Vitamin E and selenium – a vitamin and a mineral. On first glance these two compounds appear to be very different from each other. However, on closer examination it turns out that they have something in common, they are examples of antioxidants.
What is an antioxidant?
Antioxidant is a term that is seen more and more frequently in equine nutrition as we learn about their importance in human health. They are key factors in the fight against cellular level damage and inadequate levels of these important compounds can result in a decreased ability to fight off illness and chronic disease. Commonly found in fruits and vegetables they are just one of the reasons why we humans are encouraged to eat plenty of these important foods each day.
Natural antioxidant sources in the horse’s diet
Consuming enough plant material to insure abundant levels of these key nutrients is likely not a problem for horses in their native setting where a broad range of plant matter is consumed each day. However, the diets of domesticated horses can be quite bland with little variety day-to-day. Additionally, processing of forages for storage, such as making hay, can reduce the levels of these naturally occurring compounds as they are often heat sensitive. Some of them will deteriorate over time as well meaning that levels will be lower in hay stored for several months than when first baled.
While research into these compounds is not as extensive in the equine literature we do know that inadequate intakes of some of these key antioxidants can have negative implications on your horse’s performance and health.
The function of antioxidants in the horse
Before we discuss some of the antioxidants more commonly added to equine rations let’s look in more depth at what antioxidants do. The simplest definition of an antioxidant is that it is a molecule that prevents the oxidation of another molecule. Cells are made up of many molecules and in turn these molecules are made up of atoms. As you may recall from High School chemistry, atoms have a number of positively charged particles (protons) and negatively charged particles (electrons). Electrons are arranged in ring around the atom with a specific number of electrons in each ring. To improve overall stability, atoms try to maximize the number of electrons in the outer most ring and sometimes this is achieved by sharing electrons with neighboring atoms which allows molecules to bind together and create stable structures.
If these bonds break apart an odd number of electrons may be left in an atom’s outermost shell. Atoms with an odd number of electrons are inherently unstable and are known as free radicals. They take on the mission of finding an electron that can be stolen to create an even pair, and become stable, as quickly as possible. Often this electron is stolen from a neighboring molecule leaving the molecule that was attacked with an odd number of electrons such that it now becomes a free radical. If left unchecked, a cascade of stealing can occur as one molecule attacks and steals an electron from another until the reactions are stopped, which is called the oxidative process.
Because free radicals are a by-product of the oxidative processes within the cell the process is sometimes described as “oxidative damage” or “oxidative stress” and free radicals are thought of as “bad”. However, it is important to remember that free radicals are natural byproducts of the oxidation of carbohydrates, fats and proteins for use as energy—processes that are necessary for our bodies to function. This means that when horses work, free radicals are generated due to energy stores being utilized for work. Free radicals are inevitable and can also be beneficial. Oxidation is used as a way for the body to neutralize infections and viruses. Only when oxidation becomes excessive does significant damage occur, which will be more likely if inadequate levels of antioxidant compounds are available.
Antioxidant compounds are special in that they are able to donate an electron to a free radical and yet they remain equally stable whether they have an odd or even number of electrons in their outer ring. Antioxidants are like scavengers looking for oxidation reactions that they can end. Therefore, antioxidants are vital to preventing significant and long term cellular damage.
Areas of your horse’s body that may be negatively impacted by oxidation in the face of inadequate antioxidants include: muscle function, cell membrane integrity, immune function, neurological function, vision and many others.
Added sources of antioxidants
Due to their importance, some antioxidants are frequently added to commercial feeds, the most common being vitamin E and selenium. Vitamin E is a term used to describe a family of compounds known as tocopherols and tocotrienols of which there is an alpha, gamma, delta and omega form of each. Some have greater vitamin E activity than others. When found naturally occurring in the plants several forms exist and they are often referred to as mixed tocopherols.
Alpha tocopherol is preferentially absorbed by people and has the greatest body of research. It is also the form that is thought to be the most important to horses although there is some new data that suggests gamma tocopherol may also have previously unrecognized benefits. When we talk about a horse’s vitamin E requirement we are referring to the requirement for alpha-tocopherol.
Alpha tocopherol comes in different forms. The natural d-alpha tocopherol and a synthetic form known as dl-alpha tocopherol. Research in horses has shown that the natural form has greater bioavailability than the synthetic form in equine diets. This means that greater amounts of synthetic vitamin E needs to be fed each day to get the same benefit as a lessor amount of natural vitamin E will achieve. HYGAIN uses natural vitamin E to insure your horse is able to get the maximum benefit from dietary vitamin E.
