Vitamin B1 (Thiamin): the pathway in the body

Vitamin B1 (Thiamin) is part of the pathway “Vitamin B1”. This page shows the whole pathway; the station of Vitamin B1 (Thiamin) is highlighted.

Where this laboratory value sits: Thiamine in the blood — mostly in red blood cells. In the blood most of it sits as TPP inside red blood cells, while a small part circulates freely. Free thiamine that no cell absorbs is excreted by the kidneys in the urine. Source 1

In brief

Vitamin B1 (thiamine) is a water-soluble vitamin that the body cannot make itself. As thiamine pyrophosphate (TPP) it is a cofactor of enzymes that feed pyruvate into the citric acid cycle, drive the cycle and link the pentose phosphate pathway with glycolysis.

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. Thiamine in food → Into the gut cell THTR1, THTR2 The transporters THTR1 and THTR2 carry thiamine from the gut contents into the gut cell. On the blood side it leaves the cell again via a transporter. Source 2↑ supplies THTR1 (SLC19A2) and THTR2 (SLC19A3) absorb thiamine efficiently even at small amounts. Thiamine pyrophosphate made by bacteria in the large intestine is absorbed by a transporter of its own. established physiology Source 2
   ⚖ When the balance tips

   too much — Once the transporters are saturated, uptake rises no further; only a small part still enters the cell by passive diffusion.

   too little — If the transporters are occupied by drugs, less thiamine gets in; for several active substances this inhibition of THTR2 has been described in cell experiments.

   observed in studies · Source 2, 5

2. Into the gut cell → Thiamine in the blood In the blood most of it sits as TPP inside red blood cells, while a small part circulates freely. Free thiamine that no cell absorbs is excreted by the kidneys in the urine. Source 1↑ supplies The blood distributes thiamine to all tissues; heart, muscle, liver, kidneys and brain absorb it particularly readily. established physiology Source 1
   ⚖ When the balance tips

   too much — If free thiamine in the blood rises, the kidneys excrete the excess in the urine; no large store builds up.

   too little — If little follows, TPP in the red blood cells falls; the tissues then receive less thiamine for their enzymes.

   established physiology · Source 1

3. Thiamine in the cell → TPP TPK1 · ATP Thiamine pyrophosphokinase (TPK1) transfers a pyrophosphate group from ATP to thiamine. This produces TPP, the active form, which sits as a cofactor in several enzymes. Source 1↑ supplies In its enzymes TPP breaks a carbon bond next to a keto group and holds the fragment briefly until it is passed on. In this way it links glucose breakdown, the citric acid cycle and the pentose phosphate pathway. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — Once the enzymes are loaded with TPP, further TPP brings no extra activity; the amount of enzyme then sets the limit.

   too little — If little TPP is available, pyruvate dehydrogenase, α-ketoglutarate dehydrogenase and transketolase all slow down at the same time.

   established physiology · Source 1

4. Pyruvate → Acetyl-CoA PDH complex · TPP, coenzyme A Pyruvate dehydrogenase removes carbon dioxide from pyruvate and attaches the rest to coenzyme A. Besides TPP it needs lipoic acid, FAD, NAD⁺ and coenzyme A. Source 3↑ supplies Acetyl-CoA carries carbon from sugar into the citric acid cycle; the electrons gained there drive the respiratory chain, where ATP is made. established physiology Source 3
   ⚖ When the balance tips

   too much — If a lot of acetyl-CoA and NADH forms, the cell slows pyruvate dehydrogenase via a kinase that switches the enzyme off with phosphate.

   too little — If little acetyl-CoA forms from pyruvate, less carbon from sugar enters the citric acid cycle and less NADH reaches the respiratory chain.

   established physiology · Source 3

5. α-Ketoglutarate → Succinyl-CoA α-KG dehydrogenase · TPP α-Ketoglutarate dehydrogenase removes carbon dioxide and forms succinyl-CoA; NADH is produced in the process. The citric acid cycle continues from here. Source 1↑ supplies Succinyl-CoA keeps the citric acid cycle going and is also the starting material for the building block of haem. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of succinyl-CoA forms, it continues in the citric acid cycle to succinate and is processed along with the rest of the supply.

   too little — If the dehydrogenase works slowly, less succinyl-CoA and less NADH form; the respiratory chain receives fewer electrons from this point.

   established physiology · Source 1

6. Pentose phosphates → Glycolytic intermediates transketolase · TPP Transketolase makes fructose 6-phosphate and glyceraldehyde 3-phosphate from pentose phosphates. Both flow back into glycolysis. Source 4, 1↑ supplies Through this return route the cell can use sugar as needed for energy, for ribose or for NADPH. established physiology Source 4
   ⚖ When the balance tips

   too much — If many intermediates come back, they continue in glycolysis to pyruvate or are turned back into glucose 6-phosphate.

   too little — If few come back, the carbon stays bound in the pentose phosphates and the cell can use the pathway less flexibly.

   established physiology · Source 4

Further stations

• Thiamine in food — vitamin B1 Thiamine, vitamin B1, is water-soluble and is found in wholegrains, pulses and pork, for example. Part of it is present as phosphate esters; enzymes in the gut remove the phosphate before thiamine is absorbed. Source 2, 1↑ supplies The body does not make thiamine itself and keeps only a small store that turns over quickly. What it needs comes continuously from food; a small part comes from gut bacteria. established physiology Source 2, 1
  ⚖ When the balance tips

  too much — If a lot of thiamine arrives, the transporters in the small intestine are soon saturated; a small part then enters by passive diffusion and the rest passes on through the gut.

