Vitamins definition , Vitamin Classification table, Fat soluble vitamin A, Chemistry of Vitamin A, Biochemical role of vitamin A, Biochemical functions of vitamin A, RDA (Recommended dietary Allowance), Dietary sources of vitamin A, Deficiency diseases of vitamin A, Rhodopsin cycle.

2. .

3. Vitamins - Definition
• Vitamins are organic compounds required in
the diet in small amounts to perform specific
biological functions for normal maintenance
of optimum growth and health of the organism.

5. Vitamin A - Chemistry
• The fat soluble vitamin A, is present only in foods of
animal origin. However, its provitamins carotenes are found
in plants.
• Chemistry: Vitamin A and the Carotenoids consist of four
isoprenoid units joined in a head-to-tail manner and contain
five conjugated double bonds.
• They are soluble in most organic solvents but not in water.

6. Vitamin A - Chemistry
• Vitamin A and the Carotenoids are sensitive to Oxidation,
Isomerization, and Polymerization when dissolved in dilute
solution under light in the presence of Oxygen.
• Retinol, Retinal and retinoic acid are regarded as vitamers of
vitamin A.
• Retinol(Vitamin A alcohol): It is a primary alcohol
containing β - ionone ring. The side chain has two isoprenoid
units.

7. Vitamin A - Chemistry
• Retinal ( Vitamin A aldehyde): This is an aldehyde form obtained
by the oxidation of retinol.
• Retinoic acid (vitamin A acid): This is produced by the oxidation
of retinal.
• β - Carotene (provitamin A): This is found in plant foods. It is
cleaved in the intestine to produce two moles of retinal. In human,
this conversion is efficient, hence β-carotene possess about one sixth
vitamin A activity compared to that of retinol.

8. Vitamin A - Chemistry

9. Vitamin A - Biochemical Role
• Vitamin A is necessary for a variety of function such as vision,
proper growth and differentiation, reproduction and maintenance of
epithelial cells.
I. Vitamin A and Vision: The biochemical function of vitamin A in
the process of vision was first elucidated by George Wald (Nobel
prize 1968).
• The events occur in the cyclic process known as Rhodopsin cycle
or Wald’s visual cycle.

10. Vitamin A - Biochemical Role
• Rods and cones: The retina of the eye possesses two types of cells
rods and cones.
• The human eye has about 10 million rods and 5 million cones.
• Rods are involved in dim light vision whereas cones are responsible
for bright light and colour vision.
• Wald’s visual cycle: Rhodopsin is a conjugated protein present in
rods.
• It contains 11 - cis retinal and the protein opsin.

11. Vitamin A - Biochemical Role
• The primary event in visual cycle, on exposure to light, is the
isomerization of 11- cis -retinol to all-trans retinal.
• The isomerization of 11- cis -retinol to all-trans retinal leads to a
conformational change in opsin which is responsible for the generation
of nerve impulse.
• The all- trans -retinal is immediately isomerized by retinal isomerase (of
retinal epithelium) to 11-cis-retinal.
• This combines with opsin to regenerate rhodopsin and complete the
visual cycle.

13. Other biochemical functions of
vitamin A
I. Retinol and Retinoic acid function almost like steroid hormones.
II. They regulate the protein synthesis and thus are involved in the
cell growth and differentiation.
III. Vitamin A is essential to maintain healthy epithelial tissue.
IV. Cartenoids (most important β- Carotene) function as
antioxidants and reduce the risk of cancers initiated by free
radicals and strong oxidants.
V. β-Carotene is found to be beneficial to prevent heart attacks.
This is also attributed to the antioxidant property

14. Other biochemical functions of
vitamin A
VI. Vitamin A is essential for normal Bone growth.
VII. Vitamin A is necessary for normal reproduction in
rats.
VIII. There is loss of appetite in Vitamin A, deficiency
and is caused by changes in the taste buds.
IX. Wound healing is also impaired as a result of
changes in the epithelial cells.

15. Recommended dietary allowance
(RDA)
• The RDA of vitamin A for adults is around 1,000
retinol equivalents (5000 IU) for man and 800 retinol
equivalents (4000 IU) for women.
• One International Unit (IU) equals to 0.3μg of retinol.
• The requirement increases in growing children,
pregnant women and lactating mothers.

17. Recommended dietary allowance (RDA)
Group Retinol
µg
β-Carotene
µg
Man 600 2400
Woman 600 2400
Pregnant woman 600 2400
Lactation 950 3800
Infants 350 1400
Children 1-6 years 400 1600
Children 7-9 years 600 2400
Adolescents 600 2400

18. Dietary sources of Vitamin A
• Animal sources contain (performed) vitamin A.
• The best sources are liver, kidney, egg, yolk, milk, cheese,
and butter, fish (cod or shark) liver oils are very rich in
vitamin A.
• Vegetable sources contain the provitamin A - Carotenes.
• Yellow and dark green vegetables and fruits are good
sources of carotenes e.g. Carrots, spinach, amaranthus,
pumpkin, mango, papaya, etc.,

20. Vitamin A deficiency
• Deficiency manifestations of the eyes:
I. Night blindness (nyctalopia) is one of the earliest
symptoms of vitamin A deficiency.
• The individuals have difficulty to see in dim light.
II. Severe deficiency of vitamin A leads to Xerophthalmia.
• This is characterised by dryness in conjunctiva and cornea
and keratinization of epithelial cells.

21. Vitamin A deficiency
• In certain areas of conjunctiva, white triangular
plaques known as Bitot’s spots are seen.
• If xerophthalmia persists for a long time, corneal
ulceration and degeneration occur.
III. This results in the destruction of cornea, a condition
referred as keratomalacia, causing total blindness is
most common in children of the developing countries.

22. Vitamin A deficiency
IVHypervitaminosis A
• Excessive consumption of vitamin A leads to toxicity.
• The symptoms of hypervitaminosis A include
dermatitis ( drying and redness of skin), enlargement
of liver, skeletal decalcification, tenderness of long
bones, loss of weight, irritability, joint pains, etc.

23. Vitamin A - Conclusion
• Beneficial Effects of β-Carotene
• Increased consumption of β-carotene is associated
with decreased incidence of heart attacks, skin
and lung cancers.
• This is attributed to the antioxidant role of β-
carotene which is independent of its role as a
precursor of vitamin A.