Vitamin A (Retinol): the pathway in the body

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

Where this laboratory value sits: Retinol on RBP4 — transport form in blood. The liver releases retinol from the store and sends it into the blood bound to retinol-binding protein (RBP4). This form is measured as retinol in serum. Source 2, 1

In brief

Vitamin A is a fat-soluble vitamin that exists as retinol, retinal and retinoic acid. In the eye, retinal forms the visual pigment rhodopsin; retinoic acid switches genes on and off via receptors in the cell nucleus that control how cells mature.

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. Vitamin A from food → Retinol in the gut cell lipases · bile acids In the gut cell, retinol from food and from beta-carotene comes together. A binding protein holds it and passes it on to the next enzyme. Source 2, 1↑ supplies The binding protein keeps retinol dissolved in the watery cell and directs it specifically to esterification. established physiology Source 1
   ⚖ When the balance tips

   too much — If more retinol arrives than LRAT esterifies, part of it is esterified by a second enzyme or released unchanged into the blood.

   too little — If little arrives, fewer retinyl esters are packed into chylomicrons and the liver receives less for its store.

   established physiology · Source 1

2. Beta-carotene → Retinol in the gut cell BCO1 In the gut cell, retinol from food and from beta-carotene comes together. A binding protein holds it and passes it on to the next enzyme. Source 2, 1↑ supplies The binding protein keeps retinol dissolved in the watery cell and directs it specifically to esterification. established physiology Source 1
   ⚖ When the balance tips

   too much — If more retinol arrives than LRAT esterifies, part of it is esterified by a second enzyme or released unchanged into the blood.

   too little — If little arrives, fewer retinyl esters are packed into chylomicrons and the liver receives less for its store.

   established physiology · Source 1

3. Retinol in the gut cell → Chylomicrons LRAT The enzyme LRAT attaches a fatty acid to retinol again. The retinyl esters enter chylomicrons, fat droplets that flow via the lymph into the blood. Source 2, 1↑ supplies Chylomicrons carry freshly absorbed vitamin A to the tissues; most of it ends up in the liver with their remnants. established physiology Source 2
   ⚖ When the balance tips

   too much — If a lot of vitamin A arrives with the chylomicrons, adipose tissue, muscle and other tissues also absorb part of it directly.

   too little — If little arrives, the liver receives less supply and its store is drawn on more heavily.

   established physiology · Source 2

4. Chylomicrons → Store in the liver The liver stores most of the body's vitamin A as retinyl esters in fat droplets of the stellate cells. This store lasts a long time. Source 2, 1↑ supplies The liver store buffers fluctuating intake: it absorbs vitamin A after meals and releases it evenly into the blood. established physiology Source 2
   ⚖ When the balance tips

   too much — If the store is very full, vitamin A in the blood rises as retinyl esters in lipoproteins, and the stellate cells change.

   too little — If the store is almost empty, the liver can no longer maintain retinol in the blood; only then does the blood level fall.

   established physiology · Source 2

5. Store in the liver → Retinol on RBP4 · RBP4 The liver releases retinol from the store and sends it into the blood bound to retinol-binding protein (RBP4). This form is measured as retinol in serum. Source 2, 1↑ supplies RBP4 keeps retinol dissolved in the blood and delivers it to the cells; together with transthyretin the complex is too large to be lost via the kidneys. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — The liver keeps retinol on RBP4 within narrow limits; extra vitamin A hardly raises this form but is stored instead.

   too little — If the liver makes little RBP4, for example when little zinc or protein is available, retinol stays in the store even though enough is stored.

   established physiology · Source 2, 6

6. Retinol in the cell → Retinal RDH10 · NAD⁺ Retinol dehydrogenases such as RDH10 oxidise retinol to retinal with NAD⁺. This step is reversible and helps decide how much retinoic acid forms. Source 3↕ both, depending on amount Retinal is an intermediate: part is oxidised further to retinoic acid and part is reduced back to retinol; in this way the cell regulates the amount of retinoic acid. established physiology Source 3
   ⚖ When the balance tips

   too much — If a lot of retinal forms, other enzymes reduce part of it back to retinol so that not too much retinoic acid is made.

