Each and every day, your horse’s body is under constant attack from particles called free radicals. These potentially dangerous molecules are formed as the result of normal body processes, especially as a result of producing energy using oxygen. Therefore, their production is increased with exercise, but they may also be created following injury, disease or exposure to certain environmental factors, such as allergens, pollutants (such as ozone, nitrogen dioxide or sulphur dioxide) or radiation.
What are Free Radicals?
Free radicals are independent yet unstable molecules that contain one or more unpaired electrons. The preferred state is for these electrons to be paired, and therefore, free radicals actively search for a partner for their unpaired electron. But in order to become stable they have to attract electrons from surrounding molecules which usually do not have ‘spare’ electrons to give – the original free radical, therefore, effectively ‘steals’ the electron, which causes the ‘donating’ molecule to become a free radical itself. This can start what is known as a chain reaction as the new unstable free radical tries to attract an electron from the molecules surrounding it and so on. Under certain circumstances this can be harnessed and be of value, for example such a process is used by our white blood cells (the cells of our immune system) to help destroy potentially harmful bacteria and viruses.
The phrase ‘reactive oxygen species’ (or ROS) is often used to describe all those free radicals and other molecules which contain one or more molecules of oxygen that are capable of causing the reactions described above. These have the potential to oxidise essential phospholipids and proteins within cell membranes, leading to cell death. Such ‘oxidative’ damage can lead to alterations in the structure of cell membranes, cell DNA and other cellular components and cause disruption of normal physiological processes (some of which may be irreversible). To combat such damage, an antioxidant defence system has evolved.
What is the Antioxidant Defence System?
Antioxidants, otherwise known as ‘free-radical scavengers’, are the body’s natural defence against oxidative damage. Antioxidants effectively neutralise free radicals, for example, by giving them the electron that they so desperately seek, BUT without becoming an active free radical themselves – preventing the chain reaction from continuing. Due to this, antioxidants play an important role in maintaining the health and integrity of body cells. In fact, the balance between free radical production and antioxidants is thought to be strongly related to lifespan.
The elaborate antioxidant defence system depends on the dietary intake of antioxidant vitamins and minerals and the production within the body of antioxidant compounds such as glutathione. Some nutrients, including zinc, manganese, iron, selenium and copper, form an integral part of antioxidant enzyme systems others such as Vitamin E, Vitamin C and glutathione act as antioxidants in their own right. There are also antioxidant enzymes (proteins), including superoxide dismutase, glutathione peroxidase and catalase.
What do we mean by Oxidative Stress and Damage?
Oxidative Stress
As the antioxidants effectively neutralise the free radicals, they tend to become changed themselves. Most commonly, they become oxidised and can no longer act as antioxidants until they are restored to their ‘un-oxidised’ or reduced state. ‘Oxidative stress’ occurs when the antioxidant defence system cannot cope with the rate of free radical production, and a large proportion of the antioxidants have been oxidised, thereby reducing the ability of the body to deal with the free radical threat.
Oxidative stress, therefore, may occur because of
- increased free radical production due to:
– increased exposure to oxidants from the environment
– or increased production within the body e.g. an increase in processes that use oxygen such as exercise
And/Or
- decreased antioxidant capacities
And/Or
- imbalance in antioxidants e.g. from diet
Diet itself can increase the oxidative load and potentially lead to oxidative stress. For example, diets rich in polyunsaturated oils increase the requirement for antioxidants, especially vitamin E. More importantly, poor nutrition may reduce the horse’s ability to mount a sufficient antioxidant defence. Non-dietary sources of oxidative stress include UV light, radiation, pollution, anaesthesia, inflammation, physical injury as well as exercise.
Oxidative Damage
If the free radical production continues the situation can then potentially lead to cell or tissue damage i.e.: – ‘Oxidative damage’. In recent years, there has been a growing awareness of the role of oxidative damage, at least in humans, in both the ageing process and the development of a number of diseases or conditions such as asthma, cancer, skin disease, arthritis, muscular atrophy, cardiovascular disease, liver disease and auto-immune disease. It is thought that oxidative damage may also be implicated in similar diseases in animals. In the horse, for example, we have recently become aware of the real importance of oxidative stress and antioxidants in lung health which we will cover in the next article.
Which are the important Dietary Antioxidants?
It is increasingly thought that nutrition may play an important part in helping to protect against oxidative stress and damage. Certain nutrients and dietary components have antioxidant properties that are important for the normal functioning of the body. However, some of these when fed in the optimum amounts and proportions may also offer a protective role against a variety of diseases which may be at least in part be associated with oxidative stress or damage. Work has suggested that a mixture of antioxidants, especially arising from natural sources, may be more beneficial than a single antioxidant in helping to support antioxidant defences.
Vitamin C
Vitamin C is believed to be one of the most important antioxidants in the fluids found outside the cells themselves. It is also important for the formation of cartilage and bone as well as for the optimal functioning of the immune system and wound healing. Recently, as will be discussed in the next article, we have shown Vitamin C to be the most important antioxidant in the fluid lining the lungs of horses and that levels 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 e.g. exercise, transport, disease, trauma or surgery may require extra. Unfortunately, not all dietary sources of Vitamin C are very available for the horse and therefore it is important that if supplementation is provided it is via a source which has been shown to be both available and stable.
Vitamin E
Vitamin E is a collective name for a number of biologically similar compounds which share the same function. Whereas 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 also play an important role in the functioning of the immune system and is believed to be important for normal growth and muscle function.
A dietary source of Vitamin E is essential as, unlike Vitamin C, 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 some but limited amounts of Vitamin E. Many horse diets that are not supplemented will not provide the amounts of Vitamin E that we now believe to be optimal.
Trace elements
Selenium is an essential part of the antioxidant enzyme glutathione and as such plays an essential role in cellular antioxidant defences. Pasture content of selenium can be very variable as some plants may contain very high levels but most pastures, especially in the UK, tend to be low to very moderate sources of selenium. Dietary sources tend to be supplements both inorganic (e.g Sodium Selenite) and organic (selenium concentrating yeasts).
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. Hays, pastures and cereals tend to provide a variable intake of these trace elements and therefore for many horses additional dietary supplementation is needed either via the compound feed or via a broad spectrum vitamin and mineral supplement.
Carotenoids
Carotenoids are responsible for the red, yellow and orange pigments found in plant foods especially fruits and vegetables and in the tissues of animals which eat the plants. Some carotenoids are precursors of Vitamin A, but this property is unrelated to their antioxidant activity. Carotenoids such as β-carotene (found in green grass etc.), lutein and lycopene (found in tomatoes) act as powerful antioxidants. Very mature hays, especially stemmy hays, tend to be very low in B-carotene as do most cereals (apart from yellow maize).
Are there any interactions between Antioxidants?
In addition, to their individual effects, dietary antioxidants may assist each other, for example Vitamin C may assist Vitamin E by recycling Vitamin E once it has been used up in neutralising certain free radicals. Vitamin E can help protect ß-carotene from oxidation, and may therefore have a sparing effect on this antioxidant. Vitamin E and the mineral selenium appear to act to work together as parts of a multi-component antioxidant defence system.
What about herbs?
Herbs and spices can be very valuable sources of antioxidants, however they may also have a number of other properties which need to be taken into consideration when adding them to your horse’s diet. At least 30 food spices and herbs have been suggested to possess antioxidant properties. For example, eugenol is the active antioxidant compound in garlic and flavenoids are found in rosemary. Both of these antioxidants may be important to help maintain health in particular of the respiratory system which will be discussed in our next article on the lung.
For further information on:
Vitamin C:
Vitamin E:
