Oxidative stress

Oxidative stress describes a state in which reactive oxygen species outstrip antioxidant defences, leaving unrepaired damage to lipids, proteins and DNA. It accumulates with smoking, pollution, poor diet, sleep deprivation and chronic disease.

It matters because it is implicated in ageing and in the development of cardiovascular disease, neurodegeneration and cancer — though it is far more likely a contributing mechanism than a single cause. The free-radical theory of ageing, in its original strong form, has not held up.

In practice the transient oxidative stress of a hard workout is a stimulus, not a harm: it triggers the upregulation of the body's own antioxidant enzymes. The chronic, unrelieved version driven by lifestyle is the version worth reducing.