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Chemistry, and biochemical role, rda, deficiency diseases of vitamin a for ug
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- 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 functionsof 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 functionsof 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 ofVitamin 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 IVHypervitaminosisA • 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.