Research in rats has shown that vitamin E is vital for membrane repair in working muscles. As muscles contract damage occurs to the membranes around the muscle cells. In healthy rats with adequate dietary vitamin E, damage to membranes was fixed in minutes but this was not the case in rats fed vitamin E deficient diets. These rats had lower running ability and took more frequent rest breaks.
Working muscles utilize more oxygen, and therefore produce more free radicals. To avoid damage to muscle cells during exercise, adequate levels of antioxidants must be available to counter all the extra free radicals that are being generated. This is why, as a horse’s workload increases, his requirement for vitamin E goes up, too. Signs of oxidative damage in working horses include muscle soreness and stiffness and slower-than-expected recovery from intense exercise.
While the level of vitamin E provided in commercial feed is enough to meet requirements for most horses when fed properly, individual variation between horses in their ability to utilize dietary vitamin E is great. For this reason it is generally advised that performance horses have their vitamin E levels checked in order to determine whether additional vitamin E supplementation is required.
Selenium is often thought of in combination with vitamin E as it is a potent antioxidant that plays a vital role as a component of the enzyme that turns harmful peroxides (single oxygen to oxygen bonds) in to harmless alcohols before it can do significant damage to cell membranes. Vitamin E and selenium work together to protect muscle tissue, with Vitamin E helping to reduce the formation of peroxides in the cell membrane and selenium removing peroxides that do occur from the fluid surrounding cells.
When one or other of these nutrients is deficient the other must work harder to protect cells. Additionally, selenium plays a number of other important roles in the body although they are not antioxidant in nature. These include roles in thyroid function and immune function. Many of the selenium deficiency issues that used to be seen in horses are less common today due to the practice of adding selenium to commercial feeds fed to horses. It is still advisable to check your horse’s selenium levels especially if you are in a selenium deficient area and feeding forages low in selenium.
Additionally, not all forms of selenium are as bioavailable as others which may impact tissue levels. The inorganic forms sodium selenite and sodium selenate are less bioavailable than the organic selenium yeast. Not only is selenium yeast absorbed better from the digestive tract it is also taken up in to muscle tissue more readily than the inorganic forms. Hygain uses organic selenium in our feed formulations.
Less commonly considered antioxidants in the equine diet are vitamin A or beta-carotene and vitamin C. Unlike humans, horses actually make vitamin C in their liver and therefore do not have a dietary requirement. Limited data suggests that there may be a benefit to supplementing senior horses with additional vitamin C as well as horses with soft tissue injuries that need to build collagen. Additional vitamin C may also be beneficial for horses suffering from allergies and respiratory conditions due to its role in antihistamine control.
Care should be taken when removing a supplemental source of vitamin C from the ration to do so gradually. Supplementation may cause the horse to produce less vitamin C of his own leaving him with a short fall if the supplemental source is removed suddenly.
Vitamin A is rarely considered by most horse owners due to the fact that the precursor beta-carotene is abundant in fresh pasture and other green forage. However, levels of beta-carotene drop quite dramatically (almost 50 percent after a year) in stored forages which could result in a shortfall in horses fed aged forages. Critical for normal immune function, vision, reproductive health, and bone health among other functions, vitamin A is routinely added to commercial feeds and supplements.
Like vitamin E, it is a fat-soluble vitamin which means that it can be stored in the liver for months at a time. Vitamin A does not in fact exist in plants, they contain carotenes which the horse absorbs and converts to vitamin A as needed. Care must be taken when combining feeds and supplements that provide vitamin A because in its vitamin A form the horse has less ability to regulate how much is absorbed and it is possible to create a vitamin A toxicity.
Because free radicals can engage in so many different types of chemical reactions, it is good to have a wide array of antioxidants in the diet to counter them. Research is indicating potential benefits of other antioxidant compounds such as supplementing Resveratrol to horses with joint pain. However, data in horses is lacking for many antioxidants touted as beneficial to humans and results cannot always be accurately extrapolated to determine expected outcomes in horses.
For now, insure that you are correctly feeding commercial feeds that provide the most bioavailable sources of key antioxidants at levels balanced to meet, but not excessively exceed, your horse’s daily requirement. If you believe that you are seeing conditions that may be linked to an antioxidant deficiency talk with your veterinarian about testing your horse’s levels and then adjust the diet accordingly.