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  Discussion on Vitamin A: deficiency and dosages
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Posted on Monday, Apr 2, 2001 - 12:37 pm:

Dear Dr. O,

In January I posted a query for a retail source for unlined nylon sheets and mentioned I had apparently eliminated my horse’s wintertime “itchies” (also reduced his eye tearing) with Vitamin A. You responded asking if there was any reason to believe vitamin A was deficient in my horse’s diet before treatment. You mentioned that causes would be a fairly long term lack of access to green forage (grass or well put up fresh hay).

I added to that post an explanation for my delay in answering your inquiry and have directed people to locations for my answer.

As to long term lack of green forage, that’s not the case. No grass available but I do feed good hay. (As to “well-put up fresh hay” my personal definition of “fresh” is hay consumed within 30 days of baling. And I buy first-cut alfalfa from a farmer who cuts high [decreases yield but decreases sand/dirt content], bales at 1:00 or 2:00 a.m. [higher humidity, less dust], and uses good weed control.) The hay is bright green, brimming with leaves, and stored away from all rain & snow.

My horses are generally very easy keepers (one began having hot-weather weight loss 2 summers ago but after the first summer of that I figured what calorie increase to allow starting mid-June to successfully offset weight loss). Because my horses mostly have been extremely easy keepers their daily ration in terms of food weight per day is apparently lower than “industry standards”, usually around 1% to 1.2% of their body weight, depending on the time of year. (Summertime weight loss excepted, any more food than that turns them into blimps.) I keep daily feed records calculating calorie intake every time a ration is changed to insure adequate dietary energy and protein. Because of the low daily food-weight allowance I have for years fed a mult-vitamin/mineral supplement (Sho Glo by Manna Pro---the cheapest I’ve found providing NRC recommendations) to make sure the horses get what they ostensibly need. Sho Glo contains 37,500 IU per ounce (per scoop) which under ordinary circumstances should be enough.

As for suspecting a Vitamin A deficiency, Cornell University really provided the clue. The horse with the tearing problem was a couple of months past 16 when this all started (in my opinion now a critical piece of info, though I didn’t realize it then). The vet could find nothing wrong---no foreign body, no blocked duct, no infection. (An eye ointment was prescribed and used for awhile just to see if it’d help; it didn’t.) The tearing worsened over several months, and then the other eye (the right one) started tearing also. About this same time I noticed the whites of the eyes becoming red, first in the outer corner of the left eye (the first to start tearing), then the inner corner, then the other eye began developing the same problem.

Didn’t see any point in going back to the vet; I instead checked out Cornell University’s “signs” page (symptoms page) at their vet site. I typed in “tearing” and a list of 317 afflictions came up. The site includes diseases for just about every animal---swine, goats, birds, reptiles, you name it---so I was able to ignore most of what was on the list. When I got down to vitamins, lo & behold, Vitamin A deficiency was listed as a possible cause for excessive tearing in equines. That’s when I remembered reading something in Lon Lewis’ book “Care & Feeding of the Horse” involving tearing and Vitamin A. I bailed out of the Cornell site and got Lewis’ book out. Sure enough he states a Vitamin A deficiency can cause excessive tearing. He also addresses the issue of rapid oxidation of vitamins by minerals and he got me to thinking.

All nutrients degrade over time. And I really didn’t know how long a particular container of Sho Glo sat on a retailer’s shelf when I bought it (I purchase the product in large sacks, which takes 200 days to use). Possibly eventual oxidation combined with a long shelf time between manufacture and feeding might result in reduced vitamin activity. (I have since rejected the “shelf life” idea about Sho Glo because of something that happened hair-coat wise with my other horse this past February; I now very much doubt there’s anything compromised about Sho Glo.)

Additionally, the web site for Horse Interactive (www.thehorse.com/0698/summer coat0698.html) states “Fats help the absorption of fat-soluble vitamins, such as vitamin A… Vitamin A is needed for epithelial cells. A deficiency may be the cause of a dry, lusterless coat. Don Kapper, Equine Nutritionist of Buckeye Equine Nutrition, explained, ‘Vitamin A is high in fresh forages. But when you bale hay…the half-life is 60 days. When hay is six months old, all the vitamin A has been oxidized. The horse can store vitamin A in the liver to last a minimum of six months. So around January, February, and March, we see skin problems. The liver stores are used up, and if the vitamin isn't provided in the grain mix to complement hay, you see problems occur.’ In the spring, horses which return to lush pasture consume a high vitamin A content. Kapper said, ‘You see all the hair shed off and a beautiful slick coat underneath. The vitamin A content is the equivalent of beta carotene, and there's 4-5% fat in fresh pasture. The horse slicks out and shines up.’”

