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| Vitamin B12 |
What it is
In the exciting period of nutritional research in the 1920s and
1930s it was found that liver extracts and other concentrations
of animal origin stimulated the growth of farm animals and could
be used as a treatment for human pernicious anaemia. For many
years the active principle was known as the animal protein factor
and its chemical nature eluded scientists. Then, in 1948, a crystal-line
material with the same characteristics was isolated from liver
and called vitamin B12. Its chemical structure was clarified in
1955 but it was not synthesised until 1973. Until then supplies
of vitamin B12 were prepared by extraction from fermentation products
and this is still the most economical method of commercial production.
The chemical structure of vitamin B12 is the most complex of all
the vitamins. The basic unit is a corrin nucleus which consists
of a ring structure of four 5-membered rings joined by corner
nitrogen atoms. In the active centre of the corrin nucleus is
a cobalt atom. Thus, vitamin B12 is the only vitamin to contain
a mineral element. The main material with vitamin B12 activity
is believed to be cyanocobalamin which has a -CN group attached
to the cobalt atom. There seems to be some doubt if this is a
true feature of the vitamin or whether it is an artefact. Other
groups such as -OH or -NO3 can also be attached to the molecule
without affecting its vitamin status. However, there are many
other corrinoids which appear analogous with cyanocobalamin but
have very little, or no, vitamin B12 activity.
What it does
Vitamin B12 is an essential part of several enzyme systems. Most
of these involve the transfer or synthesis of single-carbon units.
Thus, vitamin B12 is responsible for a number of basic metabolic
functions in association with other vitamins such as folic acid.
The most important tasks relate to the metabolism of proteins
but it also features in the metabolism of fats and of carbohydrates.
Under normal feed conditions vitamin B12 is probably linked to
peptides or even to protein. This link is broken during digestion.
The released vitamin B12 molecule cannot be absorbed through the
intestinal wall without a carrier. Various products, collectively
known as the intrinsic factor, have been shown to carry vitamin
B12. These vary between species but most appear to be glycoproteins.
Only cats appear to be able to absorb vitamin B12 without the
intervention of the intrinsic factor.
The physiological activities of vitamin B12 are very closely inter-related
with those of folic acid but the actual mechanisms are poorly
understood. It is known that one activity is the formation of
labile methyl groups which play a significant part in the biosynthesis
of methionine which, in turn, affects the synthesis of body proteins.
There is good reason to believe that the impairment of protein
synthesis is the principal cause of the growth depression which
is frequently observed in animals deficient of vitamin B12. The
cobalt atom appears to be responsible for the transmethylating
capacity of cobalamin because the methyl- cobalt derivative is
formed.
One interesting and important function of vitamin B12 is the metabolism
of propionate products of dietary or metabolic origin. Propionate
is converted into succinate in the Kreb’s cycle. Propionate contains
three, and succinate four, carbon atoms. The extra methyl group
is supplied through methylmalonyl-CoA which is activated by methylmalonyl-CoA
isomerase, a vitamin B12-dependent enzyme.
Vitamin B12 appears to exert a calming influence on horses, particularly
those which are fresh or frisky, or generally excitable. This
hyperactive condition may be due to a generous supply of biotin
(vitamin H) and 1 mg/day of vitamin B12 can act as an ”antidote”.
If too much is given
Although excess vitamin B12 is not excreted rapidly, no toxicity
or unpleasant reactions have been reported in any species of animal
even at extremely high dosage levels. Many organs are capable
of storing it for short periods; 30-60% of the body’s reserves
are held in the liver. There is no destruction of vitamin B12
during metabolism but a regular excretion via the kidneys and
through the bile into faeces.
Biosynthesis
Many bacteria are capable of synthesising vitamin B12 when sufficient
cobalt is available. Bacteria inhabiting the lower parts of the
digestive tracts of horses can synthesise corrinoids when the
diet contains sufficient cobalt. However it has been shown that
most of these corrinoids are vitamin B12 analogues with no biological
activity. As the cobalt content in the diet is increased more
vitamin B12 analogues are produced and true vitamin B12 production
may actually fall. This suggests that, even in a cobalt-deficient
area, there is a limit to the amount of cobalt which should be
supplied. However, it is doubtful whether horses benefit to any
major extent from the vitamin production in the lower gut because
it is beyond the area where absorption into the system takes place.
Antagonists
For reasons which are still not understood, the action of vitamin
B12 is adversely affected by the presence of high levels of niacin
or of oxidising agents. If these are present in the feed additional
vitamin B12 is required. Oestrogens also antagonise the action
of vitamin B12.
How it is measured
The most accurate assay methods are biochemical, based on test
organisms such as Euglena gracilis or Lactobacillus leichmannii.
