Oxidative stress is a physiological loss of homeostasis in a biological system due to production of oxidizing species beyond the level that can be immediately salvaged or consequentially repaired. It can be manifest acutely or chronically and be a factor in disease.It is a normal physiological response and the association with disease processes does not make it necessarily a bad thing. For example the immune system uses production of reactive oxygen species to kill pathogens and some products of oxidization are important signalling molecules. The literature is full of studies in which the observation of evidence of oxidative stress in an illness has lead to interventions to reduce the observed oxidative stress intermediates with no clinical outcome improvement. This does not mean that oxidative stress is always good, and indeed it can be correlated with biological age of organisms, and disease processes as diverse as cataracts, atherosclerosis, Parkinson's disease, autoimmune disease, disorders of copper metabolism and iron metabolism such as aceruloplasminemia and Wilson's disease, toxin exposure, deficiency states (eg of selenium, scurvy) and cancer. There are multiple potential paradoxes related to the complexity of biological pathways and that the human organism has evolved to cope with oxidative stress, even with respect to specific issues with its biochemical pathways compared to other mammals. For example acute intermittent but repetitive low level oxidative stress may decrease biological age (eg ischaemic preconditioning and hormesis) and most other animals can synthesise ascorbic acid.
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