Vitamin E in Equine Nutrition: Three Questions

Of all of the vitamins required by horses for their well-being, vitamin E has found a place of prominence among horse owners, likely because of its antioxidant properties that help performance horses rebound more quickly from exercise. Behind the scenes, in research settings across the world, scientists have looked closely at vitamin E and how best to supplement it to horses. World-renowned researcher Stephanie Valberg, D.V.M., Ph.D., answers three questions about this essential nutrient.

What are the current equine dietary recommendations for vitamin E?

The 2007 National Research Council (NRC) recommendations, as outlined in Nutrient Requirements for Horses, Sixth Edition, for dietary vitamin E requirements in horses are provided in IU/kg feed based upon an assumed total daily dietary forage intake of 2-2.5% of the horse’s body weight. For adult horses, the recommended dietary amount ranges from 1-2 IU/kg/day, amounting to 500-1,000 IU/day for a 500-kg horse.

Horses obtain the highest amount of vitamin E from grazing pasture, ranging from 350-3,000 IU of vitamin E/day when consuming 1.7% of their body weight on a 90% dry matter diet of pasture. This range is variable because the vitamin E available on pasture is subject to change based on the time of year.

Vitamin E concentrations degrade when feedstuffs are processed and stored. In one study, a significant decrease was noted between the first and fifth cutting of alfalfa hay, and another study demonstrated that storage losses can reach 50% in one month. As evidence, a seasonal variation in plasma vitamin E concentrations was found with increased plasma vitamin E in the summer when horses grazed on fresh pasture and were fed fresh hay compared to the winter when horses were fed dried hay and oats.

Can you provide some background on supplementing horses with vitamin E?

Depending on the availability of grass and fresh hay, a horse may consume less than the daily-recommended amount of vitamin E, and therefore, alpha-tocopherol. In such cases, many alpha-tocopherol supplements are available for horses.

Synthetic. There are two types of synthetic vitamin E, all-rac-alpha-tocopherol and dl-alpha-tocopherol. All-rac-alpha-tocopherol acetate is accepted as the International Standard (1 mg = 1 international unit). Forms of synthetic vitamin E are available as powdered or pelleted supplements. The all-rac-alpha-tocopherol acetate form is highly dependent on adequate bile salts to generate hydrolysis and subsequent absorption of alpha-tocopherol into the plasma.

Natural. There are natural forms of RRR-alpha-tocopherol available for supplementation in horses.  RRR-alpha-tocopherol is available as a micellized liquid form and as an esterified form (acetate). In order for alpha-tocopherol acetates to be utilized in the body, the ester has to be removed and the alpha-tocopherol made water-soluble by the action of bile salts (micellization).

The difference between natural and synthetic alpha-tocopherol in the horse was recently demonstrated, where cerebrospinal fluid (CSF) concentrations of alpha-tocopherol were significantly elevated above baseline values after supplementation with the natural alpha-tocopherol, but not following supplementation of synthetic at equivalent high dosages (10,000 IU/500kg horse/day). Increased CSF alpha-tocopherol concentrations have also been demonstrated with 5,000/ IU/500kg horse per day of natural alpha-tocopherol. In addition, although serum values of alpha-tocopherol increased significantly from baseline values with both the natural and synthetic form of vitamin E, serum alpha-tocopherol concentrations were significantly higher in the group supplemented with the natural compared to the synthetic. These values were approximately two times that of concentrations obtained using synthetic, which is in agreement with other studies showing that the activity of natural is between 1.36 and 2 IU/mg higher than synthetic.

A more recent study showed that, with 5,000 IU/day/500 kg horse of powdered alpha-tocopheryl acetate formulations, it often takes 56 days or more to re-establish normal alpha tocopherol concentrations in the serum. Serum alpha-tocopherol drops rapidly when horses are switched from liquid alpha-tocopherol to powdered alpha-tocopheryl acetate at the same 5,000 IU/day/horse dose.

The argument for supplementing horses with the natural form of alpha-tocopherol arises from the knowledge that this form is the most biologically available, most readily absorbed, and has the most potent antioxidant activity.

Is there danger in supplementing healthy horses?

In healthy young and middle-aged horses with adequate dietary vitamin E intake, research-based evidence for the need for or harmful effect of additional alpha-tocopherol supplementation (>500 IU/day) is scarce. Vitamin E, unlike other fat-soluble vitamins, does not accumulate in the body to a toxic level due to protective mechanisms. Excessive supplementation of alpha-tocopherol may increase plasma/serum and tissue concentrations to a certain level; however, there is strong evidence that tissues will become saturated and additional alpha-tocopherol will be metabolized and/or excreted.

In healthy exercising horses, high dosage of alpha-tocopherol supplementation (ten times the NRC requirement) was shown to be potentially detrimental to beta-carotene absorption, and this high dose was not recommended. Healthy horses should receive the NRC dietary recommendation of vitamin E. Additional supplementation above this requirement does not appear necessary in healthy horses.

Recommended Reading by the Authors

The information provided in this paper was extracted from the following article: Finno, C.J., and S.J. Valberg. 2012. A comparative review of vitamin E and associated equine disorders. J. Vet. Intern. Med. 26:1251-1266.

For the rest of this Q&A with Dr. Valberg, check out the Kentucky Equine Research Conference 2018 Proceedings.