Weight-loss surgery has become one of the most effective treatments for severe obesity and type 2 diabetes. Many people who undergo these operations lose large amounts of weight and see major improvements in their blood sugar levels.
Some patients even experience remission of type 2 diabetes, meaning their blood sugar returns to near-normal levels without needing as much medication.
However, doctors have long noticed that the results are not the same for everyone. Some patients improve dramatically after surgery, while others experience more limited benefits. Scientists have been trying to understand why these differences happen.
Now, a new study led by researchers at the University of Gothenburg suggests that gut bacteria may play a much larger role than previously thought.
The findings were published in the journal Nature Metabolism.
Researchers discovered that changes in the gut microbiome after bariatric surgery were strongly linked to long-term improvements in metabolism and blood sugar control.
The gut microbiome refers to the enormous community of bacteria, viruses, and other microorganisms living inside the digestive system. Scientists increasingly believe that these tiny organisms affect many aspects of health, including digestion, immunity, weight control, inflammation, and metabolism.
In recent years, researchers have also found growing evidence that gut bacteria may influence type 2 diabetes.
The new study focused on two of the most common weight-loss surgeries performed worldwide: gastric bypass and sleeve gastrectomy.
Gastric bypass changes the digestive system by creating a small stomach pouch and rerouting part of the small intestine. Sleeve gastrectomy removes a large portion of the stomach, leaving behind a smaller tube-shaped stomach.
Both procedures reduce food intake and often lead to substantial weight loss. They are also known to improve blood sugar levels, but scientists now believe the benefits go beyond weight loss alone.
To better understand what happens after surgery, researchers analyzed the gut microbiota of patients before and after their procedures. They then tracked blood sugar control and metabolic health for as long as five years after surgery.
The researchers used advanced genetic analysis methods called metagenomics to study the microbial genes present inside the gut. This allowed them to identify not only which bacteria were present, but also what functions those bacteria were capable of performing.
The results showed that patients who experienced the greatest improvements in blood sugar control also showed larger changes in their gut bacteria.
These patients tended to have greater bacterial diversity and higher microbial gene richness. In simple terms, their gut ecosystems became more varied and biologically active.
Researchers also found stronger evidence of bacterial fermentation processes, especially the production of substances such as butyric acid.
Butyric acid is a short-chain fatty acid produced when certain gut bacteria break down dietary fiber. Previous research has shown that butyric acid may support gut health, reduce inflammation, and improve the body’s ability to regulate blood sugar.
According to the researchers, this suggests that the gut microbiome itself may actively contribute to diabetes improvement after surgery rather than simply responding to weight loss.
Valentina Tremaroli, a microbiologist at the University of Gothenburg, explained that bariatric surgery appears to reshape the gut ecosystem in many different ways. When this remodeling increases bacterial diversity, it is often linked with better metabolic health.
The researchers also observed some differences between the two types of surgery.
Gastric bypass tended to create more consistent changes in the gut microbiome across different patients. Sleeve gastrectomy, however, produced more individualized responses, meaning patients’ gut bacteria changed in more varied ways.
The study’s researchers believe these findings could eventually help doctors predict which patients are most likely to benefit from surgery and develop more personalized treatment strategies.
The findings also raise the possibility that future treatments may directly target the microbiome itself.
Fredrik Bäckhed, professor at the University of Gothenburg, said the study suggests gut bacteria are not simply passive bystanders. Instead, they may actively influence whether diabetes improves after surgery.
This could eventually lead to new therapies using diet changes, probiotics, or other microbiome-based approaches to reproduce some of the benefits of bariatric surgery without needing an operation.
The study highlights how much scientists still have to learn about the relationship between gut bacteria and human health.
For many years, obesity and diabetes were mainly viewed as problems involving body weight, insulin, diet, and exercise. Now researchers increasingly understand that trillions of microorganisms inside the body may also play a major role.
Looking closely at the findings, the study appears especially strong because it followed patients over several years and used highly detailed genetic analysis techniques.
However, researchers also note that more studies are needed before microbiome-targeted treatments become part of standard diabetes care.
Scientists still need to determine exactly which bacterial changes are most important and whether those changes directly cause better blood sugar control or are simply associated with it.
Even so, the research offers exciting new clues about why some people experience dramatic diabetes improvements after bariatric surgery while others do not.
The findings suggest that future obesity and diabetes treatments may eventually focus not only on weight loss itself, but also on reshaping the gut microbiome to improve metabolic health.
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