{"product_id":"how-metformin-works-a-patients-guide-to-the-mechanisms-of-action-of-the-worlds-most-widely-used-diabetes-drug","title":"How Metformin Works: A Patient's Guide to the Mechanisms of Action of the World's Most Widely Used Diabetes Drug","description":"\u003cp\u003eMetformin is one of the most widely prescribed medications for type 2 diabetes, yet the exact ways it works in the body have remained surprisingly complex and heavily debated for more than 60 years. This comprehensive review, published in \u003cem\u003eDiabetologia\u003c\/em\u003e, brings together the current scientific evidence showing that metformin works through multiple different mechanisms — affecting the liver, the intestines, the gut microbiome, and the body's cellular energy systems. For patients, understanding these mechanisms helps clarify why the drug is so effective, why some people experience digestive side effects, and how ongoing research is working to improve diabetes treatment.\u003c\/p\u003e\n\n\u003ch1\u003eHow Metformin Works: A Patient's Guide to the Mechanisms of Action of the World's Most Widely Used Diabetes Drug\u003c\/h1\u003e\n\n\u003ch2\u003eTable of Contents\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003e\u003ca href=\"#ddn-key-points\"\u003eKey Points\u003c\/a\u003e\u003c\/li\u003e\n\n  \u003cli\u003e\u003ca href=\"#introduction\"\u003eIntroduction: A 60-Year-Old Drug That Still Holds Mysteries\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#origins\"\u003eThe Origins of Metformin: From a Medieval Herbal Remedy\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#body\"\u003eHow Metformin Travels Through the Body\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#liver\"\u003eMetformin and the Liver: The Traditional View\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#mitochondria\"\u003eThe Mitochondrial Connection: How Metformin Affects Cellular Energy\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#ampk\"\u003eAMPK: The Cellular Energy Sensor and Metformin's Key Partner\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#ampk-effects\"\u003eAMPK-Dependent and AMPK-Independent Effects on Glucose Production\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#intestines\"\u003eMetformin and the Intestines: A Growing Focus\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#intolerance\"\u003eMetformin Intolerance and Digestive Side Effects\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#inflammation\"\u003eInflammation, Ageing, and the Microbiome: Surprising New Connections\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#implications\"\u003eWhat This Means for Patients: Clinical Implications\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#limitations\"\u003eLimitations of the Research: What We Still Don't Know\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#recommendations\"\u003eRecommendations for Patients\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#ddn-faq\"\u003eFrequently Asked Questions\u003c\/a\u003e\u003c\/li\u003e\n\u003cli\u003e\u003ca href=\"#source\"\u003eSource Information\u003c\/a\u003e\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003c!-- ddn:keypoints:start --\u003e\n\u003ch2 id=\"ddn-key-points\"\u003eKey Points\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003eMetformin lowers blood sugar through multiple mechanisms in the liver, intestines, gut microbiome, and cellular energy systems.\u003c\/li\u003e\n\u003cli\u003eThe gut is a major target: a delayed-release form retained in the gut is as effective as the standard form.\u003c\/li\u003e\n\u003cli\u003eGastrointestinal side effects affect 20–30% of patients; slow-release formulations may reduce them.\u003c\/li\u003e\n\u003cli\u003eMetformin changes gut bacteria, consistently increasing Akkermansia and Escherichia and decreasing Intestinibacter, but whether this causes benefits is unknown.\u003c\/li\u003e\n\u003cli\u003eMetformin suppresses inflammation markers like the neutrophil-to-lymphocyte ratio and cytokine CCL11 in type 2 diabetes.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c!-- ddn:keypoints:end --\u003e\n\n\n\u003ch2 id=\"introduction\"\u003eIntroduction: A 60-Year-Old Drug That Still Holds Mysteries\u003c\/h2\u003e\n\n\u003cp\u003eMetformin is a widely used drug that provides clear, well-documented benefits for blood glucose (sugar) metabolism and for reducing diabetes-related complications. However, the mechanisms underlying these benefits are complex and still not fully understood. The review authors, from the University of Dundee in Scotland, explain that while we have learned a great deal, the full picture of how this drug works in the body remains a work in progress.\u003c\/p\u003e\n\n\u003cp\u003ePhysiologically, metformin has been shown to reduce hepatic glucose production — meaning it decreases the amount of sugar the liver releases into the bloodstream. Yet not all of its effects can be explained by this single mechanism. There is increasing evidence pointing to a key role for the gut in metformin's benefits.\u003c\/p\u003e\n\n\u003cp\u003eAt the molecular level, research findings vary depending on the doses of metformin used and the duration of treatment. There are clear differences between acute (short-term) and chronic (long-term) administration. The drug works through multiple pathways, including both AMPK-dependent and AMPK-independent mechanisms; by inhibiting mitochondrial respiration (the energy-producing machinery of cells); by inhibiting an enzyme called mitochondrial glycerophosphate dehydrogenase (mGPD); and through a mechanism involving the lysosome — the cell's recycling center.