Carey A. Williams, Ph.D., Equine Extension Specialist; Lesleyann E. Atherly, Rutgers University, Cook College, Animal Science Research Student; Jessica D. Hirsch, Rutgers University, Cook College, Animal Science Research Student
Oxidation and Oxidative Stress
Oxidation is defined as one of the processes by which energy is obtained from the diet. During this process, nutrients are broken down and converted into energy for normal metabolic function. The browning of an apple or rusting of metal is a common example of oxidation in our everyday life. The rate of oxidation depends on the amount of activity that is occurring. At rest, the rate of oxidation is at its lowest level. However, during stress, exercise, growth, pregnancy, or lactation, the rate of oxidation is elevated because the body is rapidly breaking down nutrients (like protein, carbohydrates, and fat) to produce energy needed during these times. During these metabolic processes if the regulatory systems in the body are overwhelmed, oxidative stress can occur. Oxidative stress causes an overabundance of circulating molecules called reactive oxygen species (ROS), sometimes commonly referred to as “free radicals.” It is important to note that all free radicals are ROS, but not all ROS are free radicals. These free radicals are normally produced from oxygen metabolism (see equation below).
When we inhale, we take in oxygen (O2), in our cells the oxygen uses an electron (e-) and a hydrogen (H+) to form water (H2O) and carbon dioxide (CO2), then the carbon dioxide gets exhaled. The free radicals shown in the equation include superoxide radical (O2•), hydroxyl radical (HO•), and hydrogen peroxide (H2O2). They contain oxygen, but they are much more reactive than the oxygen in the air we breathe. Free radicals are ROS with a missing electron, therefore, making them unstable molecules. They circulate throughout the body searching for electrons, hoping to achieve stability.
Are these free radicals or ROS beneficial or detrimental to your horse? In reasonable amounts they’re necessary. They are needed for proper function of the immune system, as they aid the destruction of invading foreign organisms. On the other hand, larger amounts of circulating free radicals are harmful. They can cause tissue damage and cell death by destroying cell proteins, DNA, and fatty acids. An excess of ROS leads to fatigue and damage of vital tissues such as muscle, nervous tissue, and skin. This can lead to illness due to a decrease in immune function, lameness due to destruction of muscle tissue, and other nervous system related problems.
The Role of Antioxidants
Thankfully, there is a way to combat serious damage from these ROS. Antioxidants such as vitamin E, vitamin C, glutathione, and selenium, to name a few, all have protective action against this damage.
Antioxidants protect your horse from ROS by (see Figure 1):
- Scavenging them or rendering them inactive (see Figure 2)
- Inhibiting excess ROS production
- Promoting repair of damaged tissues and cells
The various antioxidants work together to achieve all of this and more. So where do they come from? Some antioxidants come from your horse’s diet, and some are synthesized in the body. Therefore, it is crucial that your horse is healthy, and has a balanced diet that provides nutrients, including essential vitamins and minerals. Let’s take a closer look at some common antioxidants.
Vitamin E is the most important antioxidant. Vitamin E is a fat-soluble vitamin, and it protects cell membranes from damage by free radicals. Cell membranes are comprised of lipid molecules. These lipid molecules are very reactive with ROS, making cell membranes highly susceptible to ROS damage.
Vitamin E is essential in your horse’s diet:
- Deficiency can cause uncoordination and various muscle and nervous disorders. Without vitamin E the membranes of these cells become permeable and eventually are
destroyed, thus destroying the cell.
- Vitamin E has been proven to protect against equine protozoal myeloencephalitis (EPM), equine degenerative myelonen- cephalopathy (EDM), and tying up in exercising horses.
- It is especially important for exercising horses, as exercise can:
- induce oxidative stress and ROS damage, as well as decrease circulating vitamin E levels,
- in turn cause muscle problems and an overall decrease in endurance capacity and performance.
- Supplementing vitamin E before stressing the horse (e.g. traveling, competition) could potentially be beneficial to your horse by increasing immune function and protecting muscle and nervous cells (see fact sheet FS656, “Are you ‘Stressing Out’ Your Horse?” for more on stress).
Where can sources of vitamin E be found?
- Many forages and pasture grasses are high in vitamin E
- However, vitamin E content will decrease with the age of the plant, as well as with processing (heat, bailing, grinding, etc.).
