Vitamin A Flashcards
What are the active forms of Vitamin A?
Collectively called retinoids:
* Retinol
* Retinal
* Retinoic acid
What is the function of carotenoids?
They are provitamins derived from plants which can be cleaved to form retinal
How is Vitamin A abosorbed and transported?
Bile and dietary fat are required for absorption and transportation as chylomicrons
How is Vitamin A stored in the body?
Stored in the liver and adipose tissue as retinol-esters
How is Vitamin A transported to other parts of the body?
Transported from the liver to other tissues in the blood bound to Retinol Binding Protein (RBP)
Procedure converting β-carotene to active components
Purpose of Vitamin A
Vitamin A is a component of the visual pigments of rod & cone cell
Whch active form of Vitamin A is used in the eyes?
Retinal
Describe the steps of Wald’s visual cycle
11-cis Retinal + Opsin protein = Rhodopsin (in rods) & iodopsin (in cones)
When light falls on rhodopsin, 11-cis retinal is isomerized to all-trans, which dissociates from the opsin triggering a sequence of intracellular events, leading to the generation of the nerve impulse
11-cis retinal is regenerated by isomerization completing the visual cycle
Functions of different Vitamin A forms
11-Cis–Retinal: Involved in visual cycle
Retinoic acid: It is a gene regulator thus behaves like steroid hormones. Regulates Epithelial cell growth and differentiation. Prevents the synthesis of high molecular weight forms of Keratin
Retinyl Phosphate: Needed for normal growth regulation and for mucus secretion
Retinol: Maintenance of reproduction
Carotenoids: Function as antioxidants and may reduce the risk of cancer
What are the roles of vitamin A in the eyes?
Prevents xerophthalmia
Phototransduction
Aids cellular differentiation and integrity in the eye
Normal foetal development of the eye also requires adequate vitamin A intake and stores
When may deficiency symptoms arise?
Several months once liver stores are used up
When may deficiency symptoms arise?
Several months once liver stores are used up
Signs and symptoms of deficiency
Early signs:
Nyctalopia
Mild deficiency:
Rough skin (lack of mucus secretions and excess keratin synthesis)
Increased susceptibility to infections (required for differentiation of immune cells)
Retarded skeletal growth
Sterility in males and still births in females
Bitot’s spots (white triangular plaques formed by increased conjunctiva thickness)
Severe deficiency:
Xerophthalmia (drying and keratinization of conjunctiva and cornea)
Kerotomalacia (corneal softening and opacity whihc may cause permanent blindess)
Vitamin A deficiency is the most common cause of blindness
How can vitamin A deficiency be assessed?
Retinol binding protein level
Vitamin A level in blood
Dark adaptation test (time for eyes to adapt to see objects in dim light)
How frequently should vitamin A be supplemented in cases of deficiency?
Every 6 months
Food sources containing vitamin A
Preformed vitamin A - Food of animal origin like liver, fish, eggs and dairy products
Plants contain provitamin A - Carotenoids including carrots, spinach, broccoli, apricots and other dark green and yellow vegetables and fruits. Not destroyed by cooking but are light sensitive
Colostrum is a rich source of vitamin A and should be given to the baby
Some foods can be fortified with Vitamin A
Adequate fat in the diet is needed for absorption of carotene
Recommeded dietary allowance of vitamin A
1000 RE/day (men)
800 RE/day (women)
RE = Retinol equivalents
1 RE = ?
1RE = 1µg retinol or 6 µg beta-carotene
Why are more carotenoids required for 1RE than retinol?
Carotenoids are absorbed less well than preformed retinoid
Conversion of carotenoids to retinol is not efficient
When might requirement of vitamin A be higher?
Preganancy and lactation
When might vitamin A become toxic/hypervitaminosis A?
Large amounts of preformed vitamin A or vitamin supplements taken over months or years
Does not occur on ingestion of carotenoids in large amounts
What effects may hypervitaminosis A have?
Increased cerebrospinal fluid pressure causes headache, double vision, nausea, ataxia and anorexia, loss of hair, irritability, itching & peeling of skin, swelling over long bones and liver enlargement
Teratogenic effect: Excess vitamin A consumption in the first trimester may cause congenital malformations in the developing fetus like microcephaly, hair lip, deafness.
Both deficiency and excess of vitamin A cause birth defects
What are the therapeutic uses of vitamin A?
Measles — Reduces complications and mortality
Dermatology — Retinoic acid and synthetic oxidative metabolites (isotretinoin) has been used for many hyperkeratotic and hyperproliferative skin disorders. 13-cis-retinoic acid reduces the proliferation of sebaceous glands so is used in acne treatment.
Acute promyelocytic leukaemia — All-trans retinoic acid (ATRA, or tretinoin), a synthetic oxidative metabolite of retinoic acid has been used