\u003c\/p\u003e\n\n\u003cp\u003eIn the last 10 years, the scientific community has moved from a simple picture — that metformin improves blood glucose by acting on the liver through AMPK activation — to a much more complex picture that reflects the drug's multiple modes of action. As the authors conclude: \"More work is required to truly understand how this drug works in its target population: individuals with type 2 diabetes.\"\u003c\/p\u003e\n\n\u003ch2 id=\"origins\"\u003eThe Origins of Metformin: From a Medieval Herbal Remedy\u003c\/h2\u003e\n\n\u003cp\u003eMetformin has an unusual origin story compared with most modern drugs. It is derived from a natural product called galegine, which comes from the plant \u003cem\u003eGalega officinalis\u003c\/em\u003e (also known as goat's rue or French lilac). This plant was used in herbal medicine in medieval Europe, long before modern pharmaceutical development existed.\u003c\/p\u003e\n\n\u003cp\u003eGalegine was tested as a glucose-lowering agent in humans in the 1920s but was found to be too toxic. Around the same time, two synthetic derivatives of galegine — metformin and phenformin — were first synthesised and tested. However, they were not introduced into clinical use until the 1950s. Phenformin was later withdrawn from diabetes treatment in most countries because of the dangerous side effect of lactic acidosis (a build-up of lactic acid in the body).\u003c\/p\u003e\n\n\u003cp\u003eChemically, galegine is an isoprenyl derivative of guanidine, while metformin (dimethylbiguanide) and phenformin (phenethylbiguanide) are biguanides — compounds containing two coupled molecules of guanidine with additional chemical substitutions.\u003c\/p\u003e\n\n\u003cp\u003eUnlike most modern drugs, metformin was not designed to target a particular pathway or disease mechanism. It was established as a safe and effective therapy before detailed mechanistic studies became possible, and despite 60 years of clinical use, its molecular mechanisms of action remain much debated.\u003c\/p\u003e\n\n\u003ch2 id=\"body\"\u003eHow Metformin Travels Through the Body\u003c\/h2\u003e\n\n\u003cp\u003eUnderstanding how metformin behaves in the body is essential to understanding how it works. When humans take immediate-release metformin by mouth, approximately \u003cstrong\u003e70% of the dose is absorbed from the small intestine\u003c\/strong\u003e, with the remainder passing into the colon before being excreted in faeces. Metformin is excreted in urine unchanged, with no breakdown products (metabolites) reported.\u003c\/p\u003e\n\n\u003cp\u003eConcentrations of metformin in human blood plasma are typically in the low micromolar range — for example, \u003cstrong\u003e8–24 μmol\/l\u003c\/strong\u003e. However, concentrations in the jejunum (a part of the small intestine) are \u003cstrong\u003e30–300 times higher\u003c\/strong\u003e than in the blood.\u003c\/p\u003e\n\n\u003cp\u003eA recent positron emission tomography (PET) study using a radioactively labelled form of metformin ([11C]metformin) demonstrated that, after oral dosing, metformin becomes highly concentrated in the intestines, liver, kidneys and bladder — reflecting its route of elimination — with only slow accumulation in muscle. In this study, the hepatic (liver) tissue-to-systemic blood activity ratio was approximately \u003cstrong\u003e5\u003c\/strong\u003e following oral dosing, demonstrating that much greater metformin concentrations are achieved in the liver than in the blood plasma. Extrapolating from systemic concentrations, researchers estimate that hepatic concentrations after an oral dose are approximately \u003cstrong\u003e50–100 μmol\/l\u003c\/strong\u003e.\u003c\/p\u003e\n\n\u003cp\u003eIn rats given metformin intravenously, accumulation was also observed in the pancreas and adipose tissue (fat tissue) at a concentration approximately half of that seen in the liver. The human pharmacokinetic data points to the liver, kidneys and intestines as the key target organs of metformin. In this review, the authors primarily focus on the liver and intestines, particularly regarding the beneficial impacts of metformin on metabolism and inflammation.\u003c\/p\u003e\n\n\u003ch2 id=\"liver\"\u003eMetformin and the Liver: The Traditional View\u003c\/h2\u003e\n\n\u003cp\u003eMetformin is traditionally thought to act on the liver to improve blood glucose levels, and several lines of evidence support this view.\u003c\/p\u003e\n\n\u003cp\u003eFirst, in mice lacking the organic cation transporter 1 (OCT1) — a protein that transports metformin into liver cells — metformin is ineffective at improving blood glucose after high-fat feeding. Without OCT1, the liver takes up little or no metformin, and the drug's glucose-lowering effect is lost.\u003c\/p\u003e\n\n\u003cp\u003eSecond, tracer studies in humans show that metformin lowers hepatic glucose production, with minimal impact on peripheral insulin-mediated glucose uptake (the ability of muscles and other tissues to take up sugar from the blood in response to insulin). However, when only placebo-controlled studies were analysed, the impact of metformin on endogenous glucose production (EGP) was not significant unless concomitant drug-induced reductions in plasma insulin were used to \"adjust\" the EGP measurement. This suggests that the liver effect may be more subtle than once thought.\u003c\/p\u003e\n\n\u003cp\u003eThird, multiple studies in mouse hepatocytes (liver cells) and transgenic mice provide evidence for a role of metformin in reducing hepatic gluconeogenesis (the production of new glucose by the liver) and\/or improving insulin sensitivity.