Should endurance athletes use anti-oxidant supplements?Taking anti-oxidant supplements, such as 1000mg of vitamin C or 500IU of vitamin E, on a daily basis is common practice for many endurance athletes. Consuming functional foods containing high levels of polyphenols with anti-oxidant properties, such as cherry juice, green tea or pomegranate has also become popular in recent years. This practice is understandable, as it is well known that training promotes free radical activity in muscles, leading to oxidative stress which contributes to both muscular fatigue and muscle damage. Logically, supplementing the diet with additional anti-oxidant nutrients should help to reduce that fatigue and damage by ‘quenching’ the free radicals generated.

But is it necessary to supplement anti-oxidant nutrients and might they actually do more harm than good? This has been the subject of debate among research scientists for the last 10 years or so, and it remains highly controversial. While intense prolonged exercise has been shown to promote oxidative stress (ie disturbance in the pro-oxidant-anti-oxidant balance, leading to oxidative damage in cells), it is important to realise that exercise-induced oxidative stress is not only a short-lived event, but it is becoming clear that this occurrence of free radical production in muscle is required for training adaptations to take place in the muscle fibres. It has been hypothesized that reducing the ‘cellular stress’ of exercise sessions, by supplementing anti-oxidant nutrients, may inhibit these adaptations to training, such as improvements in oxidative energy metabolism and in the function of the body’s own anti-oxidant defence mechanisms.1

There have been many studies looking at the question of whether supplementing high doses of vitamin C and/or vitamin E has a beneficial effect on exercise performance and recovery, and no firm conclusions can be drawn as yet. Reviewing these studies is beyond the scope of this blog, so if you would like more details of these studies and their results, I suggest accessing these two papers: Nikolaidis et al (2012) 2 and Draeger et al (2014)3, both of which are freely available to view. Nikolaidis et al concludes that high doses of anti-oxidant vitamin supplementation cannot be recommended, particularly on a daily basis for a long period of time, while Draeger et al remarks that there is no consistent evidence that supplementation reduces oxidative stress and ensures better results in exercise. It’s worth noting that the Australian Institute of Sport has recently reclassified supplements for athletes, and both vitamins C and E and polyphenol-rich foods such as tart cherry juice are classified under ‘Class B: Deserving of further research and could be considered for provision to athletes under a research protocol or case-managed monitoring situation.’ See this website for more on the AIS classification scheme.

There is also insufficient evidence from studies to conclude that polyphenol supplementation can improve endurance performance. But trials of polyphenol supplementation, eg tart cherry juice, for use in recovery from muscle-damaging exercise protocols have been more promising. It is important to recognise that these benefits may not be related to the anti-oxidant activity of polyphenols, but to an alternative mechanism. There are also some indications that polyphenol supplementation is able to increase the capacity to quench free radicals, but it’s not yet clear that this holds any beneficial effects for athletes. 4

So, if science appears to be leaning away from high dose anti-oxidant supplements for athletes, what about anti-oxidants in food? Is this an excuse to avoid your fruit and vegetables due to their high vitamin C content? Absolutely not; your diet should be your main source of anti-oxidant nutrients which at the level found in food certainly have a positive role to play in health, particularly when it comes to having an effective immune system, but also to support your body’s natural anti-oxidant defences which require a certain amount of nutrients like vitamin C and E in order to be activated. So my recommendation would be to consume around 5-7 servings of colourful vegetables each day. Try three in a lunchtime salad, one at breakfast or as part of a snack, and two to three more cooked as part of your evening meal. In addition, eat 2-4 servings of fruit including some that are rich in vitamin C or polyphenols such as oranges, kiwi fruit and berries, with no more than one of these servings coming from a juice. Lastly, include some avocado, nuts or olive oil in your diet each day to provide your requirement for vitamin E.

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sportBased in London, she also works as a Registered Nutritional Therapist, conducting onetoone consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit



1 Powers SJ & Jackson MJ (2008). Exercise induced oxidative stress: cellular mechanisms and impact on muscle force production. Physiol Rev. 88: 1243-1276.

2 Nikolaidis et al. (2012). Does vitamin C and E supplementation impair the favourable adaptations of regular exercise? Oxid Med Cell Longev 707941.

3 Draeger et al (2014). Controversies of antioxidant supplementation in exercise: ergogenic or ergolytic effect in humans? J Int Soc Sports Nutr, 11(1):4.

4 Myburgh KH (2014). Polyphenol supplementation: benefits for exercise performance or oxidative stress? Sports Med. 44 (Supp 1): S57-70.