Page 79 - Speedhorse June 2019

A healthy horse will never get too much vitamin A while eating natural feeds like for- ages (grasses and hay). The only time a horse might suffer from toxicity is when people feed too much vitamin A, such as in supplements. “Vitamin E is nearly impossible to overdose, but vitamin A can cause problems with exces- sive intakes. One research study fed horses 100 times the vitamin A requirement, and another group of horses 1,000 times the dietary requirement. The growing horses receiving 100 times the requirement appeared clinically normal, but had slower growth, lower weight, and a dull hair coat. It did affect them, but not as severely as the horses receiving 1,000 times the recommended dose,” says Crandell. It took time for these affects to show up – the researchers fed these doses for 15 weeks.
“After that length of time, the horses receiv- ing 1,000 times their requirement had a rough hair coat, poor muscle tone, and depression. After 20 weeks of that high dose, the horses were losing their hair (coming off in clumps) and their skin (some areas sloughing away), and had severe depression. They spent most of their time lying down. They had internal hemorrhaging, and death was the final result,” she says.
Most horses would never receive that much vitamin A for that long a time, but most com- mercial concentrates and many supplements con- tain vitamin A. If a person doubled these up too much – feeding a lot of concentrates plus various supplements, a horse could end up with toxic lev- els. Some people also give their mares or young horses injections of vitamin A or a vitamin A and E combination to try to prevent deficiencies. “A one-time injection is usually fine. It’s when the daily dietary intake is excessive that you’d tend to see problems over time,” she explains.
“What you might notice first is that the young horse isn’t growing or gaining weight as well as you’d expect, and has a rough hair coat. A young, growing horse may develop orthopedic diseases. Sometimes the bones become brittle because they don’t have the proper structure, and the cartilage is also affected,” she says.
Since excess vitamin A affects bones, there
is not a proper turnover of bone cells. “In places where nerves go through bones – such as the spinal cord, or where the optic nerve goes through – there is narrowing, which can put pressure on those nerves. Pressure on the spinal cord could cause ataxia, and the horse might want to lie down all the time. If there is pressure on the optic nerve, this could affect vision,” says Crandell.
Vision can be impaired by either a defi- ciency or an excess of vitamin A. One of the common symptoms of vitamin A deficiency is night blindness since vitamin A is crucial for
healthy eyes, and there can also be some adverse effects in the eye with vitamin A toxicity.
The scary thing when feeding too many supplements and creating an excess of vitamin A in a horse, is that you can overdo
it and not see direct signs. “By the time you start seeing signs of toxicity, it is too late; a lot of damage has already been done. The high vitamin A levels may also start interfering with other nutrients in the body, such as vita- min E, an important antioxidant,” she says.
Most people won’t be feeding too much vitamin A, but it could happen in certain situa- tions. “Perhaps you are using a commercial feed that’s fortified with vitamin A, and feeding a lot of it. We see this commonly with racehorses because they are eating so much concentrate. Perhaps the feeding rate for a certain commer- cial concentrate is 5 to 10 pounds daily, and some racehorses might be getting 20 pounds. Then they might be getting a vitamin supple- ment in addition to the feed, and this is how a horse could get the overdose. If you are feeding a horse 20 pounds of a commercial feed and adding a vitamin supplement, you could easily be overdosing vitamin A,” says Crandell.
“The recommended maximum, as listed
by the National Research Council (NRC), is 16,000 International Units (IU) of vitamin A per kilogram of diet. The typical average size horse is eating about 10 kilograms of food, so this would be about 160,000 IU per day,” she explains.
“The typical concentrate feed will have about 11,000 IU of vitamin A per kilogram.
So, if you read labels you can figure out how much vitamin A you are feeding. If a horse is on green pasture, you never have to worry about
a deficiency because of all the carotene. If the horse has high intake of vitamin A (in the form of vitamin A – as in a concentrate feed or supple- ment) the enzyme that converts carotene to vitamin A will decrease its activity. So, having a horse on green forage while you are feeding high levels of vitamin A will not create toxicity; the green feed won’t make it worse, in other words.” The biggest problem would occur if you feed way too much supplemental vitamin A.
“In humans, vitamin A toxicity can be more of an issue. People have died after eat- ing polar bear liver, for instance, because it is so high in vitamin A.”
Keep in mind that symptoms of toxicity are similar to symptoms of deficiency. “If you suspect a vitamin A problem, it’s wise to check the diet,” says Crandell.
Adding a vitamin supplement could take vitamin A levels too high.
VITAMIN A TOXICITY
body doesn’t have enough protein, for instance, many things are affected, and one of those effects is that the body can’t trans- port vitamin A around the body. It needs those retinal-binding proteins that carry it in the bloodstream,” says Crandell.
There must also be enough fat in the diet to facilitate movement and absorption for fat-soluble vitamins like vitamin A and E. “Carotene and vitamin A need fat to trans- port them and to be absorbed. The typical diet of a horse grazing at pasture does not include much fat; grass is usually between 2-5% fat. This is enough for what the horse needs as far as fat-soluble vitamins are con- cerned,” she explains.
Zinc also interacts with vitamin A. “If a horse is deficient in zinc, that horse is often deficient in circulating vitamin A, as well. We think it’s because zinc is necessary for produc- tion of retinal-binding protein. Zinc is also a component in hundreds of important enzymes in the body, so adequate amounts of this trace mineral are crucial. If there are good zinc lev- els in the bloodstream, there are usually good levels of vitamin A as well. Zinc and vitamin A are not antagonistic,” says Crandell.
“Copper, on the other hand, has an inverse relationship with vitamin A. High copper levels affect stability of vitamin A, but copper and zinc are also involved in the enzyme that converts beta-carotene to retinol (vitamin A) in the digestive tract.”
Vitamin E works hand-in-hand with vitamin A. “They work together to a certain point, so if the vitamin A is a little low and vitamin E levels are higher, the vitamin E can compensate; you’d be less apt to see some of the problems that might occur with vitamin A deficiency. If you get too much vitamin A, however, it starts affecting vita- min E’s ability to protect body cells against oxidation. Excess vitamin A also decreases the body’s ability to absorb vitamin E in the digestive tract,” she explains.
The only time a horse might suffer from toxicity is when people feed too much vitamin A, such as in supplements.
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EQUINE HEALTH