\u003c\/p\u003e\n\n\u003ch2 id=\"mitochondria\"\u003eThe Mitochondrial Connection: How Metformin Affects Cellular Energy\u003c\/h2\u003e\n\n\u003cp\u003eTo understand metformin's effects at the cellular level, it helps to know a bit about mitochondria — the tiny structures inside cells that act as power plants, generating the energy molecule ATP (adenosine triphosphate) that powers nearly all cellular activities.\u003c\/p\u003e\n\n\u003cp\u003eGluconeogenesis is an energy-intensive process: it consumes \u003cstrong\u003esix ATP equivalents for each molecule of glucose synthesised\u003c\/strong\u003e. This means that liver cells (hepatocytes) need to balance their demand for ATP with their supply, with the supply primarily provided by mitochondria.\u003c\/p\u003e\n\n\u003cp\u003eMetformin accumulates within mitochondria at concentrations up to \u003cstrong\u003e1000-fold higher\u003c\/strong\u003e than in the extracellular (outside-cell) environment. This happens because metformin carries a positive charge, and the membrane potentials across both the plasma membrane and the mitochondrial inner membrane (positive on the outside) drive metformin into the cell and subsequently into the mitochondria.\u003c\/p\u003e\n\n\u003cp\u003eThe most intensively studied mitochondrial action of metformin is the \u003cstrong\u003einhibition of Complex I of the respiratory chain\u003c\/strong\u003e — the first protein complex in the chain of reactions that produce ATP. This inhibition suppresses ATP production.\u003c\/p\u003e\n\n\u003cp\u003eA persistent criticism of this mechanism has been the high extracellular concentrations (in the millimolar range) required to observe rapid effects. However, the authors note that lower concentrations of metformin (\u003cstrong\u003e50–100 μmol\/l\u003c\/strong\u003e) do inhibit Complex I in rat hepatoma (liver cancer) cells known as H4IIE cells after several hours of exposure. This delay was attributed to the slow uptake of metformin by mitochondria, which has recently been observed experimentally.\u003c\/p\u003e\n\n\u003cp\u003eInterestingly, some studies do not detect any changes in cellular ADP:ATP ratios after metformin treatment, although such changes can be observed with phenformin. In cells carrying out gluconeogenesis, the concomitant suppression of this energy-consuming pathway might explain the modest effects on ADP:ATP ratios. Other consequences of respiratory chain inhibition besides reduced ATP production — such as changes in the NAD+:NADH ratio (molecules involved in cellular energy transfer) — may also contribute to metformin's effects on gluconeogenesis.\u003c\/p\u003e\n\n\u003ch3\u003eAn Alternative Mitochondrial Target: mGPD\u003c\/h3\u003e\n\n\u003cp\u003eRecently, an alternative mitochondrial target of metformin has been proposed. Endogenous glucose production (primarily by the liver) was inhibited after just \u003cstrong\u003e1 hour of intravenous metformin administration\u003c\/strong\u003e to rats. This was associated with an increase in the lactate:pyruvate ratio, suggesting a problem with the re-oxidation of cytoplasmic NADH (a molecule involved in energy metabolism).\u003c\/p\u003e\n\n\u003cp\u003eThe glycerophosphate shuttle is one of two systems that carry reducing equivalents from the cytoplasm into the mitochondrion for re-oxidation. In cell-free assays, metformin was found to inhibit \u003cstrong\u003emitochondrial glycerophosphate dehydrogenase (mGPD)\u003c\/strong\u003e, a key component of this shuttle.\u003c\/p\u003e\n\n\u003cp\u003eSupporting this mechanism, giving mice antisense oligonucleotides against mGPD (which block the production of the protein) or using a global mouse knockout (mice genetically engineered to lack the gene) lowered EGP and completely abolished the effects of metformin on plasma glucose and EGP.\u003c\/p\u003e\n\n\u003cp\u003eHowever, as discussed by other researchers, inhibition of the glycerophosphate shuttle alone may not be sufficient for a sustained impact on gluconeogenesis, because the malate\/aspartate shuttle (the other system that carries reducing equivalents) will compensate unless the mitochondrial membrane potential (maintained by the respiratory chain) also becomes suppressed. Thus, the relative contributions of mGPD inhibition and Complex I inhibition in metformin's glucose-lowering effects still need to be established, as does the possible role of its less understood effects on membrane properties and on interactions and oxidation of amino acid-bound copper ions.\u003c\/p\u003e\n\n\u003ch2 id=\"ampk\"\u003eAMPK: The Cellular Energy Sensor and Metformin's Key Partner\u003c\/h2\u003e\n\n\u003cp\u003eInhibition of mitochondrial function can also explain metformin's ability to activate a crucial cellular energy sensor called \u003cstrong\u003eAMP-activated protein kinase (AMPK)\u003c\/strong\u003e.\u003c\/p\u003e\n\n\u003cp\u003eAMPK acts as a cellular fuel gauge. When it is activated by increases in AMP:ATP and ADP:ATP ratios — indicators that cellular energy balance is compromised — AMPK works to restore energy homeostasis in two ways:\u003c\/p\u003e\n\u003cul\u003e\n  \u003cli\u003eSwitching \u003cstrong\u003eon\u003c\/strong\u003e catabolic pathways that generate ATP (breaking down nutrients for energy)\u003c\/li\u003e\n  \u003cli\u003eSwitching \u003cstrong\u003eoff\u003c\/strong\u003e anabolic processes that consume ATP (building up cellular nutrient stores)\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eBecause metformin causes a switch from synthesis of cellular nutrient stores to their breakdown, the idea that AMPK might be involved in metformin's action became attractive. In \u003cstrong\u003e2001\u003c\/strong\u003e, metformin was first reported to activate AMPK in rat hepatocytes and in rat liver in vivo.