Antioxidant nutrition: your ‘best practice guidelines’ for maximum performance

The recommendations on antioxidant nutrition for endurance athletes are continually changing. Andrew Hamilton looks at the recent research and come up with some ‘best practice’ guidelines for enhancing performance

There’s an old Russian proverb that says ‘life has to be lived forwards but can only be understood backwards’. It’s a bit like that with sports nutrition and in particular antioxidants, where new knowledge has changed our understanding of earlier findings. In the beginning, antioxidant supplements could do no wrong. But then new evidence emerged suggesting that some antioxidants could be worse than useless and actually harm performance! It’s been over three years since we last looked at this topic and much has changed. In this article therefore, we’re going to explore some of the very latest research and see what it means for maximising your endurance performance.

Understanding antioxidants

Before we look at the latest research, let’s recap on the story so far. When scientists discovered that certain nutrients in the diet – vitamins such as A, C and E, minerals such as selenium and naturally occurring plant compounds known as ‘phytochemicals’ – had the ability to protect cells in the body from free radical damage (see box 1 and figure 1), there was a lot of excitement about the potential of these nutrients for human health.

But since free radicals and free radical damage occur as a result of using oxygen to produce energy in the body, it wasn’t long before scientists began to wonder whether athletes (who consume much larger amounts of oxygen to fuel their training) could benefit from high intakes of antioxidant nutrients. The thinking was simple: if exercise produces increased free radical damage in the body – for example in the muscles that are working during exercise – can an increased intake of antioxidants ameliorate that damage, helping to protect muscle tissue from excessive breakdown, thereby speeding recovery?


Free radical damage describes the damage that occurs within cells – for example to cell membranes and DNA (see figure 1) – at a molecular level as a result of ‘free radicals’. These free radicals are fleeting but extremely reactive chemical species, which unavoidably occur during oxygen metabolism when fats, proteins and carbohydrates are combined with oxygen in the body to produce energy (aerobic metabolism). For this reason they are sometimes called ‘reactive oxygen species’ (ROS) or ‘oxygen radicals’.

Although our cells have very efficient antioxidant defence systems to quench and neutralise harmful free radicals, these systems are not 100% efficient, and over time biochemical damage gradually accumulates, leading to a reduction in cellular function. Most scientists now believe that accumulated cellular free radical damage lies at the heart of the ageing process and many degenerative diseases such as cancer, autoimmune diseases and Alzheimer’s disease.

Athletes process and use larger volumes of oxygen and at higher rates than the majority of the population; this explains why many scientists have assumed that they may benefit from higher intakes of antioxidant nutrients to bolster defences. Unsurprisingly perhaps, there are now dozens of antioxidant supplements on the market, some of which are aimed squarely at endurance sportsmen and women such as cyclists, runners, swimmers, triathletes etc. The reasoning is that these types of athletes might need greater antioxidant support because during long training sessions, their bodies process a lot more oxygen and generate more free radicals than the average couch potato.


Schematic and simplified representation of free radical damage. The free radical is generated through normal oxygen metabolism – for example in the muscle during aerobic exercise. Missing one electron, this free radical is highly reactive and looking to ‘steal’ an electron from anywhere it can. It encounters a strand of DNA and steals an electron to become stable. However, removing the electron from the DNA molecule has resulted in damaged DNA, which now does not function as it should.

The rush to research

It was following the discovery back in 1985 that exercise increases free radical production in muscles, that the interest in antioxidant nutrition ballooned. Over the next 30 years, the rate of research into exercise and antioxidants would increase 27-fold (see figure 2)! And with such a huge and growing amount of research, some of the initial assumptions about the role of antioxidants in human health and exercise were overturned.


Annual rates of publications on exercise and antioxidants. Bars depict numbers of reports published in individual years; arrow highlights the year exercise was shown to increase free radical content in muscle.

