Vitamin D: the pathway in the body · Biomarker & Blood Analysis

This page shows the biochemical pathway behind the laboratory value Vitamin D, vitamin D3 (25-OH-cholecalciferol): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Vitamin D is a fat-soluble hormone precursor formed in the skin under UVB light or taken in with food. The liver and kidney convert it into calcitriol, which controls the reading of certain genes in the cell nucleus. Calcitriol raises the calcium level in the blood.

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. 7-Dehydrocholesterol → Previtamin D3 · UVB light UVB light breaks open one ring of the molecule. This forms previtamin D3, a purely light-driven step that involves no enzyme. With long sun exposure, surplus previtamin D3 is converted into inactive forms. Source 3↑ supplies Previtamin D3 is an unstable intermediate: warmth rearranges it into vitamin D3, while further UVB light converts it into inactive side forms such as lumisterol and tachysterol. established physiology Source 3
   ⚖ When the balance tips

   too much — If the skin stays in sunlight for a long time, UVB converts further previtamin D3 into lumisterol and tachysterol. This is why sunlight alone does not produce unlimited amounts of vitamin D3.

   too little — If little UVB light reaches the skin, little previtamin D3 forms; in winter the sun stands so low at northern latitudes that hardly any UVB light reaches the ground.

   established physiology · Source 3, 1

2. Previtamin D3 → Vitamin D3 · Body heat The warmth of the skin rearranges previtamin D3 into vitamin D3. Vitamin D from food reaches the same point in the pathway via the gut. Part of it can be stored in fat tissue. Source 3, 1↑ supplies Vitamin D3 itself has little action; it is the starting material for all further forms. Vitamin D-binding protein carries it from the skin into the blood, and part of it is stored in fat tissue. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of vitamin D3 arrives, part of it is stored in fat tissue and released slowly from there; the rest reaches the liver and is converted into calcidiol.

   too little — If little vitamin D3 arrives from skin and food, the liver has less starting material, and less calcidiol forms.

   established physiology · Source 1

3. Vitamin D3 → 25-OH-vitamin D3 CYP2R1 (liver) · NADPH In the liver, mainly the enzyme CYP2R1 attaches an OH group. This forms calcidiol, the form that circulates in the blood bound to vitamin D-binding protein. Calcidiol is the pool from which calcitriol is made. Source 1, 4↑ supplies Calcidiol is the pool in the blood: bound to the binding protein, it circulates for several weeks and is available to the kidney and other tissues for making calcitriol. It binds the receptor only weakly itself. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If a very large amount of calcidiol builds up, it occupies the receptor itself despite weak binding; the gut then absorbs more calcium, and blood calcium rises. CYP24A1 breaks down more at the same time.

   too little — If little calcidiol is available, the kidney has less starting material. Calcitriol formation holds up at first, because parathyroid hormone rises and drives CYP27B1 harder.

   established physiology · Source 1, 2

4. Calcitriol → VDR in the nucleus · VDR and RXR Calcitriol binds to the vitamin D receptor. Together with its partner RXR, the receptor attaches to specific sections of DNA. The more receptors are occupied, the more strongly the genes respond. Source 2↑ supplies Only the pair of VDR and RXR turns the hormone into a signal to the genes. How strongly a cell responds depends on how much receptor it carries and how much calcitriol arrives. established physiology Source 2, 6
   ⚖ When the balance tips

   too much — If more calcitriol is present, more receptors are occupied, and the target genes are read more strongly; among them the gene for CYP24A1, which breaks calcitriol down again.

   too little — If few receptors are occupied, the target genes are read more weakly; calcium then reaches the blood mainly by the slower route between the gut cells.

   established physiology · Source 2

5. VDR in the nucleus → Target genes The receptor pair switches the reading of certain genes on or off. These include genes for calcium transport, bone remodelling and immune cells. In this way calcitriol also slows PTH production and triggers its own breakdown. Source 2↕ both, depending on amount The receptor pair acts in both directions: genes for TRPV6, calbindin and CYP24A1 are read more strongly, the genes for parathyroid hormone and CYP27B1 more weakly. Calcitriol thus regulates its own amount. established physiology Source 2, 1
   ⚖ When the balance tips

   too much — If the target genes are switched on strongly, calcium transport rises, and at the same time more CYP24A1 forms, which breaks down calcitriol; the formation of parathyroid hormone is throttled.

   too little — If the target genes are switched on weakly, the brake on the parathyroid hormone gene falls away; the parathyroid glands make more parathyroid hormone, and fewer calcium transporters form.

   established physiology · Source 2, 1

6. Target genes → Calcium transport TRPV6, calbindin Two of these gene products are the channel TRPV6 and the transport protein calbindin. They move calcium from the gut into the cell and on into the blood. With little calcitriol, the gut absorbs less calcium. Source 2↑ supplies TRPV6 lets calcium into the gut cell, calbindin carries it through the cell, and on the opposite side a calcium pump moves it into the blood. Calcitriol turns this route up when calcium is scarce. established physiology Source 2
   ⚖ When the balance tips

   too much — If a lot of calcium is absorbed via this route, the blood calcium level rises; parathyroid hormone falls and with it the formation of calcitriol.