\u003c\/p\u003e\n\n\u003cp\u003eWhile high concentrations (\u003cstrong\u003e500 μmol\/l\u003c\/strong\u003e) of metformin were required to observe AMPK activation after brief (1 hour) treatment of cells, significant effects were observed after incubation for much longer periods with only \u003cstrong\u003e20 μmol\/l\u003c\/strong\u003e metformin — a concentration more compatible with the levels of the drug found in the portal vein (the blood vessel that carries blood from the digestive organs to the liver).\u003c\/p\u003e\n\n\u003cp\u003eSupporting the idea that biguanides act by increasing cellular AMP:ATP and ADP:ATP ratios, AMPK was not activated by either metformin or phenformin in cells expressing an AMPK mutant that is insensitive to changes in AMP or ADP.\u003c\/p\u003e\n\n\u003cp\u003eHowever, AMPK can also be activated by glucose starvation and by low concentrations of metformin through a different mechanism — one that involves the formation of a complex with the proteins Axin and LAMTOR1 (late endosomal\/lysosomal adaptor, MAPK and mTOR activator 1), the latter being a lysosomal protein. Thus, metformin might also activate AMPK through a mechanism involving the lysosome (the cell's waste-disposal and recycling center), rather than the mitochondrion. This adds another layer of complexity to the picture.\u003c\/p\u003e\n\n\u003ch2 id=\"ampk-effects\"\u003eAMPK-Dependent and AMPK-Independent Effects on Glucose Production\u003c\/h2\u003e\n\n\u003cp\u003eOne of the key questions in metformin research has been: which of its effects depend on AMPK, and which do not?\u003c\/p\u003e\n\n\u003cp\u003eThe first pharmacological activator of AMPK to be developed was a compound called \u003cstrong\u003eAICAR\u003c\/strong\u003e (5-aminoimidazole-4-carboxamide ribonucleoside). AICAR is taken up into cells and phosphorylated to a nucleotide called ZMP (5-amino-4-imidazolecarboxamide riboside 5′-monophosphate), which mimics all the effects of AMP on the AMPK system.\u003c\/p\u003e\n\n\u003cp\u003eThe finding that AICAR downregulated expression of the gluconeogenic enzymes PEPCK (phosphoenolpyruvate carboxykinase) and G6Pase (glucose-6-phosphatase) initially supported the idea that AMPK activation might be responsible for metformin's ability to inhibit hepatic glucose production.\u003c\/p\u003e\n\n\u003cp\u003eHowever, there is an important caveat: ZMP also modulates other AMP-sensitive enzymes, such as fructose-1,6-bisphosphatase — a key enzyme of gluconeogenesis that is allosterically inhibited (meaning its activity is directly blocked) by both AMP and ZMP.\u003c\/p\u003e\n\n\u003ch3\u003eThe AMPK-Independent Acute Effects\u003c\/h3\u003e\n\n\u003cp\u003eConvincing evidence for AMPK-independent effects came from experiments using genetically modified mice. Acute treatment with metformin or AICAR inhibited glucose production equally well in hepatocytes from control mice and from mice lacking both AMPK catalytic subunits in the liver. Metformin also acutely improved glucose tolerance in both mouse strains. Metformin did increase cellular AMP:ATP ratios in hepatocytes, consistent with inhibition of the respiratory chain.\u003c\/p\u003e\n\n\u003cp\u003eThese findings suggest that the acute inhibition of glucose production by metformin or AICAR was likely due to inhibition of fructose-1,6-bisphosphatase by AMP or ZMP, respectively. However, expression of messenger RNA (mRNA) encoding G6Pase and PEPCK was also reduced by AICAR and metformin in both control and AMPK-null hepatocytes, suggesting that the effect on gene expression might also be AMPK-independent.\u003c\/p\u003e\n\n\u003cp\u003eA potential explanation for this came with a report that \u003cstrong\u003eadenylate cyclase\u003c\/strong\u003e — the enzyme that generates cAMP (a key signalling molecule) in response to the starvation hormone glucagon in mouse hepatocytes — is, like fructose-1,6-bisphosphatase, inhibited by AMP. Thus, AMP itself might have an additional AMPK-independent effect: lowering cAMP and thereby reducing the expression of gluconeogenic enzymes.\u003c\/p\u003e\n\n\u003cp\u003eMore recently, another group has proposed an AMPK-dependent mechanism by which metformin reduces cAMP. Treatment of mouse hepatocytes with a more specific AMPK activator reduced glucagon-induced cAMP levels. This was traced to the direct AMPK-mediated phosphorylation of a specific enzyme called \u003cstrong\u003ecAMP-specific 3′,5′-cyclic phosphodiesterase 4B (PDE4B)\u003c\/strong\u003e, which triggers the breakdown of cAMP.\u003c\/p\u003e\n\n\u003cp\u003eSo, while controversies remain, it seems certain that some of the acute effects of metformin on hepatic glucose production are AMPK-independent, with inhibition of fructose-1,6-bisphosphatase by AMP being one likely explanation.\u003c\/p\u003e\n\n\u003ch3\u003eThe AMPK-Dependent Long-Term Effects\u003c\/h3\u003e\n\n\u003cp\u003eA major long-term, clinically relevant effect of metformin is to \u003cstrong\u003eenhance hepatic insulin sensitivity\u003c\/strong\u003e, and mouse studies suggest that this effect is mediated by AMPK.\u003c\/p\u003e\n\n\u003cp\u003eAMPK acutely inhibits fat synthesis and activates fat oxidation in the liver by directly phosphorylating (adding a chemical phosphate group to) the two isoforms of an enzyme called acetyl-CoA carboxylase (ACC1 and ACC2) at equivalent serine residues.\u003c\/p\u003e\n\n\u003cp\u003eTo test the importance of this specific phosphorylation, researchers created \"knock-in\" mice in which both serine residues were replaced by non-phosphorylatable alanine residues (ACC1-S79A and ACC2-S212A). Consistent with the prediction that this would enhance fat synthesis and reduce fat oxidation, these mice — although not obese — had elevated diacylglycerol and triacylglycerol (types of fat) levels in the liver and muscle.