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.

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.

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.

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.

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.

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) (PubChem, NCBI, 2026) · Public domain; Vitamin A: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Vitamin A blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); 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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Representative model, for educational purposes only.