Metformin, the world’s most prescribed diabetes drug, has long puzzled researchers with its wide-ranging effects beyond glucose control. Now, a new study from Kobe University provides the first clinical evidence that metformin alters blood metal levels in humans, potentially explaining some of its broader benefits.

The research, published in BMJ Open Diabetes Research & Care, analyzed around 200 patients with type 2 diabetes treated at Kobe University Hospital. Half were receiving metformin and half were not. Blood serum samples were tested for levels of copper, iron, zinc, and metal deficiency indicators.

According to study leader Professor Wataru Ogawa, these changes are clinically meaningful: “Decreases in copper and iron concentrations and an increase in zinc concentration are all considered to be associated with improved glucose tolerance and prevention of complications.”

Metformin’s ability to bind metals, particularly copper, had been demonstrated in earlier chemical studies, but this is the first time such effects have been shown in humans. The findings may help explain why the drug has beneficial effects on cancer, inflammation, and atherosclerosis, in addition to lowering blood sugar.