\u003c\/p\u003e\n\n\u003cp\u003eThe consequences of this fat accumulation were striking:\u003c\/p\u003e\n\u003cul\u003e\n  \u003cli\u003eHyperglycaemia (high blood sugar)\u003c\/li\u003e\n  \u003cli\u003eHyperinsulinaemia (high blood insulin levels)\u003c\/li\u003e\n  \u003cli\u003eGlucose intolerance (impaired ability to handle sugar)\u003c\/li\u003e\n  \u003cli\u003eInsulin resistance (the body's cells responding poorly to insulin)\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThese effects were present even when the mice were fed a normal chow diet. When control mice were placed on a high-fat diet for 6 weeks, they became just as hyperglycaemic and glucose intolerant as the knock-in mice.\u003c\/p\u003e\n\n\u003cp\u003eThe most revealing finding came when both groups were treated with metformin for 6 weeks:\u003c\/p\u003e\n\u003cul\u003e\n  \u003cli\u003eThe metabolic measures of the high-fat-fed control mice \u003cstrong\u003esubstantially improved\u003c\/strong\u003e with metformin treatment.\u003c\/li\u003e\n  \u003cli\u003eThe metabolic measures of the knock-in mice were \u003cstrong\u003ecompletely unaffected\u003c\/strong\u003e by metformin.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThese intriguing results suggest that metformin enhances insulin sensitivity — at least in mice — by phosphorylation of ACC1 and ACC2. Since ACC phosphorylation is abolished by AMPK knockout, the long-term insulin-sensitising effects of metformin appear to be mediated entirely by AMPK.\u003c\/p\u003e\n\n\u003ch2 id=\"intestines\"\u003eMetformin and the Intestines: A Growing Focus\u003c\/h2\u003e\n\n\u003cp\u003eIt has been known for some time that the intestines may be a target organ for metformin. The drug increases anaerobic glucose metabolism (sugar breakdown without oxygen) in enterocytes (intestinal lining cells), resulting in reduced net glucose uptake and increased lactate delivery to the liver.\u003c\/p\u003e\n\n\u003cp\u003eSeveral recent studies have led to a renewed interest in the gut as a major site of action for metformin. Three lines of evidence highlight that the liver may not be as important for metformin's action in people with type 2 diabetes as commonly assumed:\u003c\/p\u003e\n\n\u003col\u003e\n  \u003cli\u003e\n\u003cstrong\u003eThe glucose-lowering effect of metformin can only be partially explained by a reduction in EGP\u003c\/strong\u003e, suggesting that other glucose-lowering mechanisms are at work.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eGenetic studies in humans\u003c\/strong\u003e have established that loss-of-function variants in SLC22A1 (the gene encoding OCT1), which reduce hepatic uptake of metformin, \u003cstrong\u003edo not impact the efficacy of metformin in lowering HbA\u003csub\u003e1c\u003c\/sub\u003e\u003c\/strong\u003e (a long-term marker of blood sugar control) in individuals with type 2 diabetes.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eA delayed-release formulation of metformin\u003c\/strong\u003e that is largely retained in the gut, with minimal systemic absorption, is \u003cstrong\u003eas effective at lowering blood glucose as the standard immediate-release formulation\u003c\/strong\u003e in individuals with type 2 diabetes.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003ch3\u003eHow Does Metformin Act on the Gut?\u003c\/h3\u003e\n\n\u003cp\u003eThere are a number of proposed mechanisms for how metformin affects glucose metabolism through actions on the intestines:\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIncreased glucose utilisation by the gut.\u003c\/strong\u003e Metformin increases glucose use by the gut itself — an effect visible on PET imaging. Metformin-treated patients show considerable intestinal fluorodeoxyglucose (FDG) uptake, especially in the colon. A recent mouse study established that colonic FDG uptake was not increased after 48 hours of metformin treatment but was increased after \u003cstrong\u003e30 days\u003c\/strong\u003e of treatment. This effect persisted even after a 48-hour washout period (a break from the drug). The increase in FDG uptake was paralleled by an increase in AMPK phosphorylation. Notably, this effect was only seen in colonic enterocytes where luminal glucose (glucose in the gut contents) was almost completely absent — suggesting that metformin increases the colon's uptake and metabolism of glucose from the bloodstream.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eIncreased GLP-1 secretion.\u003c\/strong\u003e Metformin may also impact glucose metabolism by increasing secretion of \u003cstrong\u003eglucagon-like peptide-1 (GLP-1)\u003c\/strong\u003e — an important gut hormone that stimulates insulin release and suppresses appetite. This effect has been described for both immediate-release and delayed-release metformin formulations.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eGut–brain–liver crosstalk.\u003c\/strong\u003e A further intriguing gut-mediated mechanism was identified in rats. This involves a pathway linking duodenal metformin exposure to suppression of hepatic glucose production. The signal travels via the \u003cstrong\u003enucleus tractus solitarius\u003c\/strong\u003e (a region in the brainstem) and \u003cstrong\u003evagal efferents\u003c\/strong\u003e (nerve fibres from the vagus nerve), through AMPK and GLP-1 receptor activation. This \"gut–brain–liver crosstalk\" represents a completely new way of thinking about how metformin works.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eAlteration of the intestinal microbiome.