The Importance of Vitamin E in Horse Health | Equine Science Matters™

Vitamin E is a powerful antioxidant that plays an important role in maintaining cell health and supporting the immune system. It also helps protect cells from the effects of free radicals and other reactive oxygen species produced during exercise. 

As exercise intensity and duration increase, the production of reactive oxygen species can also increase. If the body's antioxidant defences cannot adequately manage these compounds, oxidative stress can occur, potentially damaging cell membranes, proteins and DNA. However, not all exercise-induced oxidative stress is harmful. In fact, a certain level of oxidative stress is an important stimulus for the adaptations we want to see from exercise and training. The aim is therefore not to eliminate oxidative stress, but to maintain an appropriate balance.  

Vitamin E is an essential vitamin, meaning it must be obtained from the diet. Requirements can vary according to factors including life stage, workload and diet. Horses undertaking prolonged or strenuous exercise, those consuming higher-fat diets, and horses with certain muscle or neurological conditions may have increased requirements.  

During the summer, horses with good access to fresh, growing pasture would generally obtain a useful source of vitamin E from their forage. However, vitamin E concentrations in forage can vary considerably depending on growing conditions, maturity and harvesting. Drought and poor grass growth can therefore reduce the vitamin E available from grazing. Vitamin E also declines during hay storage, meaning horses receiving predominantly conserved forage may have lower dietary intakes than those grazing fresh pasture.  

When considering supplementation, the form of vitamin E is an important factor. Vitamin E occurs naturally as eight related compounds, known as vitamers: four tocopherols and four tocotrienols. Alpha-tocopherol is the form most relevant to the horse's requirements, and supplements commonly provide either RRR-alpha-tocopherol, often described as natural vitamin E, or all-rac-alpha-tocopherol, the synthetic form. Research in horses suggests that RRR-alpha-tocopherol (natural) is more bioavailable than synthetic forms, although the significance of this depends on the dose and the horse's individual requirements.  

Vitamin E is often combined with selenium, another essential nutrient. Selenium is required to produce glutathione peroxidase enzymes, which work alongside vitamin E as part of the body's antioxidant defence system, helping to protect cells and their membranes from oxidative damage.  

However, more is not necessarily better when it comes to selenium. Unlike vitamin E, selenium has a relatively narrow safety margin, and excessive intake can be harmful. Providing additional selenium above dietary requirements does not necessarily provide additional antioxidant benefit. It is therefore important to consider the total amount of selenium supplied by forage, hard feed, balancers and supplements before adding more to the diet.  

For horses already receiving a fortified hard feed or balancer at the recommended feeding rate, selenium intake may already be adequate. In these situations, supplementing vitamin E alone can be a useful way of increasing vitamin E intake without unnecessarily increasing selenium, particularly where higher levels of vitamin E are being considered for horses with elevated requirements. 

As with all vitamins and minerals, supplementation should be considered as part of the whole diet, rather than in isolation. Forage analysis and a review of all feed and supplements can help establish how much vitamin E and selenium a horse is already receiving and whether additional supplementation is appropriate. 
 
Conclusion 

Vitamin E is an important antioxidant that helps protect cells from oxidative damage and supports normal muscle and immune function. Exercise increases the production of reactive oxygen species, although some oxidative stress is a normal and beneficial part of training adaptation. Fresh grass is an important source of vitamin E, but levels can decline with forage maturity, poor growing conditions and hay storage. Horses with limited access to fresh pasture or increased requirements may therefore benefit from supplementation. The form of vitamin E matters, with natural RRR-alpha-tocopherol (natural) generally more bioavailable than synthetic forms. Selenium works alongside vitamin E as part of the antioxidant defence system, but because excessive selenium can be harmful, total dietary intake should always be considered before supplementing.