Vitamin A

Question 1

What are the active forms of Vitamin A?

Answer 1

Collectively called retinoids:
* Retinol
* Retinal
* Retinoic acid

Question 2

What is the function of carotenoids?

Answer 2

They are provitamins derived from plants which can be cleaved to form retinal

Question 3

How is Vitamin A abosorbed and transported?

Answer 3

Bile and dietary fat are required for absorption and transportation as chylomicrons

Question 4

How is Vitamin A stored in the body?

Answer 4

Stored in the liver and adipose tissue as retinol-esters

Question 5

How is Vitamin A transported to other parts of the body?

Answer 5

Transported from the liver to other tissues in the blood bound to Retinol Binding Protein (RBP)

Question 6

Procedure converting β-carotene to active components

Answer 6

Question 7

Purpose of Vitamin A

Answer 7

Vitamin A is a component of the visual pigments of rod & cone cell

Question 8

Whch active form of Vitamin A is used in the eyes?

Answer 8

Retinal

Question 9

Describe the steps of Wald’s visual cycle

Answer 9

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

Question 10

Functions of different Vitamin A forms

Answer 10

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

Question 11

What are the roles of vitamin A in the eyes?

Answer 11

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

Question 12

When may deficiency symptoms arise?

Answer 12

Several months once liver stores are used up

Question 13

When may deficiency symptoms arise?

Answer 13

Several months once liver stores are used up

Question 14

Signs and symptoms of deficiency

Answer 14

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

Question 15

How can vitamin A deficiency be assessed?

Answer 15

Retinol binding protein level
Vitamin A level in blood
Dark adaptation test (time for eyes to adapt to see objects in dim light)

Question 16

How frequently should vitamin A be supplemented in cases of deficiency?

Answer 16

Every 6 months

Question 17

Food sources containing vitamin A

Answer 17

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

Question 18

Recommeded dietary allowance of vitamin A

Answer 18

1000 RE/day (men)
800 RE/day (women)

RE = Retinol equivalents

Question 19

1 RE = ?

Answer 19

1RE = 1µg retinol or 6 µg beta-carotene

Question 20

Why are more carotenoids required for 1RE than retinol?

Answer 20

Carotenoids are absorbed less well than preformed retinoid
Conversion of carotenoids to retinol is not efficient

Question 21

When might requirement of vitamin A be higher?

Answer 21

Preganancy and lactation

Question 22

When might vitamin A become toxic/hypervitaminosis A?

Answer 22

Large amounts of preformed vitamin A or vitamin supplements taken over months or years
Does not occur on ingestion of carotenoids in large amounts

Question 23

What effects may hypervitaminosis A have?

Answer 23

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

Question 24

What are the therapeutic uses of vitamin A?

Answer 24

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