Metformin: the pathway in the body

Metformin is part of the pathway “Metformin”. This page shows the whole pathway; the station of Metformin is highlighted.

Where this laboratory value sits: Metformin — biguanide, as a tablet. Metformin belongs to the biguanides. At body pH the molecule carries a positive charge; it dissolves in water and therefore needs transporters to cross cell membranes. In the liver it lowers the formation of new glucose. Source 1, 8

In brief

Metformin is a biguanide. The body does not convert it; inside the cell it gathers in the mitochondrion and slows the first complex of the respiratory chain. It binds nothing fast — the build-up follows the charge gradient and falls back again. The liver then forms less glucose.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Metformin → In the small bowel From the gut contents metformin passes through transporters of the gut cells into the blood. Part of it stays in the gut wall and the gut contents; the label gives an availability of about one half. The transporters can become saturated. Source 1, 8↑ supplies The transporters of the gut cells open the way into the blood. The part that stays in the gut wall is highly concentrated there; in studies, gut cells then absorb more glucose and form more lactate. observed in studies Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of metformin is present in the gut, the transporters are saturated; a larger share stays in the gut contents and the gut wall instead of reaching the blood.

   too little — If little metformin is present in the gut, a larger share of it reaches the blood, because the transporters are not saturated.

   established physiology · Source 1

2. In the small bowel → Metformin in the blood In the blood metformin barely binds to protein. The body does not convert it: it leaves unchanged through the kidney, where transporters move it from the blood into the urine. How long it stays is set by the kidney alone. Source 1, 8↑ supplies In the blood metformin is unbound and so reaches the liver, gut and kidney. How long it stays in the body is set by the kidney alone, because the body does not break it down. established physiology Source 1, 8
   ⚖ When the balance tips

   too much — If a lot is present in the blood, transporters of the kidney move more of it into the urine; if the kidney works more slowly, it stays longer in the blood, as there is no other way out.

   too little — If little is present in the blood, little enters the liver cell through OCT1 either, and hardly any gathers there.

   established physiology · Source 1, 8, 2

3. Metformin in the blood → In the liver cell The transporter OCT1 brings the charged molecule into the liver cell. Without this carrier it would hardly cross the membrane. Only here can it slow glucose formation. Source 1, 2↑ supplies OCT1 determines how much metformin reaches the liver cell. Only there can it slow the formation of new glucose. observed in studies Source 2
   ⚖ When the balance tips

   too much — If OCT1 works hard, more metformin enters the liver cell, and it gathers there more strongly than in the blood.

   too little — If OCT1 works little, for instance with inherited less active variants, less enters the liver cell; whether the effect on blood glucose is then weaker is inconsistent across studies.

   contested · Source 2, 1

4. In the liver cell → In the matrix · positive charge An electrical gradient lies across the inner mitochondrial membrane. A positively charged molecule follows it and gathers in the matrix — metformin sits there at a higher concentration than outside the cell. This brings it within reach of complex I. Source 2, 3↑ supplies The gradient draws metformin into the matrix, where it reaches several times the concentration outside the cell. Only this build-up brings it within reach of complex I. observed in studies Source 2, 3
   ⚖ When the balance tips

   too much — If the gradient is large, more metformin is drawn in and complex I is slowed more strongly; the gradient itself then falls, and the inflow eases.

   too little — If the gradient is small, for instance in cells with little respiration, hardly any metformin gathers in the matrix, and complex I stays largely free.

   observed in studies · Source 2, 3

5. NADH in the matrix → Proton gradient complex I · FMN, coenzyme Q10 Complex I accepts the electrons from NADH and passes them on to coenzyme Q10. In doing so it pumps protons out of the matrix; this creates the gradient across the inner membrane. It drives the ATP synthase. Source 4↑ supplies The proton gradient is the energy store of the mitochondria. It drives the ATP synthase and at the same time draws positively charged molecules such as metformin into the matrix. established physiology Source 4, 2
   ⚖ When the balance tips

   too much — If the gradient is high and little ATP is used, the respiratory chain runs more slowly, and more reactive oxygen species form at complex I.

   too little — If the gradient is low, it drives the ATP synthase more weakly, and the cell makes less ATP; metformin is then also drawn into the matrix less.

   observed in studies · Source 4, 2

6. Proton gradient → ATP ATP synthase · ADP, phosphate The protons flow back into the matrix through the ATP synthase. Its turning motion joins ADP and phosphate into ATP. When ATP falls, AMP and ADP rise. Source 4↑ supplies ATP is the energy carrier of the cell. In the liver the formation of new glucose also needs a lot of it. established physiology Source 4, 3
   ⚖ When the balance tips

   too much — If there is a lot of ATP, AMP and ADP stay low, AMPK stays switched off, and the cell builds up glucose and fats.

   too little — If little ATP is rebuilt, AMP and ADP rise; in the liver they inhibit steps of glucose formation and switch on AMPK.