The results are usually quoted in mg/kg feed or in mg/100ml blood.
Less commonly used measures of vitamin B12 are the USP (United
States Pharmacopoeia) unit, which is the same as 1mcg and the
LLD (Lactobacillus lactis Dorner) unit.
| Approximately 11,000 LLD units |
= |
1 USP unit |
|
= |
1mcg vitamin B12. |
Assessment of status
It is extremely difficult to make an accurate assessment of the
vitamin B12 status of an animal. The vitamin is largely present
in the liver and other organs so that a blood sample is unlikely
to be indicative of the overall status unless the animal is almost
depleted of vitamin B12. There is also the chemical problem of
identifying biologically-active vitamin B12 corrinoids and separating
them from inactive corrinoids.
There are considerable doubts regarding the accuracy of some of
the existing blood assay methods. Levels of serum vitamin B12
below 200 mg/ml or liver levels below 0.1 mcg/g wet weight indicate
deficiency. The excretion of methylmalonic acid (MMA) in the urine
can be used to indicate deficiency or adequacy of total supplies
since its rate of excretion during vitamin B12 deficiency is 5-12
times normal. Serum MMA can also be used. An alternative urinary
metabolite used as an indicator of vitamin B12 deficiency is formiminoglutamic
acid (FIGLU) since daily excretion is increased 30 times over
normal.
Relationships with other ingredients
Because of its widespread function as a methyl group carrier,
vitamin B12 interacts with numerous other micro-ingredients in
various metabolic activities. One of its major relationships is
with folic acid where there are several combined activities in
nucleic acid and methionine synthesis. Various vitamins also affect
the rate of absorption of vitamin B12 from the intestine, which
is never very efficient at the best of times; folic acid deficiency
increases absorption whereas a vitamin B6 deficiency decreases
it. Biotin is active with vitamin B12 in methylmalonyl CoA metabolism
and so is pantothenic acid which has a vitamin B12-sparing action
in this function. Vitamins E and B1 are also synergistic, particularly
in blood cell production.
Vitamin B12 improves the uptake and utilisation of carotene from
the intestine and the function of vitamin A in maintaining the
integrity of mucosal and epithelial cells.
Calcium, copper and ferrous iron also act with vitamin B12, improving
absorption from the intestine and the efficiency of its metabolic
functions.
Requirements and allowances
Since it is extremely difficult to assess the true vitamin B12
status of animals the accuracy of any measurement of requirement
is doubtful. In addition to the determination of vitamin B12 status,
the many interactions with other micro-ingredients produce variable
requirements depending upon supplies of methionine, choline, folic
acid, thiamine and biotin, for example, and the needs increase
when the production of propionates during digestion is greater.
Most research on the vitamin B12 requirements arrives at a requirement
of 10 mg/kg feed dry matter.
In the absence of other information, 10 mcg/kg should be regarded
as the minimum to be supplied in the feed. There is evidence of
beneficial effects such as the optimization of growth rate when
vitamin B12 supplies are supplemented to give a total in the ration
of 30 mcg/kg. This should be regarded as the optimum allowance
although it should be increased to 120 mcg/kg for hyperactive
and performance horses. In the following suggestions for feed
supplementation it has been assumed that only minimal amounts
of animal protein products are likely to be included in horse
feed formulations. Should the animal protein proportion exceed
5% of the mix the optimum recommendations can be reduced by 5
mcg /kg.
| |
|
µg / kg |
|
µg / day |
| Adult performance horses in training |
|
120
|
|
1200 |
| Adult performance horses in light work |
|
120 |
|
800 |
| Mares & stallions |
|
30 |
|
120 |
| Young horses 1-2 years |
|
25 |
|
75 |
| Foals & young horses less than 1 year |
|
30 |
|
30-90 |
Contents of feed ingredients
Products of vegetable origin contain little or no vitamin B12.
The amounts present in animal protein materials are widely variable
because of losses during processing. Average figures from tables
are liable to be very misleading and any particular sample of
a feed may have a widely different vitamin B12 content from that
shown in tables.
Stability
The very large molecule of vitamin B12 results in a general instability.
It is affected by heat, light, acids, alkalis and oxidising agents.
It is not seriously affected by moisture. During normal feed mixing
and pelleting operations losses of vitamin B12 are of the order
of 10%. If the feed is extruded losses may be much greater and
can exceed 30%. The pure crystalline cobalamin and powder dilutions
are relatively stable to air and heat but are attacked by light,
particularly in the UV wavelengths. It is therefore essential
to keep vitamin B12 products in light-tight containers, preferably
in a cool, dry place. The recommendations for supplementation
include an allowance of 20% for losses during processing and storage.
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