Vitamin D

Question 1

Vitamin D - Structures

Answer 1

Precursors…

•   7-dehydrocholesterol - in animal tissues/skin;
•   Ergosterol - from plants;
•   UV light converts these two pro-vitamins into vitamins D2 & D3, respectively;
•   Thus, exposure of animals to sunlight for minutes to a hour a day eliminates the need for a dietary source of this vitamin.

Question 2

Vitamin D - Functions

Answer 2

Maintenance of plasma Ca++ and P levels;
-  Allows normal bone mineralization;
-  Prevent tetany (repeated prolonged contraction
of muscles)

Question 3

How does Vitamin D maintain plasma Ca++ and P levels?

Answer 3

-  Stimulates Ca and P absorption via 
intestine, reabsorption via kidneys; 
bone resorption to release these 
minerals; 
 -  Absorption via a vit. D-dependent Ca 
binding proteins.

Question 4

Vitamin D - Metabolism

Answer 4

1.   Liver converts D2 or D3 to 25-hydroxyvitamin D3 [25-(OH)D3]
   - This is the main circulating form of vit. D; but it is inactive;

2.  Activation requires an additional hydroxylation to
1, 25-(OH)2D3;
- This final step occurs primarily in the kidneys;
(also happens in bone, intestine, placenta, immune cells)
- Circulates in the blood where it is free to bind specific vit. D receptors in the nucleus of target tissues; thus… 1,25-(OH)2D3 acts as a hormone.

3.  Hydroxylation at 24-position can also occur in kidneys; serves as an inactivation and disposal pathway.

Question 5

Vitamin D - Requirements for Cattle

Answer 5

1 I.U. = 0.025 microgram vit. D (40 IU = 1 ug)

Cattle = 300 I.U./kg diet

(Vitamin D additions are normally unnecessary in the cattle rations if animals are exposed to sunlight. A deficiency might occur in winter during long periods of cloudy weather.)

Question 6

Vitamin D - Requirements for Pigs and Chickens

Answer 6

1 I.U. = 0.025 microgram vit. D (40 IU = 1 ug)

Pigs & chickens = 150-200 I.U./kg diet

Vitamin D2 sources: irradiated yeast & dehydrated alfalfa meal.
Sources of D3: fish-oils, fish meals & the skin by sunshine.
Swine exposed to sunshine in summer should not need dietary vit. D.
During the winter months and in confinement rearing, it is important that rations be supplemented with vitamin D.

Question 7

Vitamin D - Deficiency (5)

Answer 7

1.  Abnormal skeletal development
(rickets, lameness, bowed legs)

2.  Susceptibility varies among species…
chicken > pigs > cow

3.   Rapid growth rate increases susceptibility;
4.   Can be prevented with a few minutes of sunlight/day; (a bit more for dark pigmented breeds)
5.   Sunlight during the summer, but not in the winter, is able to support skin-based production of vitamin D.

Question 8

Vitamin D - Toxicity

Answer 8

•   Causes abnormal deposition of Ca in soft tissues
  (e. g., kidneys, lungs, aorta);
•   Can lead to death, usually from kidney failure;
•   Human infants show toxicity with as little as 10-times the requirement (3,000-4,000 IU/day);
•   Grazing cattle and sheep that eat plants that contain glycosides of 1, 25-(OH)2D3 have shown signs of toxicity…
•   Florida shrub - Cestrum diurnum
•   German/Austrian plant - Trisetum flavescens
•   Argentinian plant - Solarium malacoxylon