In 1991, Helmut Sies published a paper that gave a name to something biologists had been circling for years. He defined oxidative stress as an imbalance between the production of reactive oxygen species and the biological capacity to detoxify them or repair the resulting damage. The definition was precise enough to be useful and broad enough to apply across virtually every disease and ageing process researchers cared to investigate.
That breadth is both the concept’s strength and its limitation. Oxidative stress appears in the literature on cancer, cardiovascular disease, neurodegeneration, diabetes, chronic inflammation, and ageing. It is connected to almost everything, which makes it difficult to talk about causation with confidence. What the research does establish clearly is that the balance between reactive oxygen species and antioxidant defences matters enormously, and that chronic disruption of that balance has measurable consequences at the cellular level.
What Reactive Oxygen Species Actually Are
Reactive oxygen species are molecules or ions containing oxygen that have an unpaired electron, making them highly reactive with surrounding molecules. The main species are superoxide (O₂⋅⁻), hydrogen peroxide (H₂O₂), and the hydroxyl radical (⋅OH). They are produced continuously as byproducts of mitochondrial energy production — the electron transport chain, which generates ATP, inevitably leaks some electrons onto oxygen, producing superoxide.
At low concentrations, these reactive species are not damaging. They are signals. Hydrogen peroxide in particular is a well-characterised signalling molecule that activates protective pathways including NRF2, regulates immune cell behaviour, and participates in wound healing. The distinction between signalling concentrations and damaging concentrations is real and important. Oxidative stress is not the presence of reactive species but their accumulation beyond what the cell’s regulatory and repair systems can manage.
What Chronic Oxidative Stress Does to Cells
When reactive species exceed the cell’s buffering capacity, they react with whatever is nearby. DNA is vulnerable: reactive species can modify the bases of DNA, causing mutations if the damage is not repaired before the cell divides. The 8-hydroxydeoxyguanosine (8-OHdG) modification is one of the most studied markers of oxidative DNA damage and is measurable in urine and blood. Elevated 8-OHdG is consistently associated with ageing, cancer, and a range of chronic diseases.
Proteins are also targets. Oxidative modification of protein side chains can change the protein’s shape and function. Carbonylation — the addition of a carbonyl group to an amino acid residue — is a common oxidative protein modification and is used as a biomarker of protein oxidative damage. Carbonylated proteins are harder to degrade by the proteasome, the cell’s protein recycling system, meaning they tend to accumulate. Accumulated, non-functional proteins are a feature of ageing tissue.
Cell membranes are targets too. The polyunsaturated fatty acids in membrane phospholipids are susceptible to lipid peroxidation, a chain reaction in which a reactive species removes a hydrogen atom from a fatty acid, creating a lipid radical that attacks adjacent fatty acids. The product, malondialdehyde (MDA), is another standard oxidative stress biomarker. Lipid peroxidation changes membrane fluidity and permeability, disrupting the cell’s ability to regulate what moves in and out.
What Tips the Balance
Several factors consistently push cells toward net oxidative stress.
Mitochondrial inefficiency with age increases electron leakage and therefore superoxide production. Simultaneously, the antioxidant defences regulated by NRF2 — glutathione, superoxide dismutase, catalase — decline as NRF2 pathway responsiveness falls. The net result is more reactive species being produced and less capacity to manage them.
Chronic psychological stress elevates cortisol and other hormones that increase ROS production. The effect is measurable: studies comparing chronically stressed populations with controls consistently find higher oxidative stress biomarkers in the stressed groups.
Environmental exposures impose oxidative burden directly. Cigarette smoke contains enormous concentrations of reactive species and also depletes glutathione. Air pollution particulates trigger inflammatory responses that generate reactive oxygen species in lung tissue and, via systemic inflammation, throughout the body. Heavy metals interfere with antioxidant enzymes directly.
Poor sleep is consistently associated with elevated oxidative markers. The brain, which has high metabolic activity and therefore high baseline ROS production, performs significant antioxidant clearance during sleep. Disrupting that window has measurable consequences.
What Remains Unknown
The relationship between oxidative stress and disease causation is more complicated than early research implied. For most conditions, it is not clear whether elevated oxidative stress is a primary driver, a secondary consequence, or both simultaneously. Antioxidant intervention trials have consistently been more disappointing than the mechanistic research predicted. Several large randomised trials found no benefit from antioxidant supplementation against cancer and cardiovascular disease, and a few found harm.
The most likely explanation is that systemic antioxidant supplementation is too blunt an instrument. It raises antioxidant levels everywhere indiscriminately, including in contexts where reactive species are performing useful signalling functions. A drug or intervention that could selectively reduce oxidative stress in specific tissues or specific cellular compartments — the mitochondria, for example — might produce different results. Several mitochondria-targeted antioxidants are in development, but their clinical profile is not yet established.
The question of whether oxidative stress biomarkers can reliably predict individual disease risk also remains unresolved. Population-level associations are robust, but individual variation is high and the biomarkers are difficult to standardise across laboratories.
Why It Matters
Oxidative stress is the mechanism through which many of the most common cellular insults — ageing, environmental toxins, chronic inflammation, poor sleep, inadequate physical activity — translate into cellular damage. Understanding it does not mean treating reactive oxygen species as enemies to be eliminated. It means understanding that cells operate within a balance, and that the goal is to support the systems that maintain that balance rather than to override them with high-dose supplements.
The lifestyle factors that consistently show up in longevity research — exercise, dietary quality, sleep, stress management — all influence oxidative balance through documented molecular mechanisms. The science is not yet complete, but the direction is clear enough to be practically useful.
Matt Elliott is the editor of Redox News Today, an independent publication covering peer-reviewed research on cellular health, redox signalling, and related biomedical science.