   too little — If little retinal forms, little retinoic acid is made, and the retinoic acid-dependent genes are read less often.

   established physiology · Source 3

7. Retinal → Retinoic acid RALDH · NAD⁺ Retinaldehyde dehydrogenases (RALDH) oxidise retinal to retinoic acid with NAD⁺. This step is irreversible. Source 3↕ both, depending on amount Retinoic acid acts like a hormone inside the cell: it switches genes on and off. The amount matters, because formation by RALDH and breakdown by CYP26 keep it within narrow limits. established physiology Source 3, 4
   ⚖ When the balance tips

   too much — If retinoic acid builds up, it switches on the genes of the CYP26 enzymes that break it down; in this way the cell keeps the amount in check.

   too little — If little retinoic acid forms, cells make less CYP26 and break down less of it; even so, the target genes are then read less often.

   established physiology · Source 3

8. Retinol in the cell → 11-cis-retinal LRAT, RPE65, RDH5 In the pigment epithelium of the retina, retinol is esterified, twisted and oxidised to 11-cis-retinal. This angled form fits into the visual pigment. Source 5↑ supplies 11-cis-retinal is the light-sensitive part of the visual pigment; without supply from this cycle, rods and cones could not become ready again after a light stimulus. established physiology Source 5
   ⚖ When the balance tips

   too much — If more retinal accumulates in the eye than the cycle can process, two molecules can combine to form by-products that build up in the pigment epithelium.

   too little — If little retinol reaches the eye, less 11-cis-retinal is formed and the visual pigment is rebuilt more slowly after exposure to light.

   observed in studies · Source 5

9. 11-cis-retinal → Rhodopsin · opsin 11-cis-retinal binds to the protein opsin, forming rhodopsin. When light strikes it, retinal straightens and the signal goes to the nerve cells. Source 5↑ supplies Rhodopsin makes vision in dim light possible: a single particle of light can flip one molecule and trigger a signal. established physiology Source 5
   ⚖ When the balance tips

   too much — Once all opsin is loaded with 11-cis-retinal, further retinal adds no extra light sensitivity.

   too little — If little 11-cis-retinal is available, opsin partly stays empty and the rods regain sensitivity more slowly after brightness.

   established physiology · Source 5

10. Retinoic acid → Oxidised retinoic acid CYP26 Enzymes of the CYP26 family oxidise retinoic acid to forms that have little action left and are excreted. Source 3↓ depletes Breakdown by CYP26 withdraws retinoic acid from the receptors; in this way each tissue helps decide how strongly the signal reaches it. established physiology Source 3
    ⚖ When the balance tips

    too much — If breakdown is strong, little retinoic acid remains for the receptors in that tissue.

    too little — If breakdown is weak, retinoic acid stays longer in the cell and acts longer on the receptors.

    established physiology · Source 3

11. Retinoic acid → RAR and RXR Retinoic acid enters the nucleus and binds to the receptor RAR, which forms a pair with RXR and sits at particular sites on the DNA. Source 4↑ supplies With retinoic acid bound, the receptor pair releases repressing partner proteins and recruits activating ones. established physiology Source 4
    ⚖ When the balance tips

    too much — Once the receptors are occupied, further retinoic acid brings no stronger signal; the number of receptors then sets the limit.

    too little — If little retinoic acid is available, the receptors stay bound to repressing partner proteins and their target genes stay silent.

    established physiology · Source 4

12. RAR and RXR → Gene transcription Through RAR and RXR, retinoic acid controls genes that determine how cells of the skin, mucous membranes and immune system mature, and that set patterns in embryonic development. Source 4↕ both, depending on amount The same genes respond to too little and to too much retinoic acid: cell maturation follows the amount that reaches the nucleus. established physiology Source 4
    ⚖ When the balance tips

    too much — If the signal is strong, the target genes are read more often; in embryonic development the patterns set by retinoic acid then shift.

    too little — If the signal is weak, cells of the mucous membranes mature differently and produce less mucus.

    established physiology · Source 4

Further stations

• Vitamin A from food — as retinyl esters Preformed vitamin A is found in liver, egg yolk and milk fat, mostly as retinyl esters, i.e. bound to a fatty acid. In the gut, lipases remove the fatty acid; bile acids keep it in solution. Source 2, 1↑ supplies Vitamin A is fat-soluble: it enters the micelles from which the gut cell absorbs it only together with dietary fat and bile acids. established physiology Source 2
  ⚖ When the balance tips

  too much — If a lot of preformed vitamin A arrives, most of it is absorbed; unlike beta-carotene, there is hardly any brake on this in the gut.

  too little — If little fat or bile reaches the gut, more vitamin A stays undissolved and is not absorbed.

  established physiology · Source 2

• Beta-carotene — precursor from plants Beta-carotene from carrots, pumpkin and green vegetables is a precursor. The enzyme BCO1 cleaves it in the gut cell to retinal, which is converted to retinol. Source 2↑ supplies Beta-carotene provides vitamin A as needed: the body cleaves only as much as it needs; the rest is stored as carotene in the tissues. established physiology Source 2
  ⚖ When the balance tips

  too much — If the vitamin A store is well filled, the gut cell reduces the uptake and cleavage of beta-carotene via a feedback pathway.

  too little — If little vitamin A is stored, more beta-carotene is absorbed and cleaved to retinal.

  established physiology · Source 2

• Retinol in the cell — absorbed via STRA6 Cells remove retinol from the RBP4 complex, some via the receptor STRA6. Inside, a binding protein holds it until it is processed further. Source 1, 4↑ supplies Retinol itself has little action; it is the precursor from which the cell makes retinal and retinoic acid as needed. established physiology Source 1, 4
  ⚖ When the balance tips

  too much — If more retinol enters the cell than it converts, the cell esterifies it again and builds a small store of its own.

  too little — If little arrives, less retinal and retinoic acid form, and cells with high demand, such as those of the eye, receive less.

  established physiology · Source 1

Cofactors in this pathway

• Zinc — Needed for the liver to make RBP4 and release retinol into the blood Source 6In the ORY catalogue as a laboratory value: Zink
• Bile acids — Dissolve vitamin A and beta-carotene in micelles from which the gut cell absorbs them Source 2In the ORY catalogue as a laboratory value: Gallensäuren (Stuhl)
• NAD⁺ — Accepts the electrons when retinol is oxidised to retinal and retinal to retinoic acid Source 3In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Dietary fat — Forms, with bile acids, the micelles and chylomicrons that carry vitamin A Source 2
• RBP4 — Transport protein that carries retinol from the liver to the cells Source 1
• Opsin — Protein of the rods that forms the visual pigment rhodopsin with 11-cis-retinal Source 5

Sources

1. O'Byrne SM, Blaner WS. Retinol and retinyl esters: biochemistry and physiology. J Lipid Res 2013 · PubMed 23625372
2. Blaner WS, Li Y, Brun PJ et al. Vitamin A Absorption, Storage and Mobilization. Subcell Biochem 2016 · PubMed 27830502
3. Kedishvili NY. Retinoic Acid Synthesis and Degradation. Subcell Biochem 2016 · PubMed 27830503
4. Ghyselinck NB, Duester G. Retinoic acid signaling pathways. Development 2019 · PubMed 31273085
5. Kiser PD, Golczak M, Palczewski K. Chemistry of the retinoid (visual) cycle. Chem Rev 2014 · PubMed 23905688
6. Christian P, West KP Jr. Interactions between zinc and vitamin A: an update. Am J Clin Nutr 1998 · PubMed 9701158

Whole pathway: Vitamin A

Related pathways

• APOE — chylomicrons
• Cholesterol and lipoproteins — chylomicrons
• Thyroid hormones — gene transcription
• Trans fatty acids — chylomicrons
• Triglycerides — chylomicrons

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