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 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.
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):
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:
destroyed, thus destroying the cell.
Where can sources of vitamin E be found?
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?
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:
Sources of selenium:
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.
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 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.