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| Biotin-Vitamin H |
What it is
Biotin is one of the more recently discovered vitamins. It was
first reported as ”coenzyme R” in 1935 and its chemical structure
identified in 1942. Some research articles from the 1940s refer
to biotin as vitamin B8 but it is probably better known as vitamin H (H for hair, indicating
where deficiencies are seen). For many years it was assumed that
feed contents were adequate to provide horses’ requirements. Research
during the past decade has revealed the inadequacy of these supplies.
It is now known to be an essential coenzyme in several enzyme
systems where it has a specific function in carboxylations (transfer
of enzyme-bound CO2, or CO2 fixation). Although most feed components contain biotin recent
research has shown that a large proportion is organically bound
and biologically unavailable. Only very small quantities are required
daily but the amount which is biologically available is often
insufficient to meet horses’ requirements without supplementation.
What it does
The complete biochemical role of biotin is still not fully understood.
In practical terms it is essential for life, growth, food utilisation,
maintenance of epidermal tissues, normal bone development and
reproduction. Two important enzyme functions which have been under
investigation are related, respectively, to gluconeogenesis and
fatty acid synthesis. Pyruvate carboxylase is a biotin-dependent
enzyme in the gluconeogenesis pathway where it permits the maintenance
of normal blood sugar concentration by controlling the conversion
of stored energy into glucose. Then, in fatty acid synthesis,
it controls the carboxylation of acetyl-coenzyme A to malonyl-coenzyme
A (acetyl CoA carboxylase is biotin-dependent). It also affects
protein synthesis through its influence on the nature and rate
of formation of ribonucleic acid. This seems particularly important
in controlling the rate of production and deposition of scleroproteins
(”hard” proteins such as keratin). Thus biotin is involved, directly
or indirectly, with the metabolism of carbohydrates, fats and
proteins.
If too much is given
There are no indications that even extremely large amounts of
biotin can be toxic to horses. It is possible that supplies of
100 mg or more to young foals might restrict growth but it seems
that adults can tolerate a gram a day or more with no untoward
effect because very little is stored and the excretion mechanism
through the kidneys is very efficient.
Biosynthesis
Synthesis of biotin in the lower end of the gastro-intestinal
tract provides very little usable biotin. No research has yet
demonstrated any biotin absorption from the large intestine so
any benefit from this synthesis can only be from coprophagy (eating
faeces).
How it is measured
It is very difficult to determine the biotin contents of feeds
because the amounts are very small. It is even more difficult
to assess the biotin status of livestock. The amount in the liver
has been found to give a reasonable assessment of total body supplies
while the amount circulating in blood plasma can be a useful indication.
Biotin in feeds and biological samples can only be determined
microbiologically.
Antagonists
Several substances are know which can antagonise or bind biotin.
Certain proteins such as avidin (found in egg albumen) completely
inactivate biotin. Any egg product, including hatchery waste,
should be thoroughly processed to destroy avidin before use in
feeds. Streptavidin, which is present in streptomyces moulds in
bedding and spoiled feed, can also bind biotin. There are reports
that the pesticide dieldrin can affect biotin availability in
feeds. Peroxidising fats in feeds have been shown to destroy biotin.
It has also been demonstrated recently that aflatoxin may increase
metabolic biotin requirements; other mycotoxins may also interfere
with biotin absorption or otherwise increase the apparent biotin
requirements.
Requirements and allowances
Until recently very little definitive research had been undertaken
to determine actual requirements because of the problems of estimating
the amount of truly available biotin in the feed. Some research
was undertaken using avidin as a biotin-binder to remove any traces
of feed biotin so that responses to supplementary biotin could
be measured. Unfortunately excesses of avidin produced other problems
which affected results.
Even now most published recommendations for the vitamin requirements
of horses suggest that they do not require biotin supplements.
The situation is confused by research in the last 10 to 20 years
showing that some cracked hoof conditions in horses are biotin-responsive.
