Vitamin A Flashcards Preview

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Flashcards in Vitamin A Deck (16)
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1
Q

Vitamins

A
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.
2
Q

Fat-soluble Vitamins

A

Vitamin A, D,E,K

3
Q

Vitamin A - Sources

A
  • 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;
4
Q

β-carotene

A

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

5
Q

Vitamin A - Structures

A

Chemical forms:

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

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

6
Q

Two ways Vitamin A is destroyed

A

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

7
Q

Digestion and absorption of Vitamin A

A

•Follows lipids…carried within micelles;

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

8
Q

Digestion and absorption- Carotenoids

A

•  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

9
Q

Vitamin A: Transport (2 forms)

A

•  Diet sources are transported in blood via
chylomicrons;

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

10
Q

Vitamin A: Storage

A
  •   Primarily in liver; some in adipose tissue;

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

11
Q

Vitamin A - Functions

A

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.

  1.   Resistance to infectious disease…
    - Vitamin A is essential for a normal immune response.
  2.   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

12
Q

Vitamin A deficiency

A
  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

13
Q

Vitamin A – Requirements

A
  • 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)
14
Q

Vitamin A in beef cattle rations

A
  • 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.
15
Q

Intramuscular injection of vit. A

A
  • 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.
16
Q

Vitamin A - Toxicity

A

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.)