In short
Oxidative stress is an imbalance between the production of reactive oxygen species (ROS) — chemically reactive molecules containing oxygen — and the body's antioxidant defenses, resulting in damage to lipids, proteins, and DNA.
ROS, including superoxide and hydrogen peroxide, are generated continuously as byproducts of normal metabolism, primarily from mitochondrial oxidative phosphorylation, but also from immune cell activity and exposure to pollutants, radiation, and other environmental sources. At normal, low levels ROS are not purely destructive — they also function as signaling molecules in processes like immune defense and, per the hormesis principle, exercise adaptation.
The free radical theory of aging, proposed by Denham Harman in 1956, held that accumulated oxidative damage from ROS was the primary driver of aging. This theory has been substantially revised: numerous studies giving animals high-dose antioxidant supplements have failed to extend lifespan, and some have shortened it or blunted the beneficial adaptations to exercise by blocking the hormetic signaling role of ROS. The current view treats oxidative stress as one contributing factor among the hallmarks of aging, not a single root cause that antioxidant supplementation can simply neutralize.
Clinically, oxidative stress is measured indirectly through markers like F2-isoprostanes or oxidized LDL rather than directly, and chronic oxidative stress is associated with cardiovascular disease, neurodegeneration, and cancer in observational research. Large human RCTs of antioxidant vitamins (vitamin E, beta-carotene) for disease prevention have generally been disappointing or in some cases shown harm (e.g., increased lung cancer risk with beta-carotene in smokers in the ATBC and CARET trials), which is why this glossary treats “antioxidant” claims about a supplement with caution rather than as inherently protective.
Worth remembering
- Imbalance between reactive oxygen species and antioxidant defenses, damaging lipids, proteins, and DNA.
- The free radical theory of aging is now considered incomplete — antioxidant supplement trials have largely failed to extend lifespan.
- ROS also serve a signaling role, including in exercise adaptation, which blanket antioxidant supplementation can blunt.