The early and middle years

The early research into muscle damage and antioxidants this area was rather mixed. Some studies on supplementing antioxidants in this nutrients had produced inconclusive results1 2, while others had reported positive results3 4 5. Some of the research that followed however was more convincing. For example, a 2006 American study on vitamin C supplementation and delayed onset muscle soreness (DOMS), found that taking three grams of vitamin C per day (1g morning, noon and night) for two weeks prior to a heavy exercise session and for four days afterwards, significantly reduced soreness6. Also, the subjects experienced less muscle breakdown and had lower markers of free radical damage. Another study published around the same time found that an antioxidant-supplemented drink reduced post-exercise muscle soreness and markers of muscle damage following cycling exercise at a moderate pace (70% VO2max) to exhaustion at and then again 24 hours later at slightly harder pace (80% VO2max)7.

In more recent years, a number of other studies on supplementing antioxidant nutrients in sportsmen and women have shown quite positive results in terms of reducing markers of muscle damage. For example, a study on the effects of cysteine (an amino acid building block that contains the element sulphur, which gives it powerful antioxidant activity) and the cysteine-related antioxidants taurine, alpha lipoic acid and N-acetyl cysteine (NAC) found that they boosted the activity of protective antioxidant enzymes in the body and lowered markers of free radical damage8. The protective effect of alpha lipoic acid was also confirmed by a more recent study, which found taking alpha lipoic acid, significantly reduced muscle DNA damage after resistance training9.

Other antioxidant supplements that have been studied recently and have been shown to help reduce markers of free radical damage following exercise include methylsulfonylmethane (often abbreviated to MSM) in runners 10, coenzyme Q10 (CoQ10) in ultra-distance runners and climbers11 and astaxanthin (a naturally occurring pinky-coloured compound found in yeast some fish/shellfish and the feathers of some birds) in elite young footballers12.

Antioxidants & performance

There’s no doubting that the antioxidant supplements discussed above can help reduce muscle damage in athletes. The problem is that very little if any evidence had emerged showing any performance benefits. Indeed, the next step in this story came when in 2013, when two studies were published showing that certain antioxidants not only failed to enhance endurance performance, but actually made it worse!

In one of these studies, researchers investigated the performance effects of a powerful sulphur-containing antioxidant called N-acetyl cysteine (NAC) during high-intensity interval exercise and selfpaced 10-minute time-trial performance in nine trained cyclists13. On the face of it, taking the NAC supplement produced some positive changes in metabolism; the amount of fat utilised as fuel during the last two intervals of the interval session increased, and the amount of fatiguing blood lactate produced at the end of the time trial dropped by around 24% (see figure 3). But when it came to actual performance, the NAC supplementation wasn’t just ineffective – it actually harmed performance. When they had supplemented with the placebo, the cyclists managed to maintain an average power output of 319 watts. But when they had supplemented with the NAC, this fell to just 305 watts – a very significant drop (see figure 3 also).


Left: blood lactate at the end of the 10-minute time trial fell when the cyclists supplemented with NAC (normally a good thing). Right: Average power output during the time trial fell by 4.9% when the cyclists took NAC (a bad thing!).

In the second study, Twenty-three trained female runners completed three blocks of high-intensity training for 3.5 weeks, each separated by a ‘washout period’ of just under four weeks. In each of these periods, they supplemented either with vitamin C (1000mgs), blackcurrant juice or an inert placebo drink14. The results showed that the vitamin C-supplemented block resulted in lower average running speeds during training, while the blackcurrant-supplemented block showed a slight trend to faster speeds. The researchers cautioned that athletes should not take large amounts of vitamin C routinely as it seems to diminish training adaptation.

Is it time to love free radicals?

Around the same time as the above studies, two further studies showed why taking large amounts of antioxidants might harm performance. A 2012 study showed that supplementing NAC seemed to impair the body’s ability to adapt to a training stimulus such as a bout of cycling because it interfered with the signalling pathways that enable muscles to switch on important genes required for improved aerobic metabolism15.

