|
| Vitamin D |
What it is
The discovery of vitamin D followed a similar sequence to that
of many other vitamins. The curative properties of products containing
it were known for a long time before the vitamin itself was identified.
In the middle of the 1700s it was found that cod liver oil was
an effective treatment for rickets but it was not until the beginning
of the present century that the particular substance influencing
rickets was separated and examined.
Two compounds have this effect on rickets: ergocalciferol (or
vitamin D2) which has a vegetable origin and cholecalciferol (vitamin D3) which derives from animals. In general they are not found in
these chemical forms. Plant materials contain ergosterol and animal
products 7-dehydrocholesterol which are absorbed by animals and
then converted into the active vitamin D form by the action of
ultra-violet light on the skin. Before they can be utilised further
chemical changes must take place. The first step is the formation
of the 25-hydroxy product in the liver and then further hydroxylation
to 1:25 dihydroxy cholecalciferol [1,25(OH)2CC]. While 25-OH CC can be effective, the 1,25(OH)2CC is the major active form.
Since 1,25(OH)2CC is created in the organs of animals there is controversy whether
it is a vitamin or a hormone.
What it does
Both vitamin D2 and vitamin D3 have similar activities in horses. The main function of the vitamin
is the control of the absorption, transport and deposition of
calcium and, to a lesser extent, phosphorus. The first action
in this sequence is the promotion of intestinal absorption of
calcium. This occurs mainly in the duodenum and many experiments
have shown that little calcium is absorbed in the absence of vitamin
D. By far the major activity is the deposition and mobilisation
of bone calcium. Calcium is transported in the blood plasma from
the intestine via liver and kidneys to bones and other calcified
structures and in the opposite direction from bones to the kidneys
where some may be excreted. Vitamin D controls the levels of calcium
and phosphorus in blood at a super-saturated level which permits
deposition in bone. A less than saturated level withdraws calcium
from bones. A further related activity is the control of the reabsorption
of phosphorus, calcium and sodium in the renal tubules of kidneys.
If too much is given
There can also be problems from grossly excessive supplies of
vitamin D. Since it controls calcium absorption, excess vitamin
D leads to excessive calcium in the blood (hypercalcaemia). This
extra calcium has to be dumped and may be
deposited in the heart or blood vessels, in the bone joints, in
the pericardium or in the intestinal walls. This leads to heart
failure, or stiffness or intestinal problems. The excess amount
necessary to produce such conditions varies with age. Levels greater
than ten times the allowances recommended should be avoided.
How it is measured
Vitamin D3 - cholecalciferol - is usually taken as the standard.
Recommendations are generally given in International Units which
are identical to USP units (United States Pharmacopoeia).
| 1 International unit of vitamin D |
= 0.025 mg cholecalciferol |
| 1 mg cholecalciferol |
= 40,000 I.U. vitamin D |
Assessment of status
It is exceedingly difficult to assess the vitamin D status of
a horse. Actual hypocalcaemic conditions can be identified by
X-ray radiography or by histology but little is known about the
amounts of cholecalciferol or its active metabolites in blood
and other tissues. Very sophisticated analytical techniques are
needed to extract, separate and quantify the hydroxy forms of
cholecalciferol and such assays cannot be used for routine assessments
of vitamin D status. This has made it difficult to identify vitamin
D stores in the body which are not believed to be very extensive.
Relationships with other ingredients
The relationship with calcium and phosphorus is obvious. The balance
between them is fundamentally important and a ratio of calcium
to phosphorus of between 2:1 and 1:2 must always be maintained.
While age variations occur, the normal ratio is 1.4:1 or 1.5:1.
There also appears to be some influence from sodium, potassium
and magnesium.
Other essential micro-nutrients also affect bone strength. These
include zinc, manganese, niacin, choline and biotin.
Some mycotoxins appear to interfere either with vitamin D absorption,
vitamin D metabolite production or vitamin D-induced calcium absorption,
and rickets or osteoporosis can occur under these conditions in
spite of correct Ca/P ratio and adequate dietary vitamin D.
Requirements and allowances
The minimum amounts of vitamin D required by horses are very small. Young growing horses actively depositing
new bone require proportionally more than fully grown adults.
The allowances for optimum growth have to be assessed very carefully
because of the risk of hypervitaminosis D.
In general, the allowance, measured in I.U., should be one tenth
of the amount of vitamin A. Thus, as a guide,:
| |
|
I.U. / kg |
|
I.U. / day |
| Adult performance horses in training |
|
600
|
|
6000 |
| Adult performance horses in light work |
|
800 |
|
5000 |
| Ponies, hacks & hunters |
|
800 |
|
2500 |
| Mares & stallions |
|
1000 |
|
4000 |
| Young horses 1-2 years |
|
800 |
|
2500 |
| Foals & yearlings less than 1 year |
|
1000 |
|
1000-3000 |
Stability
Both forms of vitamin D are affected by light and are made inactive
by oxidation. The close presence of heavy metal ions such as copper
greatly accelerates oxidation. Moisture, particularly in the form
of saturated air, can also assist the rapid destruction of vitamin
D.
Only protected forms of vitamin D3 should be used in feeds. Gelatine matrix or spray-drying protection
methods are equally effective in minimising oxidative destruction.
Even so, some 5% is lost during feed pelleting and a similar amount
during each month of storage. Meal mixtures and vitamin concentrate
mixes lose very little during mixing but may lose 2-5 % per month
of storage. Vitamin/mineral premixes containing aggressive trace
minerals may lose 10-20% between mixing and use within 4 weeks.
It is generally recommended that an overage of 30% vitamin D should
be added to pelleted feeds to allow for anticipated losses during
production and storage.
|
|
|