These lesions do not appear to be due to simple biotin deficiencies
but larger than usual supplies appear necessary to enable the
horse to produce strong hoof horn and improve the foot condition.
Whereas the normal daily allowance of biotin for a horse is about
1-2 mg/day the amount necessary to create the conditions for hoof
repair appears to be about 15 mg/day. It has also been found that
supplying about twice the normal requirement (2-4 mg/day) can
induce excitability and hyperactivity. Extra vitamin B12 helps to modify this effect.
All quoted requirement and allowance figures relate to biologically
available biotin and the actual feed biotin content found by assay
may be only partially available. The results of recent research
suggest that the levels of biologically available biotin in compound
feedstuffs are too low and feeds may require supplementation in
order to ensure optimum health and performance.
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Supplementary
µg / kg feed
|
|
Biotin
mg / day |
| Adult performance horses with soft or cracked hooves |
|
1500
|
|
15 |
| Adult performance horses with good hooves, in training |
|
150 |
|
1,5 |
| Adult performance horses on light work |
|
160 |
|
1,0 |
| Ponies, hunters and hacks |
|
150 |
|
0,45 |
| Mares and stallions |
|
200 |
|
0,8 |
| Young horses 1 – 2 years |
|
200 |
|
0,6 |
| Foals and young horses less than 1 year |
|
250 |
|
0,25 - 0,75 |
Contents of feed ingredients
The total amounts of biotin (available and non-available) in feeds
are very variable. Analytical variations for microbiological assays
of biotin (accuracy +20%) also increase the apparent variations
between samples.
| Feedstuff |
|
Biotin contents
µg/ kg |
| |
|
Mean |
|
Range |
| Wheat |
|
101
|
|
70-276 |
| Barley |
|
140 |
|
80-246 |
| Oat |
|
246 |
|
169-317 |
| Maize |
|
52 |
|
12-162 |
| Sorghums |
|
287 |
|
173-429 |
| Rice |
|
23 |
|
9-52 |
| Wheat middlings |
|
332 |
|
190-434 |
| Maize gluten feed |
|
139 |
|
48-281 |
| Soya been meal |
|
270 |
|
200-387 |
| Cottonseed meal |
|
230 |
|
149-285 |
| Fish meal |
|
135 |
|
11-421 |
| Herring meal |
|
158 |
|
|
| Meat and bone meal |
|
88 |
|
7-364 |
| Molasses: |
|
|
|
|
|
- beet |
|
441 |
|
43-838 |
|
- cane |
|
1080 |
|
737-1930 |
| Dried yeast |
|
462 |
|
90-1070 |
| Dried distillers' solubles |
|
400 |
|
200-680 |
| Grass meal |
|
366 |
|
227-459 |
| Lucerne meal |
|
543 |
|
196-779 |
|
-Manioc (tapioca, cassava) |
|
51 |
|
5-73 |
Biological availability
The availability of biotin from most cereal products is very low
and it may be completely unavailable. The exception is maize where
the biotin is fully available. The following table lists the average
biological availability of some feed materials.
| Feed ingredient |
|
Biotin availability
(%) |
| Wheat |
|
5
|
| Barley |
|
20 |
| Oats |
|
40 |
| Maize |
|
100 |
| Milo |
|
25 |
| Manioc (tapioca, cassava) |
|
5 |
| Rice bran |
|
25 |
| Wheat middlings |
|
6 |
| Soya been meal |
|
100 |
| Rape seed meal |
|
70 |
| Sunflower meal |
|
40 |
| Fish meal |
|
100 |
| Meat meal |
|
100 |
| Dried yeast |
|
100 |
| Skim milk powder |
|
65 |
| Whey |
|
100 |
| Grass - lucerne meal |
|
65 |
| Molasses |
|
75 |
Stability
Biotin is a very stable vitamin, normally unaffected by mill processing
including pelleting. It is affected by oxidised fat and by alkaline
pH. No overages are needed in feed production to allow for processing
losses.
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