Free radicalReactive oxygen species · ROS

Free radicals are molecules carrying an unpaired electron, which makes them chemically aggressive — they stabilise themselves by taking an electron from whatever is nearby, damaging lipids, proteins and DNA in the process. Most are produced as a normal by-product of mitochondrial energy production.

It matters because they are not simply harmful. Reactive oxygen species also act as signalling molecules, and the ones generated during exercise are part of how the body knows to adapt — building more mitochondria and its own antioxidant defences in response.

In practice this is why the old advice to take large antioxidant doses around training has been revised: several trials found high-dose vitamin C and E supplementation blunted the training adaptations it was meant to protect. Antioxidants from food do not appear to carry the same problem.