Dr Greg Whyte considers the vital role of antioxidants in preventing exercise-induced damage in your muscles. The role of endurance exercise in protecting men and women against a variety of diseased states is well understood. However, there is now a growing body of evidence suggesting that endurance exercise can itself cause damage by means of increased 'oxidative stress' - the consequences of a hugely increased oxygen consumption by working muscles and the body as a whole.

There is an apparent contradiction here, but the key point is this: all endurance activity is associated with oxidative stress, and the higher the intensity of the exercise, the greater the stress; but regular performance of such exercise generates an adaptive response which helps to protect the body against the effects of such stress. The increased oxidative stress observed during exercise is associated with an increase in production of chemicals known as free radicals and reactive oxygen species (ROS). Radicals and ROS produced as a result of exercise include superoxide (O-2), hydrogen peroxide (H2O2), nitric oxide (NO), and hydroxyl radicals (HO). These are highly active chemical species which can work on DNA, proteins and lipids in such a way as to cause molecular cell damage and cell injury, leading in turn to accelerated ageing and disease(1). Further, oxidants may contribute to muscle fatigue and injury through cellular changes in skeletal muscle.

Radicals and ROS may be produced during exercise in three ways:

1. Electron 'leak'. Exercise involves a 10-20-fold increase in whole body oxygen consumption and a staggering 100-200-fold increase in local muscle oxygen consumption. Most of this oxygen is transformed to water, but a small amount (2-4%) is converted to superoxide within the electron transport system. This superoxide is transformed into hydrogen peroxide, leading to the production of hydroxyl radicals, the most destructive of the species;
2. Ischaemia-reperfusion. Redistribution of blood to the working muscle results in hypoxia within the kidneys and in the region of the liver and spleen. In addition, high-intensity exercise leads to local muscle hypoxia (shortage of oxygen). The reoxygenation which takes place when exercise stops results in a burst of ROS;
3. Auto-oxidation of catecholamines - rising levels of the stress hormones adrenaline and noradrenaline during exercise lead to increased oxidative stress.

Complex internal protective mechanisms exist to combat the deleterious effect of radicals and ROS. The two major classes of defence mechanism are:

1. Enzymic defences. The primary antioxidant enzymes are superoxide dismutase, glutathione peroxidase, and catalase. They work by removing superoxide radicals, hydrogen peroxide (H2O2), and organic hyperoxides;
2. Non-enzymic defences. These are found within lipid (fatty) and aqueous (watery) portions of the body. The major aqueous-based antioxidants are vitamin C and reduced glutathione (GSH), while the major lipid antioxidants are vitamin E, ubiquinol (coenzyme Q10) and beta-carotene.

uring exercise the pro-oxidant/antioxidant balance shifts in favour of the former, with the rate of radical and ROS production exceeding their rate of removal by the antioxidant defence mechanisms. Therefore, to avoid or minimise skeletal muscle damage the antioxidant capacity of the cell must be increased. This increased capacity may be achieved through appropriate training, diet and the use of antioxidant nutritional supplements.

So how do you deal with those pesky free radicals?
Aerobic exercise training strengthens the antioxidant defence system by increasing the antioxidant enzymes superoxide dismutase and GSH peroxidase, thus reducing the oxidative stress of exercise. This has been demonstrated by the observed reduction in lipid peroxidation in trained athletes compared with untrained individuals in response to a given bout of exercise(2). High-intensity exercise training elicits a more profound effect on the up-regulation of antioxidant enzymes than low-intensity exercise training, with the effects of both types of training confined to oxidative skeletal muscles(3).

By contrast, restriction of physical activity has been shown to compromise antioxidant defences, increasing the susceptibility of the skeletal muscle to oxidative stress. Thus, it seems clear that regular physical activity - at least 30 minutes 4-5 times per week - is required to maintain the antioxidant defence system(1), although its impact on non-enzymic antioxidants has not yet been fully elucidated.

Antioxidant supplementation
Findings from previous studies suggest that dietary antioxidants act to reduce lipid peroxidation and reduce skeletal muscle damage. A number of recent studies have reported a reduced oxidative stress during acute exercise following antioxidant supplementation(4). Although there is only limited support for the idea that such supplementation plays a direct role in enhancing performance, the consequent reduction in oxidative stress may result in a greater long-term training effect.

There is concern, however, that antioxidants can have an opposite pro-oxidant effect under certain conditions, particularly when consumed in large dosages, as sometimes happens with vitamin C. Although the evidence suggests that dietary supplementation of antioxidants may be advantageous to athletes participating in regular heavy exercise, definitive recommendations about the type and quantity of such supplementations cannot yet be made.

Dietary antioxidants
All endurance athletes might expect to benefit from a diet rich in antioxidant vitamins found in:

1 Green leafy vegetables, particularly broccoli, spinach and lettuce
1 Root vegetables including potatoes (if not overcooked), carrots and onions
1 A wide range of citrus fruits, including oranges, bananas and exotic fruits;
1 Vegetable oils, eggs, butter and wholegrain cereals.
It is worth pointing out, though, that a diet deficient in overall Calories is unlikely to maintain an adequate antioxidant defence.

Conclusion
Endurance exercise causes increased oxidative stress, which may be exacerbated if the exercise is irregular. Exercise training, however, has a positive effect upon antioxidant enzymes that act to reduce oxidative stress during exercise. While there is little definitive evidence to suggest that antioxidant supplementation is beneficial to athletic performance, it seems clear that dietary antioxidants can reduce the oxidative damage to muscles and other tissues caused by exercise. The long-term effects of antioxidant supplementation is not fully understood at present. But the evidence suggests that it may be beneficial for individuals performing regular heavy exercise.