Researchers at Baylor College of Medicine identified in 2025 (ET) a brain pathway that explains how metformin lowers blood sugar. The team found metformin reaches the ventromedial hypothalamus (VMH) and suppresses a protein called Rap1, activating SF1 neurons that change glucose regulation. When Rap1 was removed in genetically engineered mice, metformin lost its effect while insulin and GLP-1 therapies retained activity, pointing to a distinct brain mechanism.
Metformin’s hidden brain pathway
The central finding is simple and striking: metformin is operating in the brain as well as in the liver and gut. Investigators traced the drug to the VMH and linked its glucose-lowering action to suppression of Rap1, a small protein previously tied to whole-body glucose control. Delivering tiny doses directly into the brain produced marked reductions in blood sugar at concentrations far lower than oral dosing, showing a brain response that is sensitive at much lower exposure than peripheral tissues.
How the brain mechanism was shown — methods and reactions
The work used mice with targeted genetic deletions and electrical recordings of neurons. In animals bred to lack Rap1 in the VMH and then fed a diet modeling diabetes, metformin failed to improve blood sugar, while established therapies such as insulin and GLP-1 agonists remained effective. Researchers also recorded neuron activity from the VMH and found that SF1 neurons increased firing when metformin was present — but only when Rap1 was intact.
“It’s been widely accepted that metformin lowers blood glucose primarily by reducing glucose output in the liver. Other studies have found that it acts through the gut, ” said Dr. Makoto Fukuda, associate professor of pediatrics — nutrition at Baylor College of Medicine. “We looked into the brain as it is widely recognized as a key regulator of whole-body glucose metabolism. We investigated whether and how the brain contributes to the anti-diabetic effects of metformin. “
Dr. Fukuda highlighted cellular specificity: “We also investigated which cells in the VMH were involved in mediating metformin’s effects. We found that SF1 neurons are activated when metformin is introduced into the brain, suggesting they’re directly involved in the drug’s action. ” The team emphasized that metformin’s brain action is mechanistically distinct from peripheral effects and could explain long-observed benefits that were previously unattributed to central pathways.
What’s next: implications and human testing
The discovery reframes decades of use by showing the brain as a direct site of action for metformin and opens a path to more targeted therapies. The researchers note that this finding must be validated in human studies before clinical practice changes. “These findings open the door to developing new diabetes treatments that directly target this pathway in the brain, ” Dr. Makoto Fukuda said, and he added the team will explore whether the Rap1 signaling uncovered here also explains other brain-related benefits linked to the drug.
Immediate next steps will focus on translating the mouse results into human research and on mapping the specific neurons and circuits that mediate the effect. If the VMH–Rap1–SF1 axis operates the same way in people, it could permit strategies that boost metformin’s potency or recreate its central action with new, targeted interventions.