   observed in studies · Source 5, 2

7. B12 + intrinsic factor → B12 in the gut cell Cubam receptor · calcium The receptor brings the complex into the cell. This step runs only with calcium ions at the cell surface. Under metformin, a lower uptake is described at this step. Source 6, 7↑ supplies The calcium-dependent step at the receptor is the gate for B12 into the gut cell. Under metformin, a lower uptake is described at this point. observed in studies Source 6, 7
   ⚖ When the balance tips

   too much — If a lot of B12 is brought into the cell, the gut cell releases it into the blood bound to transcobalamin after some hours.

   too little — If there is little free calcium at the cell surface, the receptor binds the complex less well; one study describes metformin as acting at this calcium step.

   observed in studies · Source 6, 7

8. B12 in the gut cell → B12 in the blood · transcobalamin The gut cell releases B12 bound to transcobalamin into the blood. In this form cells absorb it and convert it into its two active forms. In the cells it serves two enzymes as a cofactor. Source 6↑ supplies Bound to transcobalamin, B12 reaches the cells. There it becomes methylcobalamin and adenosylcobalamin, the cofactors of two enzymes. established physiology Source 6
   ⚖ When the balance tips

   too much — If there is a lot of B12 in the blood, the excess is stored in the liver or excreted via the kidney.

   too little — If there is little B12 in the blood, methionine synthase and methylmalonyl-CoA mutase work more slowly; homocysteine and methylmalonic acid build up.

   established physiology · Source 6

9. AMP and ADP → AMPK LKB1 · AMP binds AMP and ADP bind to the AMP-activated protein kinase. The kinase LKB1 attaches a phosphate to it; AMPK is switched on by that. AMPK then curbs the building of fats. Source 5↑ supplies Once switched on, AMPK resets the energy balance: it slows the building of fats and cholesterol and stimulates processes that resupply ATP, such as the breakdown of fatty acids. established physiology Source 5
   ⚖ When the balance tips

   too much — If AMPK is strongly switched on for a long time, building processes run curbed, including cell growth controlled via mTOR.

   too little — If AMPK is little active, acetyl-CoA carboxylase stays switched on, and the liver keeps forming malonyl-CoA and new fats.

   established physiology · Source 5

10. AMPK → Malonyl-CoA ACC AMPK attaches a phosphate to acetyl-CoA carboxylase and puts it to rest. That enzyme converts acetyl-CoA into malonyl-CoA; while it rests, less of it arises. Less malonyl-CoA means more fat breakdown. Source 5↑ supplies Malonyl-CoA is the building block of new fatty acids and at the same time inhibits CPT1, the gate through which fatty acids enter the mitochondrion for breakdown. established physiology Source 5
    ⚖ When the balance tips

    too much — If there is a lot of malonyl-CoA, the liver forms more new fatty acids, and CPT1 stays inhibited; fatty acids are then stored rather than broken down.

    too little — If there is little malonyl-CoA, for instance because AMPK puts the carboxylase to rest, fewer new fats form, and more fatty acids reach the mitochondrion via CPT1 for breakdown.

    established physiology · Source 5

Further stations

• Metformin — biguanide, as a tablet Metformin belongs to the biguanides. At body pH the molecule carries a positive charge; it dissolves in water and therefore needs transporters to cross cell membranes. In the liver it lowers the formation of new glucose. Source 1, 8↓ depletes Metformin lowers the formation of new glucose in the liver; the label also names a lower absorption of glucose from the gut. Its positive charge determines where it goes. established physiology Source 8
  ⚖ When the balance tips

  too much — If a lot of active substance is present, more of it gathers in the mitochondria, the respiratory chain is slowed more strongly, and the cells form more lactate through glycolysis.

  too little — If little active substance is present, the respiratory chain stays largely unslowed, and the liver forms glucose as usual.

  observed in studies · Source 2, 3

• Metformin in the matrix — gathered along the gradient In the matrix metformin meets complex I, the first link of the respiratory chain. It settles on this enzyme and slows the passing on of the electrons. The cell then makes less ATP. Source 2, 3↓ depletes At complex I metformin slows the passing on of electrons. Fewer protons are pumped, and the cell makes less ATP. observed in studies Source 2, 3
  ⚖ When the balance tips

  too much — If a lot of metformin is at complex I, NADH is used up more slowly; NADH builds up, and more pyruvate is converted to lactate.

  too little — If little metformin is at complex I, the passing on of electrons runs unslowed, and the ratio of ATP to AMP stays unchanged.

  observed in studies · Source 3, 2

• NADH in the matrix — from the citric acid cycle NADH arises in the citric acid cycle from the breakdown of sugar, fats and amino acids. It carries two electrons that are handed over at complex I. From its electrons the cell gains much of its ATP. Source 4↑ supplies NADH is the electron supplier of the respiratory chain. From its electrons the cell gains a large share of its ATP via complex I. established physiology Source 4
  ⚖ When the balance tips

  too much — If NADH builds up because complex I is slowed, steps in the citric acid cycle that need NAD⁺ slow down, and more pyruvate is converted to lactate.