  too little — If little thiamine arrives, the body draws on its small store; because thiamine is turned over continuously, this store empties within weeks.

  established physiology · Source 2, 1

• Thiamine in the cell — absorbed via THTR Tissue cells bring in thiamine with the same transporters, THTR1 and THTR2. In the cytoplasm it is converted to TPP straight away. Source 1↑ supplies Because the cell quickly converts thiamine to TPP, free thiamine stays scarce inside; this keeps up the gradient that draws new thiamine in. established physiology Source 1
  ⚖ When the balance tips

  too much — If more thiamine enters than the kinase converts, more free thiamine remains in the cell; it can leave the cell again.

  too little — If little thiamine enters the cell, less TPP forms, and all TPP-dependent enzymes receive less cofactor.

  established physiology · Source 1

• Pyruvate — from glucose breakdown Pyruvate is the end product of glycolysis in the cytoplasm. In the mitochondrion it waits for pyruvate dehydrogenase, which feeds it into the citric acid cycle. Source 3↕ both, depending on amount Pyruvate stands at a junction: with TPP in pyruvate dehydrogenase it enters energy production as acetyl-CoA; without this step it is mainly converted to lactate. established physiology Source 3
  ⚖ When the balance tips

  too much — If pyruvate builds up because pyruvate dehydrogenase is working slowly, more of it is converted to lactate and alanine.

  too little — If little pyruvate forms because little glucose is broken down, acetyl-CoA comes more from the breakdown of fatty acids.

  established physiology · Source 3, 1

• α-Ketoglutarate — in the citric acid cycle α-Ketoglutarate is an intermediate of the citric acid cycle. α-Ketoglutarate dehydrogenase converts it; it is built like pyruvate dehydrogenase and also needs TPP. Source 1, 3↕ both, depending on amount α-Ketoglutarate either carries the citric acid cycle forward or leaves it as a building block for glutamate; which path predominates also depends on how fast the TPP-dependent dehydrogenase works. established physiology Source 1, 3
  ⚖ When the balance tips

  too much — If α-ketoglutarate builds up in front of a slow dehydrogenase, more of it is converted to glutamate and the citric acid cycle runs more slowly.

  too little — If little α-ketoglutarate is available, for example because much of it flows off as glutamate, the second half of the citric acid cycle receives less supply.

  established physiology · Source 1

• Pentose phosphates — sugars with five carbons The pentose phosphate pathway produces sugars with five carbon atoms, for example for building DNA. The TPP-dependent transketolase shifts two-carbon fragments between these sugars. Source 4, 1↑ supplies Pentose phosphates supply ribose for RNA and DNA; the pathway that forms them also produces NADPH, which keeps glutathione in its active form. established physiology Source 4, 1
  ⚖ When the balance tips

  too much — If more pentose phosphates form than are needed for DNA, transketolase returns them to intermediates of glycolysis.

  too little — If transketolase works slowly, pentose phosphates build up and the exchange between the pentose phosphate pathway and glycolysis stalls.

  established physiology · Source 4, 1

Cofactors in this pathway

• Magnesium — Accompanies ATP when TPP is formed and holds TPP in the correct position in its enzymes Source 1In the ORY catalogue as a laboratory value: Magnesium
• NAD⁺ — Accepts the electrons in pyruvate and α-ketoglutarate dehydrogenase and becomes NADH Source 3In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Coenzyme A (vitamin B5) — Accepts the acetyl group in pyruvate dehydrogenase, producing acetyl-CoA Source 3
• FAD (vitamin B2) — Cofactor of the third subunit of pyruvate dehydrogenase, which recharges the enzyme after each turnover Source 3
• Lipoic acid — Carries the acetyl group from TPP on to coenzyme A in pyruvate dehydrogenase Source 3
• ATP — Supplies the pyrophosphate group that TPK1 transfers to thiamine Source 1

Sources

1. Manzetti S, Zhang J, van der Spoel D. Thiamin function, metabolism, uptake, and transport. Biochemistry 2014 · PubMed 24460461
2. Said HM, Nexo E. Gastrointestinal Handling of Water-Soluble Vitamins. Compr Physiol 2018 · PubMed 30215865
3. Patel MS, Nemeria NS, Furey W et al. The pyruvate dehydrogenase complexes: structure-based function and regulation. J Biol Chem 2014 · PubMed 24798336
4. Lonsdale D. Thiamin. Adv Food Nutr Res 2018 · PubMed 29477220
5. Li P, Zhu Z, Wang Y et al. Substrate transport and drug interaction of human thiamine transporters SLC19A2/A3. Nat Commun 2024 · PubMed 39738067

Whole pathway: Vitamin B1

Related pathways

• Branched-chain amino acids — succinyl-coa
• Citric acid cycle — succinyl-coa
• Threonine — succinyl-coa
• Vitamin B12 — succinyl-coa
• Alanine — Magnesium, NAD⁺ (Nicotinamidadenindinukleotid)

As of 2026-10-05. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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