- Sun-curing hay also decreases the content of vitamin E in the forage.
- Because vitamin E is fat soluble, it needs to be provided with fat in the diet so it can be absorbed and utilized. So if supplementing with extra vitamin E (i.e. 2,000 to 5,000 IU/day) you may want to make sure you are feeding a commercially available feed with added fat (i.e. 10 %) or a separate fat source (i.e. oil, or rice bran).
- Vitamin E supplements in high amounts are not toxic to horses. However, large doses (> 5,000 IU/day) should be carefully monitored as they may interact with other nutrients in the diet, like beta-carotene.
Another important antioxidant is vitamin C. This is a water-soluble vitamin, so it isn’t localized to the cell membrane like vitamin E. Vitamin C in its active form, ascorbic acid, is usually found inside and/or outside of cells, and confronts any free radicals it encounters in these places. It does this by quenching and stabilizing the free radicals, preventing future damage inside of the horse. Ascorbic acid can also aid in regeneration of the vitamin E radical, restoring its antioxidant capacity (see Figure 1). Together, vitamin C and E work together to protect vital tissues of your horse.
When is vitamin C needed?
- Horses in good health can synthesize vitamin C on their own; so there is no need to worry about a deficiency or providing it in the diet. The horse’s liver produces as much as the maintenance horse needs to stay healthy.
- However, when a horse is stressed (e.g. geriatric, intensely exercising, long hauls), production of vitamin C can’t keep up with its demand.
- Supplementation can help decrease the detrimental effects of the stress on the immune system. The usual recommendation is 7 to 10 grams of ascorbic acid a day during the short term around stressful situations (see fact sheet FS656, “Are you ‘Stressing Out’ Your Horse?” for more on stress).
- Remember vitamin C is water soluble so if you are supplementing too much it will be wasted, and the liver will slow down production due to the decreased need for the vitamin.
Selenium is a trace mineral found in plants. By itself, selenium does not have much antioxidant capacity. When selenium pairs up with vitamin E, it becomes a strong antioxidant.
Selenium with vitamin E will:
- Stop nerve cell damage caused by free radicals, therefore preventing nervous disorders that are caused by nerve damage and degeneration (i.e. Equine Motor Neuron Disease).
- Help in preventing muscle problems in horses (i.e. Tying-up or White Muscle Disease).
- Work by sitting on the surface of cells and scavenging ROS that pass by.
- They also counterbalance each other, so if availability of selenium is low, vitamin E picks up the slack, and vice versa.
- Selenium is also an integral component of the antioxidant enzyme glutathione peroxidase (see below).
Sources of selenium:
- Soil selenium levels vary in different regions. Many hays harvested in the alkaline soils of the rocky mountain region, for example, may be very high in selenium.
- Because of this variation, it is extremely important that you know the selenium status of your region and the region where your hay is harvested so toxicity does not occur.
- Most commercial feeds have additional selenium already in them so, adding additional is not necessary.
Selenium intake should always be monitored, and never supplemented if your horse is receiving adequate amounts. Horses require 0.3 mg per kg diet (about 3 mg/day). If the horse receives too much, it can cause selenium toxicity. The earliest signs of chronic selenium toxicity are loss of mane and tail hairs and cracks going around the circumference of the hoof that can actually cause the hoof wall to slough off. This can be caused by intakes of only 10 mg/day. Acute toxicity, due to sudden high level intakes, is also called “Blind Staggers”. Signs include apparent blindness, changes in behavior, anorexia, excessive salivation, increased heart and respiration rates due to necrosis of heart and liver tissues. Toxicity is enhanced if the rations also contain high amounts of copper, as many commercial feeds now do.
Glutathione has antioxidant properties which include reactivating vitamin C and vitamin E metabolites that have been oxidized by free radicals. Glutathione depletion in cells suppresses immune response of white blood cells; it prevents lymphocytes entering their normal life cycle and inhibits antibody activity. Glutathione is commonly included in many ‘immune boosting’ supplements on the market. Its supplementation may enhance antibody activity in immune cells, but has not been documented to be beneficial in horses specifically.
Besides vitamins and minerals, other types of antioxidants exist in the form of enzymes. Superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase all serve as free radical quenchers by providing them with electrons. These enzymes also work within cells, rather than on the surface like vitamin E and selenium. These enzymes also have a universal nature, as they can be found in many tissues, including liver, muscle, and brain.