A second study looked at how NAC supplementation affected muscle performance16. It also looked at the effects of NAC on a process known as ‘redoxsensitive signalling’ during the process of inflammation and repair following exerciseinduced muscle damage. This process is very important because it underpins what we know as ‘training adaptation’ – the ability of the muscles to rebuild and adapt to the exercise demands placed upon them, which makes them better able to cope with future bouts of exercise.

In this study, subjects performed 300 eccentric contractions of the quadriceps muscle of the frontal thigh to deliberately produce muscle damage (and soreness!). After this, they were given an inert placebo or 20mg per kilo of bodyweight of NAC. The good news was that supplementing NAC did help reduce muscle damage following the exercise. The bad news was that the NAC supplements also blunted the release of some key signalling molecules that are involved in muscle adaptation. These included a signalling molecule called ‘mTOR’, which is absolutely vital for muscle repair and growth (see figure 4).

The net result was that after eight days, recovery was only complete when the men had taken the inert placebo; when they took NAC, their recovery was impaired so that even after eight days, they still hadn’t fully regained their original strength capacity! Likewise, other studies found that other popular antioxidant supplements such as quercitin and resveratrol blunted the positive effects of exercise training – probably because they too interfere with important signalling pathways17 18.

Although free radicals and free radical damage have become dirty phrases in nutrition, it seems that Nature in her infinite wisdom has actually designed our bodies to harness the activity of these free radicals produced during exercise in order to switch on the production of molecules that orchestrate the process of muscle repair and adaptation – both in terms of strength and endurance. In other words, it now seems that we need some exerciseinduced free radical damage because without it, our muscles are unable to adapt effectively to training stimuli. But does this mean it’s time to ditch all your antioxidant supplements? Well, some of the most recent research suggests that we might be able to have our antioxidant cake and eat it!


By interfering excessively with free radical production in exercising muscles, NAC blunts the release of signalling proteins involved in training adaptation (eg mTOR). With reduced signalling, the synthesis of mitochondria (the muscle cells’ energy factories) and the formation of new blood capillaries (angiogenesis) is reduced, which means training adaptation is lessened – ie fitness gains following a training session are diminished.

New frontiers

In recent years, there’s been a flurry of research into whether fruits and fruit extracts containing high levels of phytochemicals (natural plant compounds that can help protect against free radical damage in the body) can offer an alternative route for antioxidant protection in athletes. Scientists believe that it’s the high levels of various natural phytochemical compounds found particularly (but not exclusively) in berries and cherries that make these fruits function as powerful antioxidants (see figure 5). The same compounds are also thought to act as natural anti-inflammatories in the body, which explains the reduced level of post exercise soreness reported in some studies.


There’s certainly no doubting the natural antioxidant capacity of these fruits. For example, Spanish researchers found that consuming an antioxidant-rich beverage containing black grape (81 grams per litre [g/L]), raspberry (93g/L) and redcurrant (39g/L) concentrates prior to 90 minutes of cycling resulted in no additional DNA damage; this compared with a 21% increase in damage when the fruit extracts weren’t taken19. Meanwhile, a study on rowers consuming chokeberry juice (rich in anthocyanins) showed that the juice produced a significant drop in the measures of free radical damaged induced by strenuous rowing exercise20. Similarly, studies on marathon runners and road cyclists have shown that consuming antioxidant-rich tart cherry juice prior to exercise reduces measures of muscle damage, and the amount of post exercise soreness21 22 23. But what makes these fruit extracts especially interesting is that none of them have be shown to impact on training adaptation and performance. Indeed, the evidence is that as well as providing protection, they can enhance performance too.

In a study last year, researchers looked at half-marathon performance in runners and triathletes who consumed a powdered tart cherry supplement24. Twenty-seven subjects were split into two groups; subjects in one group were given a daily capsule containing 480mgs of powdered tart cherries [CherryPURE®] for 10 days before a halfmarathon race and for two days afterwards. The subjects in the other group followed exactly the same supplementation and race schedule but instead were given an inert placebo capsule containing nothing more than rice flour. The key finding was that the athletes in the tart cherry averaged 13% faster times in the halfmarathon race (see figure 6). Even better, the tart cherry-supplemented athletes reported lower levels of post-exercise muscle soreness before the race, and experienced more rapid recovery from soreness after the race.


