Vitamin A Toxicity Can Sneak Up On You

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The margin of safety between a horse’s nutritionally required level of vitamin A and a toxic-intake level is smaller than for any other nutrient — even selenium. Despite this fact, most equine supplements and feeds are fortified with vitamin A, often in high amounts.

Vitamin A plays a major role in healthy eyes and skin, reproduction and bone formation. The horse’s diet contains primarily beta-carotene, which his body converts to vitamin A as needed. Horses can tolerate excess beta-carotene from dietary sources, but excess vitamin A is another story. The current fad of high-fat feeding makes things more worrisome, since vitamin A and beta-carotene absorption is enhanced by fat.

The National Research Council set an estimated requirement of 30 to 60 IU of vitamin A per kilogram of body weight. This means a 1,100-pound horse needs 15,000 to 30,000 IU of vitamin A per day. The upper safe limit for long-term consumption is 16,000 IU/kg of diet, or about 160,000 IU/day for a horse eating 10 kg (22 lbs.) of food per day.

Many horsemen don’t realize that:

• Excess vitamin A is stored in the liver and can accumulate to toxic levels in the body.

• Horses on pasture or getting high-quality, green hay need no vitamin A supplementation.

• Yellow or old hays won’t contain sufficient vitamin A, but there’s no need to supplement beyond 30,000 to 40,000 IU/day.

• When feeding multiple supplements as well as a commercial grain mix, you must tally up daily vitamin A intake from all sources.

Hay And A
Timothy hay contains around 8000+ vitamin A IU per pound, so 14 lbs. gives the horse 112,000 IU/day. Alfalfa has twice that amount, and fresh grass has three times as much. While vitamin A levels in hays vary according to how well they were cured, it’s safe to assume that if the hay is green it contains more than enough. Levels do drop with storage, though, starting six months after the hay was baled.

If you check the symptoms of vitamin A toxicity, it’s easy to see how they could be attributed to other causes. It’s unlikely that you’d think “vitamin A toxicity” because your older horse is unthrifty, anemic, has flaky skin and diffuse, nonspecific lameness — but you should.

Excess vitamin A in young horses is of particularly concern because of serious effects on growth and bone development. Foals are born with low levels of vitamin A in their bodies, but research has shown that Mother Nature takes care of this quite well by herself since blood levels of A and beta-carotene in the mare rise dramatically close to birth, and colostrum contains 65 times more beta-carotene than later milk.

Never dose/feed foals or weanlings products containing high levels of vitamin A or allow access to free-choice supplements high in A. We talked to two prominent equine nutritionists, Dr. Edgar Ott and Dr. Lance Baker. Both nutritionists stated they saw no reason to supplement foals with vitamin A.

The only treatment for vitamin A toxicity is to stop feeding it. If you correct the problem soon enough, no harm is likely. However, it’s time to get out pen, paper and calculator and find out how much vitamin A your horse is consuming every day in his total diet.

Also With This Article
”Symptoms Of Toxicity”