Written By Dr. Kris Hiney
This month we will begin a series examining the function of vitamins in the health and well-being of horses. We will also discuss natural sources of vitamins which occur in the horse’s normal feed, as well as different forms which are offered in supplements. Finally, we will look at the latest research on vitamins in equine nutrition. Unfortunately, there is a paucity of information regarding vitamin requirements in the equine. While recommended intakes have been established for vitamins A, D, E, thiamin and riboflavin, all others essentially fall into the category of educated guesses. Often equine nutritionists must rely on published information in other species, and extrapolate that to the equine. These suppositions may or may not be valid, but often allow the only approach available.
(If we were feeding these two hays, you would most likely need to supplement your horse with vitamin A if you were feeding the discolored hay.)
We will begin with a discussion of the fat soluble vitamins in a horse’s diet, in particular vitamin A. The fat soluble vitamins will be absorbed out of the gut of the horse along with the lipid component of the diet. While that may seem odd considering that horses naturally consume a very low amount of lipid in an all forage diet, remember that plant cells do contain waxes, sterols and other compounds that are soluble in ether. Even hay will typically contain around 2-3% crude fat on a DM basis.
While many of us know these fat soluble vitamins as their familiar names of vitamin A, D, E and K, we may not be as familiar with their scientific nomenclature. Vitamin A falls into the sub group of trans-retinols. Vitamin A, or retinol, serves a host of functions in the body, far beyond the traditional role of assisting in night vision. Vitamin A is also involved in gene expression, reproduction, embryological development and immune function. Metabolically, retinol can be converted to either retinal or irreversibly to retinoic acid. While retinal plays a role in vision, retinoic acid is more active in epithelial cells health, anti-oxidant function and gene expression. As retinol cannot be stored by itself in the body, it is stored in animal tissues as retinyl palmitate, or retinol linked by an ester bond to palmitic acid. In supplements, vitamin A typically is provided as retinyl-acetate or retinyl palmitate. In the intestine, retinyl palmitate is cleaved to just retinol. In the natural equine diet, horses primarily receive vitamin A as carotenoids, which are precursors to vitamin A synthesis in the body. The functional carotenoids include alpha, beta and gamma carotene, as well as beta cryptoxanthin. Of these beta carotene provides the highest vitamin A activity. Beta carotene is cleaved into two, to form retinal. The rate of conversion of beta carotene to vitamin A is actually dependent on vitamin A status, and will decrease if vitamin A intake is sufficient. Thus, no direct conversion ratio is actually appropriate, as the individual animal’s vitamin status alters its conversion rate. Additionally, as beta carotene intake increases, the rate of conversion to vitamin A may decrease, at least has been proved to do so in other species. Beta carotene is thus considered a very safe form of supplementation, as no toxicities have been linked to beta carotene consumption. Animals will decrease the conversion to vitamin A, thus avoiding toxicities.
With that said, we can attempt to generalize the biological activity of the different forms of vitamin A. For instance, .3 micrograms of all trans-retinol is equivalent to 1 IU, or international unit, of vitamin A. In the conversion of beta carotene to vitamin A, differing values are used for equine diets. Original estimates were 400 IU of vitamin A are created for every mg of beta carotene consumed. However, beta-carotene may have a different conversion rate between life stages of the equine. In brood mares, an estimate of 555 IU for every mg of beta carotene is used, while it is presumed to be only 333 IU of vitamin A per mg of beta carotene in growing horses. While this conversion data is actually extrapolated from studies in rats, it does appear to be reflected in horses. Mares kept on the same pastures as yearlings had higher serum retinol concentrations than the yearlings, while the yearlings had higher serum beta carotene concentrations. This does indicate that the mares were more efficient in converting beta carotene to retinol.
