Oxidative stress has become an area of increasing interest in research both in human and equine sports medicine.
The importance of antioxidants for health continues to be popular in the media with numerous products claiming their additional antioxidant will enhance health and performance for both you and your animals. Basically speaking, the imbalance between of excessive oxidants and insufficient antioxidants, contributes to day-to-day and long term physiological conditions, through oxidative stress. All horses are subject to oxidative stress and as a horse’s workload increases, dietary energy must be increased to maintain weight and condition, thus increasing free radicals (or oxidants and the need for antioxidants). Along with exercise intensity and duration, diet, age, and training program can also affect oxidative stress in the horse.
So what is oxidative stress?
Oxidative stress is an imbalance of the oxidant-to-antioxidant ratio in the body. Oxygen is essential for almost all living creatures driving metabolic processes and energy production. During this reaction, some oxygen molecules form damaging “reactive oxygen species” (ROS). These molecules are also called free radicals - unstable ions that have an unpaired electron and bounce around the cell nucleus or cell membrane. Much like a ball in a pinball machine, these free radicals try to exchange electrons from other cellular molecules they encounter. Frequently, they react with and damage DNA strands, proteins, and the fats found in cell membranes, setting off a chain reaction of other molecules with further electron exchange.
Free radicals are increased with aging, exercise and infection, but they may also be created following injury, disease or exposure to certain environmental factors, such as allergens, pollutants, anaesthesia, inflammation, or radiation. Diet itself can increase the oxidative load and potentially lead to oxidative stress. For example, diets rich in polyunsaturated oils (often found in horse feed) increase the requirement for antioxidants especially vitamin E. More importantly poor nutrition may reduce the horse’s ability to mount a sufficient antioxidant defence.
During exercise, horses inhale more oxygen to meet the oxygen demand of their muscles. These free radicals or ROS can lead to fatigue, damage DNA and contribute to degenerative changes throughout the body. Along with supporting the muscular oxygen needs, free radicals are produced and horses suffer “oxidative stress.”
Fortunately, cells have natural defence with natural antioxidants in place to combat free radicals. These free-radical scavengers donate electrons to rampaging free radicals but remain stable themselves with unpaired electrons. This antioxidant defence system depends on dietary intake of antioxidant vitamins and minerals, and subsequent production in the body.
A variety of elements are shown to actively contribute to this defence system, including antioxidant compounds (such as glutathione and vitamin C) and antioxidant enzymes (proteins). Some nutrients, including zinc, manganese, iron, selenium and copper, form an integral part of these antioxidant enzyme systems while others such as vitamin E act as antioxidants in their own right. Various plant or herbal products (such as turmeric, garlic, and ginger) are also thought to serve as antioxidants.
How do we recognise oxidative stress?
The history of your horse, including poor performance for the apparent level of fitness, poor recovery rates and excessive sweating may help us recognise that your horse is suffering from ‘Oxidative Stress’. Blood tests may also show an increase in gamma glutamyl transferase (GGT) or aspartate aminotransferase (AST), which have been shown to be markers for oxidative stress. Testing the levels of selenium and vitamin E in your horses blood can also determine if your horse is likely to be deficient in antioxidants.
So how is oxidative stress treated or managed?
Dietary supplementation, managing the horses training program and treating or managing underlying disease are the mainstays of managing oxidative stress. Specific dietary supplementation (identified through testing) currently suggests considering selenium, vitamins C and E.
Vitamin C (also known as ascorbic acid) is believed to be one of the most important antioxidants in the fluids found outside the cells themselves, and has many beneficial factors. It is important for the formation of cartilage and bone, the optimal functioning of the immune system and wound healing and in the fluid lining the lungs of horses. Levels of vitamin C are reduced in horses suffering from recurrent Airway Obstruction (RAO – previously known as COPD, “broken wind” or “heaves”) and other types of airway inflammation (e.g. bacterial infection). Unlike humans, primates (i.e. monkeys) and guinea pigs, the horse is able to synthesise its own supply of vitamin C. It has therefore been thought that the vitamin C requirements of the healthy horse will be met by tissue synthesis. However, horses that are under certain types of stress may require extra supplies. Unfortunately, not all dietary sources of vitamin C are suitable for horses and therefore it is fundamental that it is provided via an appropriate source.
Vitamin E (also known as tocopherol) is a collective name for a number of biologically similar compounds which share the same function. Where vitamin C is water soluble, vitamin E is fat soluble and is the major antioxidant involved in maintaining cell membrane integrity. Vitamin E has been suggested to play an important role in the functioning of the immune system and for normal growth and muscle function. A dietary source of vitamin E is essential as it cannot be synthesised in the body. Dietary sources of vitamin E in equine diets include fresh green forage and the oils of some grain seeds. Cereals and preserved hays contain limited amounts of Vitamin E. Many horse diets that are not supplemented, will not provide the optimal amounts of vitamin E. Vitamin E supplementation can be provided in two main forms, liquid vitamin E ‘Nano E’, or powdered vitamin E supplements.
Selenium is an essential part of an antioxidant enzyme (glutathione peroxidase) and as such plays an essential role in cellular antioxidant defences. The pasture and feed program content of selenium is variable, so horses have diets with varying concentrations. It is therefore recommended to check if your horse is getting enough by having a blood test taken. Copper, zinc and manganese are also important antioxidant trace minerals as they form integral parts of a number of antioxidant enzymes, such as superoxide dismutase, which are part of the first line defence against free radicals.
A number of conditions may lead to a decreased antioxidant defence, oxidative stress and oxidative damage, with evidence suggesting that this may also occur as a consequence of certain diseases in horses. There are limited scientific studies on the role of antioxidants in horses compared with humans, but there is increasing evidence that the supply of additional antioxidants in the diet may help in a number of different conditions or diseases. While a number of antioxidant supplements are on the market and have been promoted, the success of an antioxidant supplement will depend on which antioxidants are present, in what proportions and if they are present in forms that the horse can readily absorb. As usual, different treatments have differing levels of efficacy, stability and affordability, so we encourage you to undertake your research to find the product most suitable for you and your horse.
If you have any further questions about the oxidative stress, tests or supplementation of your horse with antioxidants, please do not hesitate to contact us at 0800 VETSOUTH.
- Heather Busby
