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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Vitamin A (retinol)

Vitamin A lets you see in dim light, keeps the linings of the eye, skin and gut healthy, and supports immunity.

Updated 2026-11-01

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## Key facts

- Active forms: 11-cis-retinal in rhodopsin (vision) and retinoic acid, which acts in the nucleus like a hormone (RAR/RXR receptors).
- The first sign of deficiency is night blindness; xerophthalmia, Bitot spots and keratomalacia follow.
- Stored in the liver (hepatic stellate, Ito, cells) and carried in blood bound to retinol-binding protein (RBP).
- In excess it is toxic and teratogenic: retinoids are contraindicated in pregnancy.

## In detail

- Sources: preformed retinol in animal foods (liver, egg yolk, whole milk) and plant carotenoids (carrot, squash, mango, leafy greens), which the gut mucosa converts to retinal.
- Absorption: like fat, inside bile-salt micelles; it reaches the liver in chylomicrons as retinyl esters.
- Visual cycle: light turns the 11-cis-retinal of rhodopsin into all-trans-retinal; that isomerisation activates transducin and closes sodium channels in the rod.
- Retinoic acid: no role in vision; it controls epithelial differentiation, embryonic development and the immune response.
- Deficiency: night blindness, dry conjunctiva and cornea, keratinised epithelia, more infections (severe measles, diarrhoea).
- Excess: raised intracranial pressure, liver damage, bone pain, dry scaly skin; fetal malformations.
- Lab: serum retinol (reflects stores poorly until they are very low); measured with RBP and C-reactive protein, because inflammation lowers it.

**Also called**
Retinol, Retinal, Retinoic acid, β-carotene (provitamin A)

**Formula**
C₂₀H₃₀O

**PubChem**
[CID 445354](https://pubchem.ncbi.nlm.nih.gov/compound/445354)

## Structure

- A β-ionone ring joined to a chain of four conjugated double bonds (an isoprenoid); the end group can be an alcohol (retinol), aldehyde (retinal) or acid (retinoic acid).
- β-carotene is two retinal molecules joined tail to tail; the gut splits it in half.

## Clinical notes

- Where deficiency is common, vitamin A supplements in young children reduce deaths from measles and diarrhoea.
- Isotretinoin (acne) and tretinoin (acute promyelocytic leukaemia) are retinoids; the first requires avoiding pregnancy.
- Fat malabsorption (cystic fibrosis, cholestasis, bariatric surgery) causes deficiency of A, D, E and K together.

## Related conditions

- [Vitamin A deficiency (night blindness and xerophthalmia)](https://durgical.com/en/conditions/vitamin-a-deficiency/)
- [Hypervitaminosis A (vitamin A toxicity)](https://durgical.com/en/conditions/hypervitaminosis-a/)

## Related

- [Vitamin E (α-tocopherol)](https://durgical.com/en/biochemistry/molecules/vitamin-e/)
- [Zinc](https://durgical.com/en/biochemistry/topics/zinc/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which form of vitamin A takes part in vision?

11-cis-retinal, bound to opsin in the rhodopsin of rods; light isomerises it to all-trans-retinal.

## What is the earliest symptom of vitamin A deficiency?

Night blindness, because rods cannot regenerate enough rhodopsin.

## Why does retinoic acid behave like a hormone?

It enters the nucleus and binds RAR/RXR receptors that switch differentiation genes on or off.

## Why does isotretinoin require avoiding pregnancy?

It is a teratogenic retinoid: it disrupts development of the fetal skull, face, heart and nervous system.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Retinol (CID 445354)](https://pubchem.ncbi.nlm.nih.gov/compound/445354) (PubChem, NCBI, 2026) · Public domain; [Vitamin A: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002400.htm) (MedlinePlus, US National Library of Medicine, 2026); [Vitamin A blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003570.htm) (MedlinePlus, US National Library of Medicine, 2026); [Vitamin A deficiency](https://www.who.int/data/nutrition/nlis/info/vitamin-a-deficiency) (World Health Organization, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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