The tart cherry runners not only achieved faster times, but they also maintained their projected race pace far better than the runners who took the placebo.

Cherries aren’t the only fruit under the spotlight. In New Zealand, scientists have been researching the effects of blackcurrant extract on endurance performance. Blackcurrants are very rich in natural antioxidants called anthocyanins, which known to influence increase peripheral blood flow. Researchers wondered therefore if (by stimulating blood flow through exercising muscles) concentrated blackcurrant extracts could enhance performance.

In an initial study, research gave cyclists six grams per day of Sujon New Zealand blackcurrant powder to experienced triathletes for seven days, after which the subjects performed a cycling test at differing intensities, during which measurements were taken25. The key finding was that when the triathletes took the blackcurrant extract, blood lactate (a measure of physiological fatigue) was lower at 40%, 50%, 60% and 70% of maximum power by 27%, 22%, 17% and 13% respectively. They also found that at rest, blackcurrant increased stroke volume and cardiac output by 25% and 26%, and decreased peripheral blood flow resistance by 16%, with no changes in blood pressure and heart rate.

With the findings of reduced lactate and improved blood flow, the obvious question was whether blackcurrant extract could enhance endurance performance. Using a double-blind, crossover design (scientifically rigorous), 14 cyclists took either a concentrated blackcurrant extract rich in anthocyanins or an inert placebo for seven days. On day seven, participants performed 30 minutes of cycling (3 x 10 minutes at 45, 55 and 65 % VO2max), followed by a 16.1 km flat-out timetrial, with lactate sampling during a 20-min passive recovery. The results showed that the blackcurrant extract not only increased fat oxidation at 65 % VO2max by 27 %, it also significantly and improved the 16.1 km time-trial performance by 2.4% (see figure 7).


In a follow up study, the same researchers tested the effects of a highly concentrated blackcurrant extract on high-intensity running performance26. Thirteen fit men performed a treadmill-running protocol to exhaustion. This consisted of stages containing 6 x 19 seconds of sprints with 15 seconds of lowintensity running between sprints. The inter-stage rest time was 1 minute and stages were repeated with increasing sprint speeds. The distance each subject could achieve before exhaustion set in was recorded. The key finding was that when taking the blackcurrant extract, the subjects were able to run 10.6% further than when taking the placebo (figure 8).


NAC alternative?

Why is it that high-dose antioxidants in tablet form (such as NAC and vitamins C and E) seem to interfere with training adaptation, while those from fruit (and vegetable) extracts don’t appear to have this unwanted effect? One theory is that high levels of external single antioxidants upset the delicate internal antioxidant systems that exist within cells.

In very simple terms, when you exercise, your own internal antioxidant defences are boosted – regardless of any antioxidants you consume in the diet. These internal defences are finely tuned to remove free radicals that become damaging to cells, but to allow some radical activity in order to activate signalling pathways and promote training adaptation. But if these delicate systems are flooded with high levels of external antioxidant – for example, vitamins C or E – they can no longer function efficiently.

In a paper published last year, the scientists from New Zealand proposed a mechanism for this effect27. Instead, they recommend consuming antioxidants that, rather than acting externally and in a sledgehammer-like fashion, are able to enhance the body’s internal antioxidant defences. So for example, rather than taking NAC (an ‘external’ source of cysteine which is rapidly released), they suggest consuming proteins naturally very rich in cysteine, which when metabolised by the body, should in theory enhance the naturally produced and ‘internal’ antioxidant called glutathione. One possible protein is hydrolysed keratin, which is very rich in cysteine (hydrolysing is required to make the keratin digestible). At the moment however, this is just a theory and we await further research!


