Free radicalReactive oxygen species · ROS

A free radical is a molecule with an unpaired electron, which makes it highly reactive as it seeks to pair that electron by taking one from something else. In biology the main ones are reactive oxygen species, produced continuously as a by-product of using oxygen to make energy.

They can damage lipids, proteins and DNA, and that damage accumulating over a lifetime became the free radical theory of ageing, one of the most influential ideas in the field and one that has aged less well than expected.

The complication is that they are also signals. Reactive oxygen species are used by the immune system to kill pathogens and by muscle to register that it has been trained, so a body that eliminated them entirely would be worse off, not better.

That explains the disappointing trials. Large studies of antioxidant supplements have generally failed to reduce disease, some have found harm, and high dose vitamin C and E around training can blunt the adaptations training is meant to produce.

The body's own defences, including glutathione and enzymes such as superoxide dismutase, are the main line, and regular exercise upregulates them. That is hormesis: a small repeated stress producing an adaptation larger than the insult.

The word survives mainly as a marketing device, attached to foods and supplements as though neutralising radicals were self evidently good in unlimited quantities.

In practice, get antioxidants from food, where they arrive in mixtures at modest doses alongside fibre and micronutrients, and do not take high dose supplements around training expecting a benefit.