\u003c\/strong\u003e A final potential gut-mediated mechanism involves changes to the trillions of bacteria that live in the intestines (the microbiome). This is outlined below in relation to inflammation, but how this contributes to the glucose-lowering and gastrointestinal side effects of metformin remains unknown.\u003c\/p\u003e\n\n\u003ch2 id=\"intolerance\"\u003eMetformin Intolerance and Digestive Side Effects\u003c\/h2\u003e\n\n\u003cp\u003eMetformin treatment is frequently associated with gastrointestinal (GI) side effects, affecting \u003cstrong\u003e20–30% of patients\u003c\/strong\u003e. Severe side effects result in metformin discontinuation in approximately \u003cstrong\u003e5% of patients\u003c\/strong\u003e. For many patients, these digestive issues are the main barrier to staying on the medication.\u003c\/p\u003e\n\n\u003cp\u003eThe mechanism by which metformin causes GI side effects remains uncertain. However, there are a number of potential explanations:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eHigh local concentrations:\u003c\/strong\u003e The side effects may simply relate to the very high concentration of metformin in intestinal enterocytes. This would explain why slow-release formulations, which disperse slowly and reduce local luminal metformin concentrations, reduce GI intolerance.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSerotonin involvement:\u003c\/strong\u003e Metformin may affect serotonin signalling — either by stimulating serotonin release from enterochromaffin cells (specialised cells in the gut lining), or by reducing serotonin transport via the serotonin transporter (SERT), resulting in increased luminal serotonin (the \"feel-good\" chemical that also has important functions in the gut).\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eGenetic factors:\u003c\/strong\u003e Genetic studies have identified key roles for \u003cstrong\u003eOCT1\u003c\/strong\u003e and \u003cstrong\u003eSERT\u003c\/strong\u003e in mediating metformin intolerance.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMicrobiome changes:\u003c\/strong\u003e A third potential mechanism of intolerance may be due to metformin's impact on the intestinal microbiome.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThe authors note that further studies are required to establish the mechanisms for metformin intolerance, as this may enable approaches to reduce or avoid the unpleasant side effects of this drug. For example, the studies reporting a role for OCT1 in metformin intolerance would support an approach whereby OCT1-interacting drugs — such as \u003cstrong\u003eproton pump inhibitors\u003c\/strong\u003e (common medications used to reduce stomach acid) — are avoided in individuals experiencing GI side effects with metformin use.\u003c\/p\u003e\n\n\u003ch2 id=\"inflammation\"\u003eInflammation, Ageing, and the Microbiome: Surprising New Connections\u003c\/h2\u003e\n\n\u003cp\u003eIn the tiny roundworm \u003cem\u003eCaenorhabditis elegans\u003c\/em\u003e, metformin lengthens lifespan through effects on intestinal microbial growth. This finding is consistent with a fascinating concept: that metformin can affect host metabolism indirectly, by changing the population of bacteria that live in the gut.\u003c\/p\u003e\n\n\u003cp\u003eRecent research in this area has produced notable findings:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003eMetformin expanded the gut population of \u003cstrong\u003eAkkermansia\u003c\/strong\u003e species in animal studies. This was linked to reduced adipose tissue (fat tissue) inflammation.\u003c\/li\u003e\n  \u003cli\u003eAcross datasets from different countries, increases in \u003cstrong\u003eEscherichia\u003c\/strong\u003e and \u003cstrong\u003eAkkermansia\u003c\/strong\u003e species and decreases in \u003cstrong\u003eIntestinibacter\u003c\/strong\u003e species were the most consistently observed effects on the microbiome.\u003c\/li\u003e\n  \u003cli\u003eThis work emphasises that microbiome changes in type 2 diabetes are predominantly associated with \u003cstrong\u003emetformin itself\u003c\/strong\u003e, rather than with type 2 diabetes itself.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eHowever, whether the microbiome changes are a cause or a consequence of the therapeutic benefit still requires further investigation.\u003c\/p\u003e\n\n\u003cp\u003eMetformin also appears to have direct effects on inflammation. These include effects on \u003cstrong\u003eNF-κB signalling\u003c\/strong\u003e (a protein complex that controls the activity of genes involved in inflammation) and on the differentiation of monocytes (a type of white blood cell) into macrophages (cells that engulf and digest debris and pathogens). Metformin suppresses proinflammatory cytokines (inflammatory signalling molecules) from these macrophages.\u003c\/p\u003e\n\n\u003cp\u003eConsistent with its anti-inflammatory effects, metformin suppresses the \u003cstrong\u003eneutrophil to lymphocyte ratio (NLR)\u003c\/strong\u003e in type 2 diabetes. NLR is a marker of inflammation that has recently gained attention. In the blood, these findings are supported by observational studies showing NLR suppression in humans with type 2 diabetes, and by randomised placebo-controlled trials showing suppression of cytokines, including C-C motif chemokine 11 (CCL11, also known as eotaxin-1), with metformin treatment.\u003c\/p\u003e\n\n\u003ch2 id=\"implications\"\u003eWhat This Means for Patients: Clinical Implications\u003c\/h2\u003e\n\n\u003cp\u003eSo what does all this complex science mean for a patient taking metformin? Several important practical implications emerge:\u003c\/p\u003e\n\n\u003col\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMetformin works through multiple pathways, not just one.