Although touted as providing health benefits, antioxidant supplementation does not guarantee improved endurance performance. In fact recent evidence suggests the contrary; that many antioxidant supplements – especially large doses of single nutrients – may harm performance by impairing the process of training adaptation. The good news is that very recent research suggests that natural sources of antioxidants from fruits and vegetables do not have this drawback – indeed, they can actually enhance your performance. With that in mind, here are some best practice recommendations:

  • Don’t take single high-dose antioxidant supplements such as NAC, vitamin C, vitamin E, quercitin, resveratrol etc. They may harm your training adaptation and reduce performance.
  • The latest evidence suggests that fruit extracts (concentrates, powders etc) rich in anthocyanins such as cherry, raspberry, blackberry, blackcurrant, strawberry etc are much better options as supplements. They help reduce muscle damage and soreness without any detrimental effects on performance. And in the case of tart cherry and blackcurrant extract, they may well improve it.
  • Although concentrated vegetable extracts (for example ‘super greens’ powders) haven’t yet been investigated in research studies on athletes, there may well be health and possibly performance benefits; studies have shown that concentrated vegetable powders can improve survival rates and health outcomes in patients undergoing surgery and improve exercise capacity in obese subjects28 29.
  • Before you consider using fruit or vegetable extracts/concentrates, boost your natural antioxidant intake naturally by ensuring your daily diet contains plenty of brightly coloured fresh fruits and vegetables. You should aim to consume a minimum of five portions a day. Choose as wide a variety of colours from each of the colour groups (see figure 9). That’s because different colours of fruit/ vegetable tend to be rich in different antioxidants; combining them will likely lead to greater overall health benefits.
  • Given the importance of the glutathione antioxidant system in the body, ensuring you consume plenty of foods rich in cysteine is recommended. These include: eggs, cottage cheese, poultry (eg chicken, duck etc), yoghurt, oats and soybeans.


See also:


  1. Eur J Appl Physiol 2004, 92(1-2): 133-8
  2. Int J Sports Med 2002, 23(1): 10-5
  3. Int J Sport Nutr Exerc Metab 2001, 11(4): 466-81
  4. Free Radic Biol Med 2004, 36(8): 966-75
  5. Am J Physiol 1990;259:R1214–9
  6. Int J of Sport Nutr Exerc Metab 2006, 16;270-280
  7. Med Sci Sports Exerc. 2006 Sep;38(9):1608-16
  8. J Physiol Sci. 2007 Dec;57(6):343-8
  9. Med Sci Sports Exerc. 2013 Aug;45(8):1469-77
  10. J Sports Med Phys Fitness. 2012 Apr;52(2):170-4
  11. Eur J Nutr. 2012 Oct;51(7):791-9
  12. J Sports Med Phys Fitness. 2012 Aug;52(4):382-92
  13. Appl Physiol Nutr Metab. 2013 Dec;38(12):1217-27
  14. Eur J Sport Sci. 2014 Volume 14 Issue 2 160-168
  15. Acta Physiol (Oxf). 2012 Mar;204(3):382-92)
  16. Am J Clin Nutr. 2013 Jul;98(1):233-45
  17. Scand J Med Sci Sports. 2013 Oct 14. doi: 10.1111/sms.12136
  18. J Physiol. 2013 Oct 15;591(Pt 20):5047-59
  19. Eur J Appl Physiol. 2005 Dec;95(5-6):543-9
  20. Int J Sport Nutr Exerc Metab, 15(1): 48-58, 2005
  21. J Int Soc Sports Nutr. 2010 May 7;7(1):17
  22. Nutrients. 2014;6(2):829–43
  23. Scand J Med Sci Sports. 2010;20(6):843–52
  24. J Int Soc Sports Nutr. 2016 May 26;13:22
  25. Int J Sport Nutr Exerc Metab. 2015 Aug;25(4):367-74
  26. Int J Sport Nutr Exerc Metab. 2015 Oct;25(5):487-93
  27. J Int Soc Sports Nutrn (2017) 14:12 DOI 10.1186/s12970017-0168-9
  28. Clin Nutr. 2017 Aug 10. pii: S0261-5614(17)30272-8
  29. Br J Nutr. 2013 Nov 14;110(9):1685-95
Share this

Follow us