- Superoxide dismutase is found in a multitude of organs; the highest activity of which is in the liver, followed by the kidney, brain, heart, and muscle.
- Glutathione peroxidase is primarily found in the red and white blood cells of mammals, which helps prevent oxidation of cell membranes by consuming free radicals in the cell. Selenium is important to the structural integrity of glutathione peroxidase, and without adequate selenium, its activity is severely handicapped.
- Glutathione reductase is essential for glutathione peroxidase to effectively stabilize free radicals and protect tissues from damage. It then reduces the oxidized glutathione to complete the cycle.
- Minerals are also a structural component of catalase, the main one being iron. An iron deficiency does slow the performance of catalase. The catalase enzyme is found mostly in liver and muscle.
What Does This Mean for Your Horse?
Any stressful condition in horses, including exercise, involves an adjustment of the antioxidants in the body to take care of the ROS produced by the increase in oxygen consumption. Horses that are especially traveling long distances and competing in several shows, races, or events in a short period of time are more prone to deficiencies in antioxidant status. This makes it even more important that the horse is on a good balanced diet with plenty of fresh green forage in the form of pasture grass or good quality hay. If necessary, an antioxidant supplement may be required; this is especially true if the horse is on limited pasture turnout.
Besides through exercise, oxidative stress can be induced by pregnancy. A growing fetus can exert an enormous amount of stress on the dam, as her body is trying to produce enough energy for herself as well as for her developing foal. Antioxidant activity can usually keep up with the demand for energy, however, during the final weeks of pregnancy before foaling, fetal development peaks. During this time, it has been shown levels of antioxidants fluctuate, so it is important to keep supplying the pregnant mare with adequate amounts of vitamin E, selenium, and other essential minerals. This should be in the form of good quality forage, pasture preferably.
As horses age, metabolic function slows and is less efficient. Efficiency of organ function also decreases. This increases susceptibility to oxidative stress and damage, thus worsening organ and tissue function. Supplementation of antioxidants is extremely important for an aging horse in order to decrease their susceptibility to oxidative damage. Older exercising horses need more antioxidants as well, because exercise can intensify their vulnerability to ROS damage. Vitamin E and C are possibly needed as supplements to a geriatric horses’ diet.
Horses in a diseased state are also vulnerable to oxidative stress. Although free radicals to some extent do aid in fighting sickness, the increased levels still need to be monitored. Sickness may also decrease food intake and absorption in the intestines. Vitamin deficiencies can occur, which can make an existing problem worse, so additional antioxidant supplementation may be necessary to fortify the normal diet in these ill horses.
Equine motor neuron disease (EMND) is a neurodegenerative disorder in the adult horse. There is a significant association between EMND and vitamin E status; lower plasma levels of alphatocopherol are found in diseased horses than in control horses. This hypothesis of vitamin E deficiency has been replaced with the newer theory that vitamin E is low due to its increased utilization of scavenging the ROS that are damaging the affected nerves.
Chronic rheumatic disease and degenerative bone and joint diseases are linked to excessive ROS production. The ROS are also capable of degrading components of the joint and this has been implicated in the pathogenesis of equine joint disease.
The Bottom Line
The main point to be concerned about is that oxidation increases as the need for energy increases, like during exercise and pregnancy. As oxidation increases, so does the production of ROS, including free radicals, which can damage vital tissues in your horse. Horses do have internal mechanisms to keep up with the increased production of ROS, such as vitamin C synthesis and antioxidant enzymes, but these internal mechanisms may not be sufficient when ROS levels rise. The best way to prevent serious damage is to keep your horse healthy with a balanced diet with the essential vitamins and minerals, but avoiding oversupplementation.
- Chew, B.P. 1996. Importance of antioxidant vitamins in immunity and health in animals. An. Feed Sci. Tech. 59:103-114.
- Clarkson, P. and Thompson, H. 2000. Antioxidants: What role do they play in physical activity and health? Am. J. of Clin. Nutr. 72 (supp.): 637S-46S.
- Thomas, Heather S. 2004. The Role of Antioxidants. The Horse Magazine.
- Williams, C.A. 2005. Are you ‘Stressing Out’ your horse? Rutgers Cooperative Extension. FS656.