Good gut bacteria may signal diabetes years ahead

Researchers have identified six types of gut bacteria that appear to signal a higher risk of developing type 2 diabetes years before any symptoms appear, according to a large Swedish study. The findings, published in Cell Reports Medicine, suggest that changes in the gut microbiome may precede the disease rather than being a consequence of it.

The study involved 4,685 Swedish adults whose stool samples were analyzed for gut bacteria. Over an average follow-up of 5 years, 383 participants developed diabetes. Researchers found common microbial patterns among those who later got the disease.

Of the 9 bacterial species tied to future diabetes risk, 6 were associated with increased risk, while 3 were linked to lower risk. The work was part of the European HealthFerm project led by Chalmers University of Technology.

Gut bacteria changes show up years before diagnosis

Because the study tracked participants over time, researchers could see microbiome differences long before diabetes was diagnosed. That timing matters: it strengthens the idea that certain bacteria may play an early role in disease development, not just reflect an existing condition.

Study author Gaël Toubon, a postdoctoral researcher at Chalmers, said previous studies mostly compared people who already had the disease with those who did not. “Finding microbial signatures years before diagnosis strengthens the idea that the gut microbiome may play a role early in disease development, rather than simply reflecting established diabetes,” he told the outlet.

The study also turned up a surprise.

Akkermansia muciniphila, a bacterium often considered beneficial for metabolic health, was higher in people who later developed diabetes.

That seems counterintuitive.

But the team thinks diet may explain it. Normally, A. muciniphila feeds on dietary fiber. When fiber intake is low, the bacterium may start breaking down the protective mucus layer lining the gut. That could weaken the gut barrier, let other bacteria interact more closely with the intestinal lining, and potentially fuel inflammation and insulin resistance.

For the people most affected by the condition—those with limited access to high-fiber foods or who struggle to change long-standing eating habits—this finding adds a layer of complexity. It suggests that even “good” gut bacteria can turn harmful depending on what a person eats. That makes diet not just a general health recommendation but a direct lever on how the microbiome behaves.

Another species, Coprococcus catus, showed a threshold effect: very low levels were linked to higher diabetes risk, but higher levels were not.

Fiber intake may determine whether bacteria help or harm

They emphasize that the findings are not yet ready for clinical use. Rikard Landberg, lead author and professor at Chalmers, said the microbiome could provide additional biological information that complements traditional risk factors like blood glucose, body weight, and family history. But he stressed that results need to be replicated in diverse populations, standardized methods must be developed, and studies must show that microbiome-based prediction improves decision-making beyond existing tools.

Because the study was observational, it cannot prove the bacteria themselves cause the disease. Further research in other populations is needed to confirm the link.

If future studies establish that certain gut bacteria actively contribute to diabetes, the microbiome could become a target for personalized prevention. Unlike genetic risk factors, gut bacteria can be influenced by lifestyle changes—especially diet.

Toubon noted that dietary fiber is a major nutrient source for many gut bacteria and can influence which microbes thrive and how they function. “When there is plenty of fiber available (through diet), gut bacteria produce compounds that help support the gut barrier and regulate metabolism and inflammation,” he said.

He added that Akkermansia muciniphila was linked to higher diabetes risk mainly among people with lower fiber intake, while the association was much weaker in those eating more fiber. “While our findings don’t prove that increasing fiber will change disease risk, they definitely add to the growing evidence supporting a fiber-rich diet for metabolic health,” Toubon said.

Landberg advised focusing on overall dietary patterns rather than individual bacteria. “Rather than focusing on individual ‘good’ or ‘bad’ bacteria, it’s better to support a healthy and diverse gut microbiome through long-term dietary and lifestyle habits, which are also known to promote metabolic health,” he said.