By: Shannon Pratt-Phillips, PhD
Find out how oxidative stress affects your horse, and what you can do to combat cell-damaging free radicals in his system.
Antioxidants protect cells from damage caused by free radicals, which are produced by many chemical reactions in the body,particularly during exercise. Peroxides and hydroxyl radicals, for example, are types of reactive oxygen species (ROS) that are produced as a consequence of cellular processes involving oxygen. When oxygen is used by the body, it is converted mostly to carbon dioxide and water, but some (about one to two per cent of oxygen) is converted to ROS.
The body keeps the levels of these ROS in check through antioxidants and free radical scavenging systems. These function through three mechanisms: they prevent ROS synthesis, inactivate oxidants, or facilitate repair of oxidative damage.
Under stressful conditions, the balance between ROS production and antioxidative protection (often called the oxidant/antioxidant equilibrium) may be disrupted, resulting in oxidative stress. These oxidative compounds can harm cells by causing cell death or damage to DNA, resulting in cancer or other diseases.
Oxidative stress is measured through the assessment of thiobarbituric acid reactive substances (TBARS, a byproduct of lipid peroxidation) or malondialdehyde (MDA) in the horse’s system. A decrease in antioxidant status suggests increased oxidative stress. Studies have found increases in oxidative stress with exercise, transport and even disease, such as equine infectious anemia (EIA), for example.
There is extensive research investigating factors affecting oxidative status in horses. Oxidative stress has been detected in both intense (racing) and endurance types of exercise. Another study found increases in MDA, with decreases in glutathione peroxidase (GPx), in horses that were transported for 12 hours. Oxidative stress markers have also been detected in horses with recurrent airway obstruction and horses with ischemiareperfusion injury, such as may occur with colic surgery. Thus, many horse owners look to supplement their horses with antioxidants in effort to prevent further damage and disease associated with oxidative compounds.
There are many types of antioxidants found within the body. Those derived nutritionally include vitamin E and C as well as beta-carotene (the precursor for vitamin A). In addition, some naturally occurring enzymes such as superoxide dismutase (SOD) and GPx, which contains the mineral selenium, function as antioxidants.
The basic nutrient requirements of horses outlined by the National Research Council’s (NRC) Nutrient Requirements of Horses (2007) include the provision of several antioxidants.
- Vitamin C (ascorbic acid) is produced naturally by the body (unlike in humans, other primates, guinea pigs and fruit bats) and, therefore, does not need to be provided for in the diet. Studies have suggested, however, that older horses have lower vitamin C status and, therefore, may benefit from vitamin C supplementation (as such, ascorbic acid is often found as an ingredient in ‘senior’ horse feeds). Vitamin C is a water-soluble antioxidant and functions within the cell cytosol and blood plasma.
- Vitamin E is a fat-soluble vitamin and functions to protect cellular membranes. Both beta-carotene and vitamin E are found naturally in fresh forages such as pasture, though their concentrations decrease with storage as hay. Cereal grains also contain smaller amounts of vitamin E and betacarotene. Beta-carotene is also found in carrots.
- Selenium is a component of the antioxidant enzyme complex glutathione peroxidase. Selenium concentration in feeds is largely dependent on the soils from which they are derived, though many commercial feeds are fortified with selenium to compensate for selenium-deficient soils.
Most horses consume a diet of pasture and/or hay, along with some commercial concentrate (either a grain mix, or even a vitamin/mineral mix). These horses should, therefore, easily meet their basic nutrient needs, and that includes their basic antioxidant requirements. If a horse is stressed, however, it is possible that he may have some additional oxidative stress and may benefit from supplemental antioxidants.
Several studies have examined the effects of various antioxidant compounds on oxidative status within the horse, and while antioxidant status can be improved, clinical benefits, particularly with respect to exercise performance, are often lacking. Vitamin E and C, beta-carotene and selenium are commonly supplemented to horses, while others such as coenzyme Q10 are less common. Two excellent scientific review articles discuss antioxidant supplementation in much greater detail: Kirschvink et al., The Veterinary Journal, 2008 and Soffler, Veterinary Clinics North America, Equine Practice, 2007.
- Vitamin E is the most commonly supplemented antioxidant, despite conflicting evidence supporting its efficacy at reducing oxidative damage caused by exercise. Exercised horses have lower vitamin E status, which, therefore, results in increased requirements identified by the NRC. In two studies, decreased vitamin E status was negatively correlated with oxidative stress markers (TBARS or MDA), suggesting that if vitamin E status is reduced, oxidative stress is higher and vice versa, which supports its supplementation.While vitamin E is relatively non-toxic, even at high concentrations, one study reported a negative effect of vitamin E supplementation on beta-carotene status (when vitamin E was administered at 10x the NRC’s recommendations).
- Vitamin C is also commonly offered to horses as an antioxidant supplement, and while one study reported a prevention of an exercise-induced increase in TBARS, another found no effect.
- Beta-carotene alone has not been studied extensively, but one study reported reduced betacarotene status in elite eventing horses.
- Selenium is often supplemented in combination with vitamin E because they function synergistically and they may reduce oxidative stress caused by exercise better in combination. In fact, antioxidants are rarely offered alone, and many horse owners offer antioxidants as a group. One study offered racing Thoroughbred horses an antioxidant mix, containing vitamins A, E and C along with zinc, copper (both of which are part of superoxide dismutase) and selenium. While some aspects of anti-oxidative capacity were improved, others were not changed.
Other antioxidants may also have therapeutic benefits, but have not been studied as extensively. N-acetyl cysteine, for example, is a glutathione analogue (and thus may benefit the glutathione peroxidase pathway), but has only been studied in vitro. Dimethyl sulfoxide (DMSO) offers some local antioxidative benefits, particularly for ischemiareperfusion injury. Synthetic versions of superoxide dismutase are also being developed and may have future benefits.
While many of the studies investigating the benefits of antioxidants on specific conditions or exercise performance have conflicting results, there is also some support to offer some supplemental antioxidants to horses that may be under stress. A well-balanced diet should meet the basic needs to maintain antioxidant status of your horse, but offering a mix of antioxidants (vitamins E and C, betacarotene, selenium) is relatively safe.
Risk of toxicity (for selenium, or negative interactions with vitamin E) only arises when multiple supplements are offered. If, for example, a complete feed is offered (which is often fortified with vitamins A, D and E) along with a vitamin/mineral or antioxidant supplement, and then if a vitamin E-selenium supplement is also offered, these potentially negative consequences may arise. It is wise to discuss all supplements with your equine nutritionist to ensure your horse is meeting his nutritional needs, and potentially maximizing his antioxidant status.