What it is
Vitamin E is an essential dietary requirement supplied by a range
of components with different activities. There are at least 8
different tocopherols with vitamin E activity. Alpha-Tocopherol
has the greatest activity and accounts for 70-90% of the total
biologically available vitamin E in most grazing and cereal-based
mixed feeds. The other tocopherols may be present in greater quantities
but they have very low vitamin E activities and are usually discounted
in assessments of vitamin E in feeds.
What it does
Vitamin E appears to have several different, but related, functions.
One of the most important is its role as an inter -and intra-cellular
antioxidant preventing the oxidation of unsaturated lipid materials
within cells. If lipid hydroperoxides are allowed to form because
the supply of tocopherols is inadequate, direct cell tissue damage
can result. The more active the cell - such as those of skeletal
and involuntary muscles - the greater the inflow of lipids for
energy supply and the greater the risk of tissue damage if vitamin
E is limited. This antioxidant property also ensures erythrocyte
stability and maintenance of the integrity of capillary blood
vessels.
Recent studies have shown that vitamin E has a regulatory action
on the pituitary-midbrain system, promoting the production of
hormones that stimulate the output of the thyroid and the adrenal
cortex.
While vitamin E is present in all tissues and organs and is essential
for the maintenance of the endocrine system and good muscle tone,
it does not have a specific role in reproduction in horses.
How it is measured
Many tables of values measure vitamin E in International Units.
The standard is: 1 mg dl-alpha-tocopheryl acetate = 1 I.U. vitamin
E.
Tocopherols form various stereo-isomers which have different activities
- this is in addition to the chemical variations between alpha-,beta-,gamma-
and delta-forms. Tocopherols found in feed ingredients are always a
specific stereo-isomer, labelled 'd' for convenience. This form
has greater biological activity than mixtures of isomers (the
racemic or 'dl' form). Each 'd' isomer is 36% more active than
the corresponding 'dl' form.
There is now considerable doubt regarding the biological values
of the various isomers but the following rule should produce a
reasonably accurate vitamin E value for a feed:
Rule: Determine specifically the alpha-tocopherol content of the feed, whether
present in feed ingredients or added as a supplement. Assume a
nil value for other tocopherols and equate mg alpha-tocopherol
with I.U.
What about total tocopherols?
Any attempt to determine vitamin E activity by measuring total
tocopherols is doomed to failure because of the wide variation
between the vitamin values of the various homologues. Furthermore
it is not possible to find a formula to calculate a vitamin E
value from an assessment of total tocopherols because the proportion
of the alpha-form varies widely between feed ingredients.
Assessment of status
Vitamin E is found in most body tissues in association with lipid
material (fat); there are no major storage sites although reproductive
organs, pituitary and adrenal glands have particularly high concentrations.
It is located primarily in the lipophilic parts of cells such
as the membranes. It is transported in the bloodstream by low
density lipoproteins and, although the amount in transport varies
with the time of recent food absorption, plasma tocopherol content
is a reasonably good indicator of status.
If muscle tissues have been damaged, such as by myopathy due to
vitamin E deficiency, they release their contents of cell enzymes
into surrounding tissues and, from there, into the blood stream.
Increased amounts of aspartic aminotransferase (AspAT) or creatine
phosphokinase (CPK) in blood plasma indicate tissue damage but
are not specifically indicative of low vitamin E status although
this assumption is often made.
Relationship with other ingredients
There is a close working relationship between vitamin E and selenium
in their functions within tissues. Selenium is an important constituent
of the enzyme glutathione peroxidase which has the important task
of removing active peroxides from cells before they oxidise the
unsaturated lipids which are protected by the tocopherols. Glutathione
peroxidase is generally found in the cell cytosol and mitochondrial
matrix while tocopherol is present in the membranes. To a small
extent vitamin E and selenium are mutually replaceable, but there
are lower limits below which substitution is ineffective.
Requirements and allowances
The amount of vitamin E necessary to prevent the occurrence of
deficiency disorders in the presence of adequate selenium is comparatively
small, increasing progressively as selenium supplies decrease.
It is therefore very difficult to establish a true requirement.
The effects of increased allowances are also difficult to quantify
because there are few effects on growth rate or performance.
Since individual horses have considerable differences in requirements
the ideal is to establish allowances which will ensure optimum
muscular activity and freedom from all forms of deficiency condition.
Horses appear to have higher vitamin E requirements in relation
to their age, weight and size than other animals. It seems likely
that the stress of muscular activity in training, racing, jumping
and endurance riding requires the input of relatively large amounts
of vitamin E to maximise intra-cellular antioxidant activity and
ensure good cell metabolism. The following guide indicates the
optimum daily allowances of vitamin E from feeds and supplements
combined:
Factors affecting allowances
- FAT
Since one of the roles of vitamin E is to prevent the oxidation
of lipids, it follows that the amount required is positively correlated
with the dietary fat content and, in particular, with the amounts
of polyunsaturated fatty acids. If large amounts of fat are added
to the diet the normal vitamin E supplement should be increased
by 30 mg/kg for every 1% of polyunsaturated fatty acids (PUFA)
in the supplementary fat. Maize and soya oil contain about 60%
PUFA whereas blended fats usually contain about 20%. Thus, for
example, the addition of 4% maize oil with 60% PUFA adds 2.4%
(4 x 60%) PUFA to the diet and requires an additional supplement
of 72 mg vitamin E per kg diet (2.4% x 30). If the ”normal” vitamin
E supplement is 200 mg/kg, then the 4% maize oil diet requires
272 mg/kg.
- STRESS
Animals under stress need more vitamin E than those in less stressful
conditions. Thoroughbreds in full race training, endurance horses,
eventers and other performance horses are frequently under stress.
This is a major reason why their vitamin E requirements are apparently
higher in relation to their size and weight than other animals.
In addition, transport, disease, poor stabling or grazing conditions
and irregular noise patterns all increase the requirements for
vitamin E.
- SELENIUM
If feed selenium supplies are low, additional vitamin E may be
required.
- POOR QUALITY FEED
Ingredients that contain peroxidising fats greatly increase the
requirement for vitamin E.
Grain stored in a moist condition loses all its natural vitamin
E and an increased supplement must be given to make good these
losses.
Mycotoxins in the feed lead to increased metabolic requirements
for vitamin E.
Stability
The tocopherols are excellent antioxidants. It therefore follows
that any condition of feed in which there is active oxygen leads
to a reduction in the true tocopherol content. The supplementary
form of vitamin E is alpha-tocopheryl acetate which is not an
antioxidant and is much more stable to moisture, heat and oxygen
than alpha-tocopherol. The acetate form is converted to tocopherol
during digestion and absorption in the intestine. All forms of
vitamin E are rapidly destroyed by alkalis.
|
