For more than six decades, metformin has been the cornerstone of treatment for type 2 diabetes. It is inexpensive, widely available and prescribed to millions of people across the world. Yet despite its long clinical history, scientists have never fully understood how the drug actually lowers blood sugar.
Now, researchers say they may finally have the answer, and it lies partly in the brain.
A study published in Science Advances by scientists from Baylor College of Medicine and international collaborators has uncovered a previously unknown brain pathway that metformin relies on to regulate blood glucose.
For decades, the dominant explanation was that metformin works mainly by reducing glucose production in the liver. Some later studies suggested the gut also plays a role.
But the new research indicates that a specific neural mechanism in the brain is essential for the drug’s action, particularly at lower doses.
The discovery centres around a small protein called Rap1, located in a region of the brain known as the ventromedial hypothalamus (VMH), an area involved in regulating metabolism and energy balance. Researchers found that metformin’s ability to lower blood sugar depends on suppressing Rap1 activity in this specific region.
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To test this, scientists used genetically engineered mice that lacked Rap1 in the VMH. When these mice were placed on a high-fat diet to mimic type 2 diabetes, the results were striking. Low doses of metformin failed to reduce their blood sugar levels. However, other diabetes medications such as insulin and GLP-1 receptor agonists continued to work normally.
This suggested that Rap1 is specifically required for metformin’s mechanism of action. “Metformin has been used for decades to treat diabetes, yet how it actually works has remained surprisingly unclear. Our findings show that the drug acts through a specific brain pathway involving the protein Rap1 in the hypothalamus, revealing a neural mechanism that appears crucial for its glucose-lowering effects,” as per a research paper published in Science Advances.
In another experiment, researchers delivered extremely small amounts of metformin directly into the brains of diabetic mice. And the result was that even doses thousands of times smaller than typical oral doses triggered a significant drop in blood sugar.
This suggests the brain responds to metformin at concentrations far lower than the liver or gut requires, indicating that neural circuits may play a more central role in glucose regulation than previously thought.
“For a long time, diabetes was viewed mainly as a disease of the pancreas, liver and muscles. But we now know the brain plays a central role in regulating metabolism, constantly sensing nutrients and hormones and adjusting blood sugar levels accordingly. While this understanding of metmorphin's connect with the brain is a breakthrough in terms of research; whether it will help us in clinically controlling the sugar in a better way we don't know and only time can tell," says Dr Sudheer Ambekar, neuro surgeon at Jaslok hospital in Mumbai.
“The brain is increasingly recognised as a command centre for metabolic control. Signals from the hypothalamus influence how the body produces and uses glucose, so discoveries linking diabetes drugs to specific brain pathways are particularly exciting.”
The researchers also identified the specific brain cells responsible for the effect.
These were SF1 neurons, located in the same region of the hypothalamus. When metformin entered the brain, these neurons became more active. Laboratory experiments measuring electrical activity in brain tissue confirmed that metformin stimulated most SF1 neurons but only when Rap1 was present.
In mice that lacked Rap1 specifically in these neurons, metformin produced no response at all.
The finding suggests that Rap1 acts as a crucial molecular switch that allows metformin to activate these neurons and regulate blood sugar levels.
Metformin has increasingly attracted attention for possible benefits beyond diabetes, including potential roles in ageing, inflammation and neurological health.
By identifying the brain pathway involved, researchers say it may now be possible to design more targeted therapies that act on the same mechanism.
Such drugs could potentially help people whose diabetes is not well controlled with current treatments.
The researchers also plan to investigate whether the Rap1 pathway could explain metformin’s suspected effects on brain ageing.
Despite the discovery, doctors say nothing changes immediately for patients taking metformin. The drug remains the first-line therapy for type 2 diabetes worldwide, thanks to its effectiveness, affordability and strong safety record. But scientifically, the findings mark an important milestone.
More than 60 years after it became a mainstay of diabetes care, researchers are finally beginning to understand why the drug works so well. And the answer, it appears, may lie as much in the brain as in the body’s metabolic organs.