\u003c\/strong\u003e This is good news for patients because it means the drug can be effective even if one pathway is not working optimally. Even patients with genetic variations that reduce metformin's uptake into the liver still experience full blood sugar-lowering benefits — likely because the gut and other mechanisms compensate.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eThe gut is a major target, not just the liver.\u003c\/strong\u003e The finding that a delayed-release metformin formulation retained largely in the gut is just as effective as the standard immediate-release version confirms that patients benefit from metformin's actions on the intestines. This knowledge is already being used to develop formulations with fewer side effects.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eGastrointestinal side effects have identifiable causes.\u003c\/strong\u003e The 20–30% of patients who experience GI side effects and the ~5% who have to stop the medication may benefit from strategies such as switching to slow-release formulations or avoiding medicines that interact with OCT1 (such as proton pump inhibitors).\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eThe microbiome connection may open new doors.\u003c\/strong\u003e The finding that metformin significantly changes gut bacteria — increasing beneficial \u003cem\u003eAkkermansia\u003c\/em\u003e species — suggests that some of the drug's benefits may come from its effects on the microbial community in the gut. This could lead to new treatment strategies in the future.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eAnti-inflammatory effects may contribute to broader health benefits.\u003c\/strong\u003e Metformin's ability to suppress inflammation (reducing NLR and inflammatory cytokines) may help explain its benefits that go beyond glucose control, including potential effects on cardiovascular health, cancer prevention and ageing.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003ch2 id=\"limitations\"\u003eLimitations of the Research: What We Still Don't Know\u003c\/h2\u003e\n\n\u003cp\u003eThe authors are transparent about the limitations of current knowledge:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMuch evidence comes from animal models.\u003c\/strong\u003e Many of the mechanistic findings come from studies in mice, rats or cell cultures. While these are powerful research tools, they do not always perfectly reflect how the drug works in humans with type 2 diabetes.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eThe dose question.\u003c\/strong\u003e Many laboratory experiments used metformin concentrations much higher than those achieved in human plasma (8–24 μmol\/l). While liver concentrations are estimated to be higher (50–100 μmol\/l), the relevance of some in vitro findings to real-world therapy remains debated.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eAcute versus chronic effects.\u003c\/strong\u003e The review highlights clear differences between acute and chronic metformin administration. Most patients take metformin chronically (long-term), and the relative contributions of the various mechanisms over time are still being worked out.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMultiple potential targets.\u003c\/strong\u003e The relative contributions of Complex I inhibition, mGPD inhibition, AMPK activation, lysosomal signalling, and gut-mediated mechanisms to metformin's overall glucose-lowering effect are still uncertain.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eMicrobiome cause or effect?\u003c\/strong\u003e While metformin clearly changes the gut microbiome, whether these changes are a cause or a consequence of its therapeutic benefit still requires investigation.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSide effects remain poorly understood.\u003c\/strong\u003e Despite being a major clinical problem, the mechanisms of GI intolerance are still not fully established.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2 id=\"recommendations\"\u003eRecommendations for Patients\u003c\/h2\u003e\n\n\u003cp\u003eBased on this review and current clinical knowledge, here are practical recommendations for patients taking or considering metformin:\u003c\/p\u003e\n\n\u003col\u003e\n  \u003cli\u003e\n\u003cstrong\u003eDo not be discouraged if metformin doesn't seem to work through one single mechanism.\u003c\/strong\u003e The drug's effectiveness comes from multiple complementary actions across the liver, gut and immune system.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eIf you experience digestive side effects, ask your healthcare provider about a slow-release (extended-release) formulation.\u003c\/strong\u003e The research shows that these formulations reduce local intestinal concentrations of the drug and are associated with less GI intolerance, while remaining equally effective at lowering blood glucose.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eReview your other medications.\u003c\/strong\u003e If you are taking proton pump inhibitors (for acid reflux or stomach ulcers) and experiencing GI side effects with metformin, talk to your doctor. The interaction may be related to OCT1, a transporter involved in both metformin uptake and metformin intolerance.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eUnderstand that the gut effects are part of the benefit.\u003c\/strong\u003e Metformin's actions on the gut — including GLP-1 secretion and microbiome changes — are now recognised as major contributors to its glucose-lowering effectiveness, not merely side effects to be tolerated.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eStay informed.