Vitamin A

Question 1

Vitamins

Answer 1

Organic substances that are essential in 
“small quantities” to promote normal 
metabolism and the overall health of an 
animal AND that must be obtained from 
foodstuffs.

Question 2

Fat-soluble Vitamins

Answer 2

Vitamin A, D,E,K

Question 3

Vitamin A - Sources

Answer 3

• A fat-soluble vitamin only found in a few animal products, such as: liver, egg yolks and milk.
• Plants do NOT contain vitamin A, but instead contain carotenoids, some of which are vitamin A precursors;
• Of the over 500 carotenoids found in
  various fruits and vegetables,
• β-carotene is the most active and consists of two vitamin A molecules covalently linked in a head-to-head configuration;

Question 4

β-carotene

Answer 4

β-carotene is the most active and consists of two vitamin A molecules covalently linked in a head-to-head configuration;

Question 5

Vitamin A - Structures

Answer 5

Chemical forms:

1.   Retinol (all trans)
2.   Retinal
3.   Retinoic acid

The conjugated double bonds in carotenoids give vegetables their characteristic yellow color;

Question 6

Two ways Vitamin A is destroyed

Answer 6

UV light attacks these double bonds, destroying vitamin A activity.
Rumen biohydrogenation of double bonds destroys vit. A activity.

Question 7

Digestion and absorption of Vitamin A

Answer 7

•Follows lipids…carried within micelles;

•  Re-esterified in mucosal cells, then
secreted in the lymph in chylomicrons.

Question 8

Digestion and absorption- Carotenoids

Answer 8

•  β-carotenoid is split into 2 retinol molecules
at intestinal mucosal in most animals;
•  Some animals readily absorb beta-carotene
intact;
•  Humans, chicken, some cattle breeds;
•  Beta-carotene can be split into retinol in liver;
•  Splitting β-carotene into retinol is inefficient
(six β-carotenes = 1 retinol)

Some carotenoids possess no vitamin A
activity (e.g., zeaxanthin & xanthophyll)

**Cats are devoid of this enzyme

Question 9

Vitamin A: Transport (2 forms)

Answer 9

•  Diet sources are transported in blood via
chylomicrons;

•  After release from liver stores, free retinol
is bound to retinol binding protein (RBP).

Question 10

Vitamin A: Storage

Answer 10

•   Primarily in liver; some in adipose tissue;

*   Stored as retinyl esters (usually esterified to palmitic acid)

Question 11

Vitamin A - Functions

Answer 11

1.  Vision…
- Retinal is a necessary structural component of
rhodopsin or visual purple , the light sensitive pigment within rod and cone cells of the retina.

2.   Resistance to infectious disease…
   - Vitamin A is essential for a normal immune response.
3.   Epithelial cell “integrity”…
   - Many epithelial cells appear to require vitamin A for proper differentiation and maintenance.

4.  Bone remodeling…
- Normal functioning of osteoblasts and osteoclasts
is dependent upon vitamin A.

5.  Reproduction…
- Vitamin A is required for sperm production;
- Similarly, normal reproductive cycles in females
require adequate availability of vitamin A

Question 12

Vitamin A deficiency

Answer 12

1.   Blindness
   - due to inability to synthesize adequate quantities of rhodopsin. Moderate deficiency leads to deficits in vision under conditions of low light (“night blindness”),
   - while severe deficiency can result in severe dryness and opacity of the cornea (xeropthalmia)

2.  Increased risk of infection;
3.  Abnormal function of many epithelial cells,
manifest by such diverse conditions as dry,
scaly skin, inadequate secretion from mucosal
surfaces;
4.  Abnormal bone growth;
5.  Infertility

Question 13

Vitamin A – Requirements

Answer 13

• Animal nutritionists provide most vitamin A from cheap synthetic sources.
• International Unit (IU) = 0.3 mcg retinol
• Retinol equivalents (RE) is used to estimate
  the amount of vitamin A in various feedstuffs
  *RE = mcg retinol + (mcg β-carotene/6) +
  (mcg other provitamin A forms/12)

Question 14

Vitamin A in beef cattle rations

Answer 14

• Greater use of milo and barley to replace corn
  has increased the incidence of vit. A def. in beef
  cattle;
• Longer storage of forages results in greater
  losses of vit. A and carotenoids;
• Cattle are much less efficient at converting
  carotenoids to vitamin A than rats, so Tables
  that show the vitamin A activity in plant sources
  can be off by ~4-fold.

Question 15

Intramuscular injection of vit. A

Answer 15

• Useful approach for herds with a history of
  vitamin A def.
• 1 million IU every 28 days to boost liver stores
  in steers;
• 4-6 million IU injected once in beef steers was
  able to maintain liver stores of vit. A;
• Not routinely recommended, but can help in a
  drought year when forage quality is expected to
  be low.

Question 16

Vitamin A - Toxicity

Answer 16

Symptoms:
Anorexia, weight loss, skin thickening, scaly
dermatitis, rough hair coat, embryonic
malformations, miscarriage, and death.

• Reports of death in humans after a
  single dose of > 500,000 IU…
  (e.g. Artic explorers eating polar bear liver.)