Vitamin A exists in two forms - retinoids found in animal foods and carotenoids found in plants. Retinoids are absorbed in the small intestine and transported to the liver where they are stored. Carotenoids are cleaved to form retinal. Vitamin A plays a key role in vision through the visual cycle where retinal combines with opsin to form rhodopsin, allowing light detection. It also regulates gene expression and tissue differentiation. Deficiency can cause night blindness and xerophthalmia while excess intake can result in dermatitis and skeletal issues.

1. Vitamin ‘A’

2. Vit A activity : is shown by retinoids and
carotenoids
Retinoids: preformed vit A found only in foods of animal origin.
They are three types:
1. Retinol: alcohol with beta ionone ring. Side chain and one OH grp. +nt
in animal tissue as retinyl ester with long chain FA.
2. Retinal: oxidation of retinol retinal ( aldehyde)
3. Retinoic acid: oxidation of retinal retinoic acid ( acidic)

3. Carotenoids : Provitamin A; found in plants.
• β-carotene and related compounds
• β-carotene (provitamin A): found in plants cleaved in intestinal
mucosa by carotene dioxygenase  retinal.
• Theoretically, produces 2 moles of retinal.
• But in practical; is inefficient in humans, β-carotene has 1/6th vitamin A
activity compared to that of retinol. (6 μg of β-carotene is equivalent to 1
μg of preformed retinol)

4. Recommended dietary allowance
• Retinol equivalents (RE) rather than International units(IU)
• 1 RE= 1 μg retinol
• Woman= 800 RE
• Man= 1000 RE
• One IU equals = 0.3 mg of retinol
• The requirement increases in growing children, pregnant
women and lactating mothers

5. Dietary sources
• Animal source contain preformed vit A
• The best sources are liver, kidney, egg yolk, milk,
cheese, butter. 🧀🍼🧀
• Fish liver oils are very rich in vitamin A. 🐟🐠
• Vegetable sources contain the provitamin A-carotene
• Yellow and dark green vegetables: tomato,carrots,
spinach, pumpkins, 🧀🍅🧀
• fruits are good sources: mango, papaya etc. 🧀

6. 👉🏻👉🏻Absorption
and
transport
of
vitamin A👍👍

7. 1. Dietary retinyl esters hydrolysed by pancreatic and intestinal
hydrolases retinol and FFA.
2. Whereas beta carotene retinal reduced to retinol
3. In intestinal mucosal cell both retinol get esterified again retinyl
esters.
4. Carried by chylomicrons  transferred to lymph
5. Retinol esters is taken up by liver and stored.
6. When needed vit A is released from liver as free retinol.
7. Retinol is bound with RBP in circulation form complex
8. At peripheral target tissue binds with specific receptor on cell
membrane enters into cell.
9. Retinol  retinoic acid enters into nucleus binds with nucleus
retinoid receptor regulate gene transcription.

9. 👀👀Rhodopsin / Wald’s visual cycle🏻
Retina>>> retinal. 🏻 🏻
Two types of cell rods cell ( dim) and cones cell ( color and
day)
rhodopsin iodopsin
11-cis retinal ( CHO- grp) + opsin (NH2- of lysine) rhodopsin
(rods) and iodopsin (in cones).

11. Wald's visual cycle
How does this light
strikes create vison in
our brain?
What is actually
happening?
What is visual cascade&
cGMP?
🏻
🏻

12. • When light strikes, the following reaction occurs in actual
💥👉🧀Visual cascade and cGMP
• This results in hyperpolarization
• Which is an excitatory response transmitted
through the neuron network to the visual cortex of
the brain
• image is formed😃😎
Breaks
cGMP
Bleachingaction

13. COLOR VISION
• Cones cell are specialized in bright and color vision
• Visual cycle comparable to rodes is seen in cones too.
• Governed by color sensitive pigments:
• Porphyropsin ( red)
• Iodopsin (green )
• Cyanopsin (blue)
• Splitting of these three pigments in different proportion results in
perception of different color by brain.
•👠🧀🌹🌼

14. Retinol ,Retinoic Acid : Regulation of Gene
Expression & Tissue Differentiation
• Retinoic acid growth, development and tissue
differentiation.
• Like steroid hormone and vit D, it binds with nuclear
receptor  that binds with response element of DNA.
• And regulate transcription of different genes.
• Two nuclear receptor: Retinoid Acid Receptor ( RARs) and
Retinoid X Receptor ( RXR).
• RAR: all trans and 9-cis
• RXR: 9-cis

16. Other
functions of vitamin A
• Cell growth and differentiation
• Maintain healthy epithelial cells
• Synthesis of certain glycoproteins required for growth and mucus
secretion
• The synthesis of transferrin, the iron transport protein
• To maintain proper immune system( fight bacterial infection)
• Cholesterol synthesis require vit A
• Carotenoid acts as anti oxidants.( cancer and heart attack)

17. Usually not immediate coz Hepatic stores can meet body
requirements : 2-4 months
In eyes: earliest symptoms night blindness ( nyctalopia)
Severe deficiency xerophthalmia
Dryness in cornea, conjunctiva and keratinization of epithelial
cells
Certain area of conjunctiva triangular plaques aka Bitot’s
spots.
If xerophthalmia persists for longer timecorneal ulceration and
degeneration occurs keratomalacia causing total blindness
Vitamin A deficiency: in eye

18. xerophthalmia
keratomalacia
Bitot’s spots.

19. Other deficiency
• Effect on growth : retardation due to impairment in skeletal formation.
• Effect on reproduction: Degeneration of germinal epithelium leads to
sterility in male
• Effect on skin and epithelial cells : rough and dry skin.
• Keratinization of epithelial cells of gastro intestinal tract, urinary tract
and respiratory tract is noticed
The plasma level of retinol binding protein is decreased in vitamin A
deficiency (negative acute phase protein)

20. Hypervitaminosis A
• There is only a limited capacity to metabolize vitamin A, and
excessive intakes lead to toxicity.
• Dermatitis
• Hepatomegaly with hyperlipidemia
• Skeletal decalcification
• Hypercalcemia and calcification of soft tissue.
• CNS ( headache, nausea, ataxia, anorexia, ↑ CSF pressure)
• Excessive dry skin and alopecia
🙏⏰🙏