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You train hard so you could benefit from more antioxidant supplementation, right? Not necessarily it

seems, because according to Andrew Hamilton, some antioxidant supplements could actually harm your exercise performance…

There’s an old Russian proverb that says “life has to be lived forwards but can only be understood backwards”, and it’s a bit like that with sports nutrition; as research is carried out, scientists make recommendations based on their best current knowledge but over the years, this can and often does change – sometimes completely!

A classic example of this is the field of antioxidant nutrition research.

When scientists discovered that certain nutrients in the diet, such as vitamins 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 “What is free radical damage?”), there was a lot of excitement about the potential of these nutrients for human health and performance.

In particular, 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.

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Antioxidants and muscle damage

The early research into this area was rather mixed. Some studies on supplementing antioxidant nutrients had produced inconclusive results(1,2) whilst others had reported positive results including:

The early research into this area was rather mixed. Some studies on supplementing antioxidant nutrients had produced inconclusive results(1,2) whilst others had reported positive results including:

  • Reduced muscle soreness after shuttle running when taking vitamin C(3);
  • Reduced exercise-induced DNA damage in immune cells in women when taking vitamins C and E(4);
  • Enhanced muscle damage repair in older runners running downhill when taking vitamin.

Some of the research that followed however, was more convincing.

For example, a 2006 American study found that 3g per day of vitamin C supplementation significantly delayed the onset of post-exercise muscle soreness(6).

Meanwhile another study published around the same time observed that an antioxidant supplemented carbohydrate/protein drink given to cyclists reduced post-exercise muscle soreness and markers of muscle damage(7).

More recent studies have suggested that supplementing certain antioxidant nutrients can reduce exercise-induced muscle damage.

The latest research suggests that far from helping, some antioxidant supplements could actually harm your performance

These include N-acetyl cysteine (NAC) and alpha lipoic acid (ALA) in resistance trainers(8,9), methylsulfonylmethane (MSM) in runners (10), and coenzyme Q10 (CoQ10) in ultra-distance runners and climbers(11).

Given these positive findings on antioxidant supplementation, it’s only natural to ask whether all active people should rush out and stock up on antioxidants.

However, if it’s increased performance you’re after, you might want to hold fire, because the latest research suggests that far from helping, some antioxidant supplements could actually harm

your performance.

Antioxidant supplements and performance

There’s no doubting that antioxidant supplements can help reduce muscle damage.

But can your performance benefit from higher levels of antioxidants swilling round the body? This is where things get muddy because to date, the findings have been very mixed, with some recent studies showing that certain antioxidants not only fail to enhance performance but actually make it worse.

In one such study on the powerful sulphurcontaining antioxidant NAC, researchers studied its effects on metabolism during high-intensity interval exercise and self-paced 10-minute time trial performance in nine trained cyclists using a double-blind, randomised crossover trial (the most scientifically rigorous of the lot)(16).

The cyclists performed two separate trials, each consisting of 6 x 5-minute bouts of

intervals at 82% of their maximum power output, followed two minutes later by a 10-minute time trial during which they had to sustain the fastest pace possible.

For seven days before each trial, the cyclists took either an inert placebo pill or a NAC supplement and

the results were then compared. On the face of it, taking the NAC supplement

produced some positive changes in metabolism:

Firstly, the amount of fat utilised as fuel during the last two intervals of the interval session

increased significantly, suggesting more efficient fat burning.

Secondly, the amount of fatiguing blood lactate produced at the end of the time trial dropped by around 24%.

However, when it came to the nitty-gritty of time trial 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 with NAC, this fell to just 305 watts, which is a very significant drop.

NAC isn’t the only antioxidant supplement that’s under the spotlight for diminishing exercise performance. A 2013 study investigated the intake of vitamin C (1000mg), blackcurrant juice (containing 15mg of vitamin C, plus 300mg of natural anthocyanins and an inert placebo on training progression, incremental running test and 5km time-trial


Twenty-three trained female runners completed three blocks of highintensity training for three-and-a-half weeks, each separated by a “washout period” of just under four weeks. In each of these periods, they supplemented either with vitamin C, blackcurrant juice or the inert placebo drink.

