Oxidative Stress Research: Beyond Free Radicals

Summary

Oxidative stress is an imbalance between reactive oxygen species production and antioxidant defence. The simple version — free radicals cause ageing, antioxidants prevent it — did not survive contact with large clinical trials. The current view treats ROS as necessary signalling molecules whose dysregulation, not mere presence, causes harm.

Last reviewed 2026-09-01

What it is

Reactive oxygen species include superoxide, hydrogen peroxide and hydroxyl radicals, generated mainly by mitochondrial electron transport and by NADPH oxidases. Oxidative stress is the state in which their production exceeds the capacity of enzymatic and non-enzymatic defences.

Denham Harman proposed the free radical theory of ageing in 1956. Between the 1990s and 2010s, large randomised trials of antioxidant vitamins failed to reduce mortality and in some cases increased it, forcing a reappraisal.

Endogenous ROS are unavoidable by-products of aerobic metabolism and are also produced deliberately by immune cells.

The failure of antioxidant supplementation is one of the clearest examples in biology of a plausible mechanism producing the wrong clinical prediction.

How it works

In plain terms

Reactive molecules are produced constantly as a by-product of using oxygen. They damage things if there are too many, but they are also how cells send certain messages — which is why mopping them all up backfires.

Technical detail

Superoxide from complexes I and III is dismutated to hydrogen peroxide by SOD1/SOD2 and reduced to water by catalase, glutathione peroxidase and peroxiredoxins. Hydrogen peroxide functions as a second messenger, reversibly oxidising cysteine residues on phosphatases and transcription factors, including the KEAP1-NRF2 axis that upregulates endogenous antioxidant genes. Chronic excess produces lipid peroxidation (measurable as malondialdehyde and F2-isoprostanes), protein carbonylation and 8-oxo-dG DNA lesions. Hormesis explains why exercise-induced ROS improve resilience while high-dose antioxidants blunt training adaptations.

Pathways involved

Mitochondrial complex I/III superoxide generation
SOD, catalase, glutathione peroxidase, peroxiredoxin defence
KEAP1-NRF2 antioxidant response element signalling
Redox signalling via reversible cysteine oxidation
Lipid peroxidation and protein carbonylation damage markers

Current research

Laboratory research

Redox signalling is well characterised biochemically; NRF2 pathway activation is a standard readout for adaptive antioxidant responses.

Animal research

Overexpressing antioxidant enzymes in mice generally does not extend lifespan, with catalase targeted to mitochondria a partial exception. Conversely, mice with elevated ROS do not reliably age faster — findings that undermined the strong version of the free radical theory.

Human research

Large randomised trials — including SELECT for vitamin E and selenium, the ATBC and CARET beta-carotene trials, and the Physicians' Health Study II — found no mortality benefit, and beta-carotene increased lung cancer incidence in smokers. Meta-analyses of antioxidant supplements have reported neutral or slightly adverse mortality effects. Exercise studies show high-dose vitamin C and E supplementation can blunt training adaptations.

Ongoing research

Interest has shifted from scavenging ROS to modulating redox signalling and supporting endogenous defence via NRF2 activators.

What is being investigated

Redox signalling in adaptation to exercise
NRF2 activation as an alternative to direct scavenging
Mitochondrially targeted antioxidants
Oxidative damage markers as ageing biomarkers
Hormesis: mild stress as a driver of resilience

These are research directions reported in the literature, not established effects or recommendations.

Risks, limitations and unknowns

Read this section before the rest

High-dose antioxidant supplementation has shown neutral-to-harmful outcomes in large trials
Beta-carotene increased lung cancer risk in smokers in two independent trials
Suppressing exercise-induced ROS can reduce training adaptations
Oxidative damage markers are unstandardised across laboratories
Marketing claims for antioxidant products routinely ignore the trial record

Evidence ratings

Laboratory studies

Strong

Redox biochemistry and signalling well established.

Animal studies

Moderate

Antioxidant overexpression generally does not extend lifespan.

Human supplement trials

Strong

Consistently negative for mortality benefit.

Redox-signalling approaches

Preliminary

Newer strategies lack outcome data.

References

[1]Free radical theory of ageing — reappraisals — PubMed, 2010–present
[2]Antioxidant supplement mortality meta-analyses — PubMed, 2007–present
[3]Exercise, ROS and adaptation — PubMed, 2009–present
Reference links open searches and records on PubMed and ClinicalTrials.gov so that every statement above can be traced to primary literature.

Frequently asked questions

What is oxidative stress?+

An imbalance in which reactive oxygen species production exceeds antioxidant defence capacity, leading to damage of lipids, proteins and DNA.

Do antioxidant supplements slow ageing?+

Large randomised trials found no mortality benefit, and some showed harm. The evidence does not support supplementation for longevity.

Why did the free radical theory fail?+

Because ROS are also essential signalling molecules. Removing them indiscriminately eliminates adaptive signalling along with damage.

Should I avoid vitamin C after exercise?+

Studies suggest high-dose vitamin C and E can blunt training adaptations. Dietary intake is a different matter from high-dose supplementation.

What is hormesis?+

The principle that mild, transient stress triggers adaptive responses that leave the system more resilient than before.

What is NRF2?+

A transcription factor that switches on endogenous antioxidant and detoxification genes when it detects oxidative stress.

How is oxidative stress measured?+

Through markers such as F2-isoprostanes, malondialdehyde, protein carbonyls and 8-oxo-dG, though standardisation across laboratories is poor.

Did beta-carotene really increase cancer risk?+

Yes — the ATBC and CARET trials both found increased lung cancer incidence in smokers receiving beta-carotene supplements.

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