Metformin Lowers Blood Sugar Through the Gut, Muscle and Liver, Not Just the Liver

Metformin is the most widely used first-line oral drug for type 2 diabetes, and it is also a prominent candidate in longevity research. Despite decades of use, its mechanism of glucose lowering remains unresolved, which complicates efforts to predict who benefits and to repurpose it for other goals.

This narrative review by Drewe, Foretz and Krähenbühl examines the mechanistic literature across human clinical studies, animal experiments and cell models. It organizes the proposed mechanisms by organ: liver, skeletal muscle, kidney and intestine. It also separates mechanisms that depend on AMPK activation from those that do not, such as inhibition of mitochondrial complex I and mitochondrial glycerophosphate dehydrogenase.

The authors report that long-term metformin treatment in humans improves glucose disposal and reduces hepatic gluconeogenesis. Liver effects have been shown in animals and cells, but often at higher doses than patients receive. In muscle, AMPK activation and anti-inflammatory actions may increase glucose uptake. Renal gluconeogenesis inhibition may add to the effect. Short-term intravenous metformin does not lower glucose in humans, which points to the gut. Proposed intestinal mechanisms include reduced glucose absorption through increased enterocyte glycolysis, greater glucose transport into the colon, and increased GLP-1 secretion that suppresses hepatic gluconeogenesis through vagal signaling. Bile acids and bacterial metabolites may also play a role.

The authors conclude that metformin's effect is multifactorial, with complex I inhibition in several organs as an important shared mechanism. This shifts attention from a liver-centered model toward a whole-body one.

The main caveats are that this is a narrative review based on the abstract, with no new data. Several mechanisms remain hypotheses, and many were seen at supraphysiologic doses. The relative contribution of each organ in patients is still uncertain.