The researchers monitored the runners’ changes in training and performance in each block (adjusting for each runner’s performance at the beginning of each training block).

The results showed that the vitamin C-supplemented block resulted in lower

average running speeds during training. Meanwhile, the effects of vitamin C and blackcurrant juice on performance in the incremental test and time trial were unclear.

However, the researchers noted a trend towards improved peak running speeds when the runners took the blackcurrant juice (a larger study would be needed to confirm this effect).

As a result of their findings, the researchers cautioned that while blackcurrant juice might enhance performance in elite athletes, they should not take large amounts of vitamin C routinely as it seems to diminish training adaptation.

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Can free radical damage be good?

If nutrients such as NAC and vitamin C are powerful antioxidants and can reduce exerciseinduced muscle damage, how is it possible they could harm your exercise performance? The first clue came from a 2012 study, which showed that antioxidants such as NAC might impair the body’s ability to adapt to a training stimulus such as a bout of cycling, because it interferes with the signalling pathways that enable muscles to switch on important genes required for improved aerobic metabolism(18).

Further evidence for this effect comes from a 2013 study, which looked at how NAC supplementation affected muscle performance and training adaptation – a measure of how efficiently and rapidly your muscles respond and adapt to training(19).

In this study, ten men performed 300 eccentric (muscle lengthening) contractions of the quadriceps muscle to deliberately produce muscle damage.

After this, they were given an inert placebo or 20mg per kilo of bodyweight of NAC.

In each trial, muscle performance was measured at baseline, immediately after exercise, two hours after exercise, and then daily for eight consecutive days. The results showed that supplementing with NAC blunted the release of some key signalling molecules that are involved in muscle adaptation.

These included a signalling molecule that is absolutely vital for muscle repair and growth called mTOR (see box “The importance of mTOR”), as well as important muscle growth factors.

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 recent studies have found that two more popular antioxidant supplements (quercitin and resveratrol) both seem to blunt the positive effects of exercise training – probably because they too interfere with important signalling pathways(20,21).

In the study on resveratrol, scientists discovered that supplementing with 250mg per day of resveratrol during a training programme produced negative effects: one) the subjects that took an inert placebo experienced a 45% greater increase in maximal oxygen uptake compared to the resveratrol group. Two) while the placebo group benefited from significantly lower blood pressure at the end of the eight weeks, there was no improvement whatsoever in the resveratrol group.

What these results suggest is that although free radical damage has many negative connotations in nutrition, it seems that Nature has designed our bodies to harness the activity of the 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. By taking supplements to artificially suppress free radical damage, you may also be suppressing your muscles’ ability to adapt to the training you put them through.

Summary and practical recommendations

Despite various health benefit claims, the jury is still very much out on antioxidant supplementation, especially for large doses of single nutrients. Not only are many of the health claims dubious, by suppressing free radical production, some of these antioxidants might impair the process of training adaptation, leading to worse exercise


In short, it now seems that we need some exercise induced free radical damage because our muscles harness it to adapt effectively to training stimuli. A much better way to boost your antioxidant intake is to consume more deeply coloured fruits and vegetables.

These are very rich in plant antioxidants, which have not been shown to harm performance – indeed, they may even enhance it (see Practical advice).

Practical Advice

  • Don’t take antioxidant supplements to enhance your performance, especially high doses of single nutrients such as NAC or vitamin C. It’s unlikely they’ll help performance and they may even harm it.
  • Juices rich in anthocyanins such as cherry, raspberry, blackberry, blackcurrant and strawberry, for example, are much better options as supplements; they appear to help reduce muscle damage and soreness without any detrimental effects on performance (indeed, they may even improve it).
  • Boost your antioxidant intake naturally by ensuring your daily diet contains plenty of brightly coloured fresh fruits and vegetables – at least five portions a day.