To muddy the waters even further, an aging horse’s digestive system utilizes nutrients less efficiently. So, at that time my thinking went like this: IF all the Vitamin A in alfalfa is gone in 6 months, and IF there’s an oxidation issue with Sho Glo, and IF there’s a shelf-life issue as well, and IF my horse is old enough to begin having digestive inefficiencies, then maybe a Vitamin A problem was present. I used Lewis’ information to figure Vitamin A needs/dosages/
excesses. Lewis states (page 46, 2nd edition of “Feeding & Care of the Horse”) that although clinical signs of excessive Vitamin A are unlikely in mature horses at intakes less than 100 times the recommended amount, 16000 IU per kg (7272 IU per lb) of diet dry matter is regarded as the upper safe limit. (1 kg = 2.2 lbs; Lewis in several places bases his numbers on a horse being fed 1.5% to 3% of body weight per day; diet dry matter numbers run generally .88% to .91%, depending on the particular “common” foodstuff; for personal purposes I use .9%). My horse is 1000# (about 454 kg). At the lowest feeding level (1.5% of BW) a 1000# horse gets at least a grand total of 15# of food daily. On a dry matter basis, that’s about 13.65# (6.2 kg) of food. When 6.2 is multiplied times 16000 the result is 99,200 IU per day as the maximum (based on food consumption of 1.5%). So the “safest” upper limit, so to speak, for a 1000# horse is probably 99,200 IU. I certainly shouldn’t be in trouble if I stay well below that.

What’s the recommended maintenance dosage? According to Lewis, 2000 to 3000 IU per kg of diet dry matter is needed for maintenance (echoed by other equine nutrition sources I checked). When 2000 is multiplied by 6.2 kg the result is 12,400 IU/day. Since we don’t all feed the same amounts of diet-dry-matter-per-kg-of-horse, Lewis also states Vitamin A needs are approx. 30 IU per kg of BW. And 30 IU/day times a BW of 454 kg is 13,620 IU/day. To my way of thinking those two numbers (12,400 & 13,620) are fairly close (within 10%) so probably either method of computation is okay. (I find Lewis’ computation formula of dosages based on BW convoluted. A lot of very, very careful reading is required to do the math. There’s a vitamin/mineral table for equines at Virginia Tech’s web site [www. ext.vt.edu/pubs/horse/406-473/table6.html] that’s easy to do the math with, and the numbers match Lewis’.)

I found cheap Vitamin A gelcaps (8000 IU) at Wal-Mart and started the horse on 2 a day, in addition to the Sho Glo (at 37,500 IU). This gave him 53,500 IU daily. I don’t have any note as to when the tearing began to slow, but I did note that a couple of months later the right eye’s excessive tearing had stopped altogether, and the left eye’s watering had reduced by about 80%. Come winter I cringed when it came time to swap out dirty blankets for clean ones (I launder equine winter garments every 2 weeks) as I anticipated damage to the front of the garment on this particular horse. (Two winters ago, after he turned 17, he began routinely rubbing the front of his blankets and sheets on his pen rails, feeders, etc. and he was breaking front buckles. I do all my own repairs and modifications to garments and equipment; had quite a time coming up with designs he couldn’t damage.) But I was somewhat stunned at the first blanket-changing that winter: no evidence at all of rubbing or scraping. After a few minutes’ hard thought a light bulb went on: the Vitamin A. In addition, the “reds” of his eyes went back to nearly a cream color. Considering that Vitamin A is necessary for healthy skin, eyes, and hair I concluded the horse probably had a Vitamin A deficiency. (His hair coat had also become dull-looking, no matter what I fed or how I groomed. His coat has regained the shine it had when he was younger.)

I can’t be absolutely certain, of course, that all this really indicates a Vitamin A deficiency. But given the cheap price of gelcaps, his ready acceptance of them (I just toss them in his supplements and he eats them), and the apparent margin of dosage safety, I’m keeping him on additional A. And I found an even cheaper source than Wal-Mart, so I’ve put my other horse on extra Vitamin A as well. The reason is this past February he began having hair coat problems that “matched” some of Don Kapper’s remarks. And I don’t believe it’s coincidence that this other horse is within striking distance of age 16…

Hope this didn’t put you to sleep,
calamity j.

Posted on Tuesday, Apr 3, 2001 - 12:13 pm:

Thanks calamity,
I don't think it is a vitmain A deficiency that was causing your horses eyes to tear, based on your description of your hay.

The man from Buckeye was incorrect or you misunderstood what he said. When he said the half like was 6 months what he meant was that after 6 months there is 1/2 as much vitamin A (actually the number is closer to 7 months). Because of the over abundance of Vit A in good quality alfalfa (up to 16,000 IU of vit A per lb DM) that good quality hay stored dry will still contain enough vitamin A for 3 half lives or longer or 21 months. But we do not need to do these calculations.

It turns out the amount of green in the hay accurately reflects the amount of vitamin A in that hay. If hay has any green color at all and makes up 3/4 of the diet it should meet the horses minimum maintenance amounts for A. Your hay sounds more like the type that may be providing 4 to 8 times this amount.

Concerning the age related effects, I cannot find any work that directly addresses this issue. However in a large study of 400 horses the opposite to your statements above was found to be true: Serum vitamin A levels were lower in the younger horses, compared to the older horses, when kept under the same conditions (Can Vet J 1994 May;35(5):297-300 The vitamin A and vitamin E status of horses raised in Alberta and Saskatchewan. Blakley BR, Bell RJ).

Don't get me wrong, I am into supplementing vitamin A and E in horses not on pasture and receiving less than good quality hay or that perform at hgih levels of effort. The next NRC convention is likely to increase the vitamin A recommendations substantially. I just don't see a good argument that vitamin A deficiency is a problem in your horses, but there may be other factors we are unaware of.
DrO
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