\u003c\/strong\u003e Research into metformin's mechanisms is ongoing, and a better understanding of how the drug works is already leading to improved formulations and strategies to reduce side effects.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eNever stop or adjust metformin without consulting your healthcare provider.\u003c\/strong\u003e While this review focuses on mechanisms of action, clinical decisions about metformin use should always be made with your medical team.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003c!-- ddn:faq:start --\u003e\n\u003ch2 id=\"ddn-faq\"\u003eFrequently Asked Questions\u003c\/h2\u003e\n\u003ch3\u003eHow does metformin work to lower blood sugar?\u003c\/h3\u003e\n\u003cp\u003eMetformin lowers blood sugar through multiple actions. It reduces glucose production by the liver, increases glucose use by the intestines, increases GLP-1 hormone secretion, changes gut bacteria, and affects cellular energy pathways. These complementary mechanisms together help control blood sugar, even if one pathway is not working optimally.\u003c\/p\u003e\n\u003ch3\u003eWhy does metformin cause digestive side effects?\u003c\/h3\u003e\n\u003cp\u003eDigestive side effects affect 20–30% of patients and are severe enough to cause about 5% to stop. Possible causes include very high metformin concentrations in intestinal cells, effects on serotonin signalling, genetic factors involving OCT1 and serotonin transporters, and changes in gut bacteria. Slow-release forms often reduce these side effects.\u003c\/p\u003e\n\u003ch3\u003eIs metformin only effective if it works on the liver?\u003c\/h3\u003e\n\u003cp\u003eNo. The gut is a major target. A delayed-release metformin formulation that stays largely in the gut lowers blood glucose just as well as the standard immediate-release form. Also, people with genetic variations reducing liver uptake still get full blood sugar benefits, likely because gut and other mechanisms compensate.\u003c\/p\u003e\n\u003ch3\u003eCan metformin change my gut bacteria?\u003c\/h3\u003e\n\u003cp\u003eYes. Studies show metformin changes the gut microbiome, most consistently increasing Akkermansia and Escherichia species and decreasing Intestinibacter species. These microbiome changes are linked mainly to metformin itself rather than to type 2 diabetes. Whether these changes are a cause or consequence of the drug's benefit is still being investigated.\u003c\/p\u003e\n\u003ch3\u003eDoes metformin have anti-inflammatory effects?\u003c\/h3\u003e\n\u003cp\u003eResearch suggests metformin suppresses inflammation. It reduces proinflammatory cytokines from macrophages and lowers the neutrophil-to-lymphocyte ratio, a marker of inflammation, in type 2 diabetes. A randomised placebo-controlled trial showed suppression of the cytokine CCL11 with metformin treatment. These effects may contribute to health benefits beyond glucose control.\u003c\/p\u003e\n\u003ch3\u003eWhat should I do if I have stomach problems from metformin?\u003c\/h3\u003e\n\u003cp\u003eAsk your healthcare provider about a slow-release or extended-release formulation; research shows these reduce local intestinal concentrations and are associated with less gastrointestinal intolerance while remaining equally effective. If you also take proton pump inhibitors and experience side effects, talk to your doctor, because a drug interaction may be involved.\u003c\/p\u003e\n\u003ch3\u003eIs metformin based on a herbal medicine?\u003c\/h3\u003e\n\u003cp\u003eMetformin is derived from galegine, a natural product from the plant Galega officinalis, also called goat's rue or French lilac. This plant was used in medieval European herbal medicine. Galegine was tested in the 1920s but was too toxic. Metformin and phenformin were later synthesised; phenformin was withdrawn due to lactic acidosis risk.\u003c\/p\u003e\n\u003c!-- ddn:faq:end --\u003e\n\n\u003ch2 id=\"source\"\u003eSource Information\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eOriginal article title:\u003c\/strong\u003e REVIEW The mechanisms of action of metformin\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors:\u003c\/strong\u003e Graham Rena, D. Grahame Hardie, and Ewan R. Pearson\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eJournal:\u003c\/strong\u003e Diabetologia (2017) 60:1577–1585\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eDOI:\u003c\/strong\u003e 10.1007\/s00125-017-4342-z\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003ePublished:\u003c\/strong\u003e Received 28 March 2017; Accepted 19 April 2017; Published online: 3 August 2017\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eAffiliations:\u003c\/strong\u003e Division of Molecular \u0026amp; Clinical Medicine, School of Medicine, University of Dundee, Dundee, UK; and Division of Cell Signalling \u0026amp; Immunology, School of Life Sciences, University of Dundee, Dundee, UK\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eAccess:\u003c\/strong\u003e This article is an open access publication under the terms of the Creative Commons licence.\u003c\/p\u003e\n\n\u003cp\u003e\u003cem\u003eThis patient-friendly article is based on peer-reviewed research. It is intended for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider regarding any questions about your medications or treatment plan.\u003c\/em\u003e\u003c\/p\u003e","brand":"DiagnosticDetectives.Com","offers":[{"title":"Default Title","offer_id":47451073314972,"sku":null,"price":0.0,"currency_code":"JPY","in_stock":true}],"url":"https:\/\/diagnosticdetectives.tw\/products\/how-metformin-works-a-patients-guide-to-the-mechanisms-of-action-of-the-worlds-most-widely-used-diabetes-drug","provider":"DiagnosticDetectives.Com","version":"1.0","type":"link"}