   too little — If little calcium is absorbed via this route, the body maintains the blood level by parathyroid hormone releasing calcium from bone and the kidney retaining more calcium.

   established physiology · Source 2, 1

7. Calcitriol → Calcitroic acid CYP24A1 · NADPH The enzyme CYP24A1 breaks down calcitriol and calcidiol step by step. The end product is calcitroic acid, which is excreted via the bile. CYP24A1 thus limits the amount of active vitamin D. Source 1, 4↓ depletes CYP24A1 is the brake of the system: it inactivates calcitriol and calcidiol and is switched on by calcitriol itself and by FGF23. Calcitroic acid no longer binds the receptor. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If CYP24A1 works hard, for example driven by FGF23, calcitriol is broken down quickly, and the calcidiol pool also empties faster.

   too little — If CYP24A1 works little, calcitriol is broken down more slowly; it acts for longer, and blood calcium can rise.

   established physiology · Source 1, 4

Further stations

• 7-Dehydrocholesterol — precursor in the skin The lower layers of the skin contain 7-dehydrocholesterol, a close relative of cholesterol. It is the starting material for vitamin D3. How much of it the skin holds helps set how much vitamin D3 can form. Source 3↑ supplies 7-Dehydrocholesterol is the raw material of the whole pathway: from it, UVB light forms previtamin D3 in the skin. How much of it the skin holds helps set how much can form under the same light. established physiology Source 3
  ⚖ When the balance tips

  too much — If the skin holds a lot of 7-dehydrocholesterol, the same amount of UVB light forms more previtamin D3; the upper limit is set in the next step, not here.

  too little — If the skin holds little 7-dehydrocholesterol, less previtamin D3 forms under the same light, and the pathway depends more on vitamin D from food.

  established physiology · Source 3

• Calcitriol — active form In the kidney, CYP27B1 attaches a second OH group, forming calcitriol. Parathyroid hormone (PTH) stimulates this enzyme; the messenger FGF23 slows it down. Calcitriol raises calcium and phosphate in the blood. Source 1, 2↑ supplies Calcitriol raises calcium and phosphate in the blood: more absorption in the gut, less loss via the kidney, release from bone. At the same time it slows its own formation and stimulates CYP24A1 to break it down. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If calcitriol builds up, the gut absorbs more calcium and phosphate, and bone releases more; the blood calcium level rises. Parathyroid hormone falls, and CYP24A1 is turned up.

  too little — If little calcitriol is available, the gut absorbs less calcium; blood calcium falls, parathyroid hormone rises and draws calcium from bone, and the bone is less mineralised.

  established physiology · Source 1, 2

Cofactors in this pathway

• Magnesium — Described as a cofactor of the enzymes that convert vitamin D in liver and kidney, and of the binding protein in blood Source 5In the ORY catalogue as a laboratory value: Magnesium
• NADPH — Electron donor for CYP2R1, CYP27B1 and CYP24A1; without it every conversion step stalls Source 1
• Iron — Haem iron at the centre of the cytochrome P450 enzymes that convert vitamin D; it activates oxygen for the OH group Source 1, 2In the ORY catalogue as a laboratory value: Eisen
• Zinc — Forms the zinc fingers with which the vitamin D receptor binds to DNA; without them the receptor cannot dock Source 6In the ORY catalogue as a laboratory value: Zink
• Vitamin A (retinoic acid) — Binds to RXR, the partner of the vitamin D receptor; only the pair binds to DNA Source 6
• Calcium — Transported across the intestinal wall via the gene products TRPV6 and calbindin; calcitriol turns this route up Source 2In the ORY catalogue as a laboratory value: Calcium (intrazellulär)

Sources

1. Bikle DD. Vitamin D metabolism, mechanism of action, and clinical applications. Chem Biol 2014 · PubMed 24529992
2. Christakos S, Dhawan P, Verstuyf A et al. Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects. Physiol Rev 2016 · PubMed 26681795
3. Holick MF, MacLaughlin JA, Clark MB et al. Photosynthesis of previtamin D3 in human skin and the physiologic consequences. Science 1980 · PubMed 6251551
4. Saponaro F, Saba A, Zucchi R. An Update on Vitamin D Metabolism. Int J Mol Sci 2020 · PubMed 32911795
5. Uwitonze AM, Razzaque MS. Role of Magnesium in Vitamin D Activation and Function. J Am Osteopath Assoc 2018 · PubMed 29480918
6. Haussler MR, Haussler CA, Jurutka PW et al. The vitamin D hormone and its nuclear receptor: molecular actions and disease states. J Endocrinol 1997 · PubMed 9379138

Related pathways

• Calprotectin — Eisen, Zink
• Estradiol — Magnesium, Eisen
• HbA1c and insulin — Magnesium, Zink
• Neopterin — Magnesium, Eisen
• 8-OHdG — Eisen, Zink

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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