers team is explicit that their findings require external validation before they can inform clinical guidelines. The cohort was drawn exclusively from Swedish adults — a relatively homogeneous population in dietary terms — and it is not established that the same nine bacteria would hold as predictive markers in populations with different ancestries, diets, or environmental exposures
The prospective design substantially strengthens the causal argument compared to prior studies, but confounding remains possible. Participants who developed diabetes may have differed from those who did not in ways not fully captured by the study’s controls. The study also cannot specify precisely how far in advance the bacterial signals are detectable — “several years” is the characterization offered, not a precise number.
“We cannot yet give that kind of dietary advice,” said Rikard Landberg, Professor at Chalmers’ Department of Life Sciences and lead author of the study, referring to specific dietary prescriptions based on microbiome results. “But, at a general level, the study’s findings support current recommendations to eat foods rich in fibre from fruit, vegetables, legumes and wholegrains.”
What the study does confirm is that diet-microbiome interaction research cannot be conducted in isolation from either component. A bacterium classified as “beneficial” in one dietary context may be classified as a risk factor in another. That is not a contradiction in the science — it is the science becoming more precise
800 Million People, a Modifiable Risk Factor
The scale of the problem gives urgency to the research. According to the World Health Organization, the number of adults living with diabetes has more than doubled since the 1990s. Today 800 million people are living with the disease, and more than 90 percent of those cases are type 2. Most type 2 diabetes is preventable or delayable with lifestyle intervention — which makes an early-warning signal that precedes clinical thresholds by years medically significant, even if its clinical application remains years away.
The full study citation: Gaël Toubon et al., Cell Reports Medicine, published July 7, 2026. DOI: 10.1016/j.xcrm.2026.102835
Frequently Asked Questions
Can gut bacteria actually predict type 2 diabetes before it develops?
Based on the Chalmers University study published today, the answer appears to be yes — with the caveat that the finding needs to be validated in additional large-scale studies before it can inform clinical practice. The study tracked 4,685 Swedish adults for an average of five years and found that nine specific gut bacteria showed distinctive patterns in people who later developed type 2 diabetes, identifiable years before any clinical threshold was crossed. The prospective study design — which follows healthy people forward in time rather than studying patients who already have the disease — allows researchers to argue that the microbiome changes preceded the illness, not the reverse.
What is Akkermansia muciniphila, and is it good or bad?
Akkermansia muciniphila is a naturally occurring gut bacterium that has attracted significant attention as a potential probiotic for metabolic health. Multiple supplement companies now sell products targeting it. Whether it is beneficial or harmful depends on your dietary fiber intake. When fiber is sufficient, the bacterium consumes it and produces short-chain fatty acids that protect the gut lining and reduce inflammation. When dietary fiber is insufficient, the bacterium switches to consuming the protective mucus layer of the intestinal wall — which can allow other bacteria to trigger inflammation linked to insulin resistance and type 2 diabetes. The Chalmers study found elevated Akkermansia muciniphila in people who later developed diabetes, which the researchers attribute to low fiber intake rather than the bacterium itself being harmful.
Should I take an Akkermansia muciniphila supplement to protect against diabetes?
Based on current evidence, taking an Akkermansia muciniphila supplement without also increasing dietary fiber intake may provide little or no benefit — and in low-fiber diets, could theoretically increase the bacterium’s mucin-degrading activity. The Chalmers finding suggests that the dietary environment is the determining variable, not the bacterium’s presence alone. The researchers recommend eating a diet rich in fiber from fruit, vegetables, legumes, and whole grains as the most direct action supported by current evidence. Consult a healthcare provider before starting any supplement regimen, particularly if you have metabolic risk factors.
How far away is a stool-based diabetes screening test?
The study authors describe a potential future in which stool microbiome testing supplements existing risk factors — blood glucose, HbA1c, BMI, family history — to improve early identification of people at risk. However, the nine bacteria identified need to be validated in additional large-scale studies across diverse populations before they could be used clinically. The researchers did not specify a timeline. A test would also require regulatory approval from health authorities before clinical use. Most researchers in the field estimate that clinical translation of microbiome biomarkers, when successful, takes a decade or more from initial discovery.
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