  too little — If little NADH is available, for instance because little fuel is broken down, complex I receives few electrons and pumps fewer protons.

  established physiology · Source 4, 3

• B12 + intrinsic factor — at the end of the small bowel Vitamin B12 reaches the last section of the small bowel bound to the protein intrinsic factor. Only in this form does the Cubam receptor at the gut cell recognise it. Without this complex hardly any B12 enters the body. Source 6↑ supplies Only as a complex with intrinsic factor does vitamin B12 enter the body through the gut. The complex is the precondition for uptake at the end of the small bowel. established physiology Source 6
  ⚖ When the balance tips

  too much — If a lot of B12 arrives with intrinsic factor, the Cubam receptors are soon saturated; a small part then passes through the gut wall without a receptor.

  too little — If little intrinsic factor or little B12 arrives, little reaches the gut cell via the receptor, and the store in the liver is gradually used up.

  established physiology · Source 6

• AMP and ADP — the other adenine forms ATP, ADP and AMP stand in a balance that adenylate kinase evens out. When less ATP is rebuilt, the balance shifts towards ADP and AMP. They signal that energy is running short. Source 5↑ supplies AMP and ADP are the signal that energy is running short. They switch on AMPK and inhibit individual enzymes that use energy. established physiology Source 5
  ⚖ When the balance tips

  too much — If AMP and ADP rise, AMPK is switched on more strongly; the cell curbs processes that use energy, such as building fats, and stimulates those that supply energy.

  too little — If AMP and ADP stay low because enough ATP is rebuilt, AMPK stays switched off, and building processes run unchecked.

  established physiology · Source 5

What this active substance affects

• Vitamin B12 — The Cubam receptor picks up B12 with intrinsic factor; this step at the end of the small bowel runs with calcium Source 6, 7
• Calcium — Carries the uptake step at the Cubam receptor; this step depends on calcium Source 7

What takes part in these steps

• Coenzyme Q10 — Accepts the electrons from NADH at complex I and carries them on through the inner membrane; this builds the gradient Source 4
• NAD⁺ — As NADH the electron source of complex I; NAD⁺ arises again from it for the citric acid cycle; if slowed, NADH backs up Source 4
• Vitamin B2 (riboflavin) — Forms the FMN in complex I — the site that accepts the electrons from NADH; without FMN no electrons are accepted Source 4
• Iron — Complex I carries iron-sulfur centres along which the electrons travel to coenzyme Q10; without them, transfer stalls Source 4

What the prescribing information states

One trial in the United States with 141 adults who received metformin and 145 who received a placebo. Listed are the reactions that occurred in more than 5 of 100 participants and were more common than on placebo. The figures apply to this trial.

How to read the table: this prescribing information lists only reactions that occurred more often on metformin than on placebo — reactions of equal or lower frequency are therefore missing. What matters is the distance within the row: for headache it is one point, for diarrhoea more than forty.

In the same section the prescribing information notes an observation that belongs on this page: in studies over 29 weeks the vitamin B12 value in the blood fell in about 7 of 100 participants who had an unremarkable value before. Reactions reported after approval are not listed here — the prescribing information states that neither a frequency nor a causal relationship can be derived from them.

Sources

1. Graham GG, Punt J et al. Clinical pharmacokinetics of metformin. Clin Pharmacokinet 2011 · PubMed 21241070
2. Rena G, Hardie DG, Pearson ER. The mechanisms of action of metformin. Diabetologia 2017 · PubMed 28776086
3. Foretz M, Guigas B et al. Metformin: from mechanisms of action to therapies. Cell Metab 2014 · PubMed 25456737
4. Sazanov LA. A giant molecular proton pump: structure and mechanism of respiratory complex I. Nat Rev Mol Cell Biol 2015 · PubMed 25991374
5. Hardie DG, Ross FA, Hawley SA. AMPK: a nutrient and energy sensor that maintains energy homeostasis. Nat Rev Mol Cell Biol 2012 · PubMed 22436748
6. Nielsen MJ, Rasmussen MR et al. Vitamin B12 transport from food to the body's cells — a sophisticated, multistep pathway. Nat Rev Gastroenterol Hepatol 2012 · PubMed 22547309
7. Bauman WA, Shaw S et al. Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin. Diabetes Care 2000 · PubMed 10977010
8. US prescribing information (United States): Metformin Hydrochloride Tablets, DailyMed, version of 21 Sept 2026, sections 12.1 Mechanism of Action and 12.3 Pharmacokinetics · Prescribing information
9. US prescribing information (United States): Metformin Hydrochloride Tablets, DailyMed, version of 21 Sept 2026, sections 6.1 Clinical Studies Experience and 6.2 Postmarketing Experience · Prescribing information

Whole pathway: Metformin

As of 2026-09-25. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
Legal notice Privacy policy All biomarkers