Natural sources of vitamin A are higher in fresh, growing forages, and are associated with the bright green color in hay. Many horse owners associate the bright color of corn with a substantial amount of vitamin A, but it actuality it contains only about 6 mg/kg of DM of beta carotene. Concentrations of beta carotene in hay can range as much as only 30 mg/kg of DM to as much as 380 mg/kg of beta carotene. Thus, corn, is typically much lower in beta carotene activity than hay. Typically, the content of beta carotene is reflected in the quality of the hay. What we typically call low quality hay, that of excessive maturity, lengthy storage, rain damaged, sun exposure etc. will be potentially deficient in vitamin A. The type of the hay also influences vitamin A content. Legume hays not only have higher concentrations of vitamin A, but it may be more available as well. We can do a quick calculation using an intermediate conversion number of 400 IU of Vitamin A/mg of beta carotene and the range of beta carotene seen in hays. Per kg, forage can vary from 1200 IU of vitamin A per kg, to as much as 152,000 IU per kg of hay. The requirement for vitamin A for a maintenance horse is recommended to be 30 IU/kg of bwt. Thus our 500 kg horse is would need 15,000 IU per day. Assuming he was eating 2% of his body weight in a low vitamin A forage (typically mature ), he would be receiving only 12,000 IU per day, which would be short of his requirement. Horses which were fed a low quality forage with no grain supplementation were depleted of their vitamin A stores within two months. Comparatively, horses which had access to pasture at the same time experienced no change in vitamin A. Therefore, horses on fresh pasture, or more brightly colored forage would easily meet his vitamin A requirement and should need little supplementation. A horse eating a high quality forage may actually be receiving the equivalent of 1,520,000 IU of vitamin A! While this may seem excessive, remember, the horse will essentially decrease the rate of vitamin A synthesis from the beta carotene in the diet.
(Despite its bright appearance, corn offers relatively little beta carotene compared to forages)
Many horse owners are also interested in the synthetic vitamin A which may be found in feeds, and how that compares with the natural carotenoids. A water soluble, synthetic beta carotene was not able to support vitamin A status to the similar extent seen in naturally occurring beta carotenoids, or in comparison to retinyl palmitate. This may be similar to trials even in humans, where water soluble supplements were not as beneficial as fat soluble. However, an alternative synthetic beta carotene source was able to increase blood concentrations in of beta carotene in the horse. Thus subtle differences in chemical composition may be key. Retinyl esters, or retinol attached by an ester bond to either short chain or long chain fatty acids, are also used in equine diets. Again, these represent the similar form to how retinol is found in the actual animals body. Due to their increased stability both retinyl acetate and retinyl palmitate have been used in feeds which allow for longer storage. If we look at these two sourced, retinyl acetate offers .344 micrograms for each IU while .550 micrograms of retinyl palmitate is needed for 1 IU of vitamin A.
(Retinyl palmitate. The storage form of retinol in animal tissues, as well as a common supplement in animal feeds.)
So how much vitamin A should a horse consume? The original information provided concerning vitamin A requirements was obtained as the concentration needed to prevent the classical deficiency diseases. Deficiencies of vitamin A are actually quite hard to produce, at least as the classical symptoms of vitamin A deficiency diseases are known. These include night blindness, hair loss, and ataxia. Certainly as has been stated before, as the content of beta carotene decreases in the diet, the animal may adapt to becoming more sensitive to absorption and assimilation into the body. Furthermore, as vitamin A is a fat soluble vitamin, it can be stored in the liver and in adipose tissue, and mobilized to support peripheral tissues when the diet is insufficient. Growing and exercising horses are recommended to receive 45 IU /kg bwt, while pregnant and lactating mares require 60 IU/kg bwt. However, there may be a difference between the amount of vitamins in the diet to prevent deficiency diseases, compared to what is optimal for overall health and well-being. It has been suggested that broodmares can benefit by receiving 400-500 mg per day of beta carotene in late gestation and early lactation. This is truly the area of future research, establishing how much should be fed to offer health benefits without exceeding either the safety margin, or simply wasting money as no additional response can be seen. Certainly fat soluble vitamins should be considered more carefully as they are also more likely to cause toxicities, as they can be stored in association with lipid, while water soluble vitamins fed in excess are typically excreted more rapidly. Many horse owners may reach for supplements too often, with little regard to actual dietary concentrations. Over-supplementation of vitamin A has actually been linked to developmental disorders in young horses. However, no direct information is available to state at which exact level vitamin A can interfere with proper bone development.
So what is the bottom line for vitamin A? If your horse is grazing fresh growing pastures or consuming high quality, bright green hay, it is probably more than adequate in vitamin A. However, if your hay is of lower quality, or your horse does not have access to pasture, you should consider a supplement or a grain that is fortified with vitamin A. If you are concerned with toxicities, remember that beta carotene is by far a safer choice. Perhaps some time in the future we will have better information as to what values are optimal for growth, reproduction or performance.