So, you may have heard of oxidative stress, free radicals and antioxidants but what are they and what is their link to running?

Free Radicals

Free radicals are unstable molecules which contain an unpaired electron. In other words, they are highly reactive molecules. In this instance, we’re talking about the oxygen we breathe. Our body uses oxygen for metabolism, to release energy from mainly glucose and fat to fuel our muscles.

However, when oxygen is used like this, it has a tendency to generate free radicals which cause cellular damage to the body. Since we breathe up to 15 times more oxygen during sustained running, a lot more free radicals can accumulate. Free radicals are associated with oxidative stress and are also thought to contribute to muscle fatigue, cell damage and inflammation which can make it harder to run the following day. Luckily our body has ways to deal with the oxidative stress caused by free radicals.

Oxidative Stress

Oxidative stress caused by free radicals is associated with cellular damage that is thought to cause chronic disease and accelerated ageing over time. Since we all breathe oxygen, everyone is subject to oxidative stress but the amount is substantially higher for all endurance activities due to the higher volume of oxygen being taken in.

However, it’s not all bad news. Research has shown that oxidative stress is beneficial to us in small amounts. This is because it can prompt your cells to become stronger and more resistant to oxidative stress over time by increasing the number of endogenous (inside the body) antioxidant enzymes produced to deal with free radicals. It can be seen as one of the stressors of exercise that weakens your body, allowing you to adapt and become stronger and more resistant to oxidative stress.

Too much oxidative stress can have harmful implications on the body. This is especially caused by endurance training. We’re talking over 60 minutes of hard sustained exercise here so think long, tough endurance events such as marathons and ironmans. The number of free radicals generated is far more than the number of antioxidants our body can produce or use to handle the oxidative stress. Studies have shown that such high levels of oxidative stress can indicate an increased risk of heart disease and cancer.

Surprisingly, anaerobic exercise such as strength training and sprinting has been found to produce similar amounts of oxidative stress in endurance training. However, the total amount of free radicals that can be generated is far higher in endurance events as no athlete will sprint for 2 hours straight, but runners may run that long and elite cyclists will cycle even longer.

Long-term studies have shown, as opposed to short-term studies that extreme endurance training, can increase antioxidant levels, helping to prevent and repair oxidative damage. It is thought to cause the stress response that leads to health-physiological adaptations.

In one study, untrained people ran five times per week, for an hour at a time, at 80% of their maximum heart rate. At the beginning of the study, lipid peroxidation and oxidative stress increased. At the end of the study, twelve weeks later, the runners had less lipid peroxidation, higher antioxidant levels, and less oxidative stress than before the study. This shows just how much our body can adapt to the oxidative stress of exercise.

The amount of oxidative stress occurring in your body is not necessarily linked directly to the volume and intensity of exercise, but rather the duration and intensity of it in relation to your relative ability. So, as you train harder and longer, your body will become more resistant to oxidative stress (allowing you to train even harder and longer). Whilst a cyclist may train for 6 hours a day and have relatively low levels of oxidative stress, your average person could cycle for half an hour and have far high levels.


Antioxidants are substances that inhibit the oxidation, effectively countering the effects of free radicals and oxidative stress. There are two ways we can get antioxidants:

  • Endogenous antioxidants (from inside the body)
  • Exogenous antioxidants (from outside the body)

Many studies have demonstrated that a diet rich in antioxidants reduces oxidative stress and the risk of developing diseases to which it contributes to such as heart disease by reducing free radical damage to the artery walls. Foods rich in antioxidants are fruit and vegetables; dark coloured foods in particular such as blackberries and blueberries.

However, there are other sources of antioxidants such as nuts, milk and salmon. A good way to ensure you get plenty of antioxidants and nutrients is to ‘taste the rainbow’ with every meal. And no, I don’t mean eat skittles. What I mean is ensure that your plate has a range of colour in it from fruit and vegetables!

Endogenous antioxidants are produced within the body and more are produced when free radicals are created to help counteract their oxidative effects.

Potential Benefits of Antioxidants for Runners

Other than reducing oxidative stress, taking antioxidant-rich foods (typically brightly coloured) such as tart cherries can help improve performance in endurance events. Tart cherries typically contain the highest amount of anthocyanins, an antioxidant which functions as an anti-inflammatory in the body and can potentially combat the inflammation induced effects of exercise allowing you to run harder for longer before fatiguing. As well as reducing inflammation, it can also reduce the amount of muscle damage suffered from running, helping to facilitate a quicker recovery.

Less Exogenous Antioxidants for Improved Performance?

It can be beneficial to have little exogenous antioxidants from your diet as free radicals stimulate physiological changes that strengthen the body’s endogenous antioxidant defences, making the body itself more resistant to free radical damage in future workouts. Antioxidants in pill form or simply high levels of exogenous antioxidants can act as a ‘crutch’ that reduces these beneficial adaptations.

The Takeaway?

When building up to endurance exercise, try to build up the period of time you exercise for and intensity gradually, to allow your body to adapt to oxidative stress and avoid too much cellular damage.

Did you know about the link between running and oxidative stress? And will you be eating more foods rich in antioxidants? Let me know down below!