Patients with type 2 diabetes have what is called insulin resistance, an inability to properly use insulin. The pancreas will make more insulin to keep blood glucose levels normal however, eventually, the pancreas can’t keep up and drugs may need to be taken. The most common drug to treat type 2 diabetes is metformin. A large team of scientists throughout Europe and China published a study in Nature showing that metformin affected gut bacteria in type 2 diabetics.

The researchers analyzed stool samples from 784 individuals with and without type 2 diabetes and looked at the effects that metformin had on gut bacteria. Metformin is usually prescribed in high doses and because it is a chronic disease, patients end up taking the drug often for many years. Based just on stool samples, they were not able to identify which sample was from a diabetic patient or control unless they took metformin. Type 2 diabetics who were on metformin had higher levels of E. coli and lower levels of I. bartletti than the controls or type 2 diabetics not taking metformin.

Studying the bacteria that changed in abundance in the gut suggested to the scientists that butyrate and propionate had elevated production. These two short chain fatty acids are associated with lowering blood glucose levels.

Importantly, this study helps explain some existing studies with conflicting results comparing gut bacteria of people with and without type 2 diabetes. This was most likely due to the fact that there were more individuals taking metformin in one study than another and this was not controlled for.

This study not only informs us on what is happening with gut microbes after taking metformin but also shines a light onto the importance of controlling for all external factors in microbiome studies, including treatments that could have confounding effects.