Vitamin A

Question 1

What are Retinoids? What are the 3 forms?

Answer 1

Retinoids are the biologically active forms of vitamin A
Exists in 3 forms:
- Retinol: an alchohol
- Retinal: an aldehyde 
- Retinoic acid

Question 2

What does the tail of a vitamin A molecule mean?

Answer 2

The tail can be in either cis or trans form and this will effect function

Question 3

What are carotenoids?

Answer 3

They are provitamins which need to be converted to be a metabolically active vitamin A.

Question 4

What are the 3 carotenoids which can be converted to vitamin A?

Answer 4

• Beta-carotene
• Alpha-carotene
• Beta-cryptoxanthin

Question 5

How is vitamin A digested? How are carotenoids digested?

Answer 5

Retinyl esters split into retinol and fatty acids in the GIT via bile and pancreatic lipase

Carotenoids are split from their food molecule by enzymes to be directly absorbed

Question 6

How is vitamin A absorbed?

Answer 6

90% of retinol is absorbed by carrier proteins in the small intestine

5-60% of carotenoids are absorbed by passive diffusion (depending on intake of fat)

Question 7

How is vitamin A converted to its active form?

Answer 7

Carotenoids are cleaved to form retinal or retinoic acid

Retinol is converted to retinal or retinoic acid

Question 8

How is vitamin A transported through the body (to liver from enterocytes)?

Answer 8

In the enterocyte retinyl esters are reformed, then packaged into chylomicrons so that they can enter the lymphatic system

Retinoic acid (water soluble) and carotenoids can directly enter the bloodstream

Question 9

How is Vitamin A transported from the liver?

Answer 9

Retinoids are bound to retinol binding protein to be transported to cells

Carotenoids are carried by VLDL

Question 10

Where is Vitamin A stored in the body?

Answer 10

90% stored in the liver (enough to last several month)

Question 11

How is Vitamin A excreted?

Answer 11

Small amounts excreted in urine

Carotenoids are excreted attached to

Question 12

What are the functions of Vitamin A?

Answer 12

Retinoids:

• Growth and development
• cell differentiation via gene expression
• vision: needed for cornea, rods and cones, light to dark adaptation
• immune function (maintains levels of killer cells and increases macrophages)
• dermatology

Carotenoids:
- protection from free radical damage

Question 13

What are the Vitamin A equivalents/ conversions?

Answer 13

1µg Retinol activity Equivalent (RE)
= 1µg all trans-retinol
= 6µg all trans beta-carotene
= 12µg alpha-carotene, beta- cryptoxanthin

1 International Unit (IU)
= 0.3µg RE
= 0.3µg trans-retinol
= 1.8µg beta carotene
= 3.6µg alpha-carotene, beta- cryptoxanthin

Question 14

What are the deficiency symptoms of Vitamin A?

Answer 14

• Night blindness
• Xerophthalmia
• Keratomalacia (involves the
  cornea, results in blindness)

Question 15

What are the symptoms of Vitamin A toxicity?

Answer 15

• Acute: nausea/GIT disruption, headaches

- Chronic: liver damage, haemorrhages, dry skin, coma

Question 16

What are good sources of Vitamin A?

Answer 16

Sources of Retinoids:

• beef
• eggs
• butter/milk

Sources of Carotenoids:

• carrots
• sweet potato
• mango
• pumpkin