A free radical is any molecule with an unpaired electron in its outer orbital, making it highly unstable and reactive. The most biologically relevant reactive oxygen species (ROS) include superoxide anion (O2-), hydrogen peroxide (H2O2), and the hydroxyl radical (OH). These species are generated pri
Free Radicals and Reactive Oxygen Species: The Chemistry of Oxidative Damage
A free radical is any molecule with an unpaired electron in its outer orbital, making it highly unstable and reactive. The most biologically relevant reactive oxygen species (ROS) include superoxide anion (O2-), hydrogen peroxide (H2O2), and the hydroxyl radical (OH). These species are generated primarily as byproducts of mitochondrial oxidative phosphorylation — the process by which cells produce ATP. Estimates suggest that approximately 0.4-4% of oxygen consumed by mitochondria is partially reduced to superoxide rather than being fully reduced to water during the electron transport chain’s normal operation.
At physiological levels, ROS serve essential signalling functions. Hydrogen peroxide at low concentrations activates proliferative signalling pathways and supports the oxidative burst that neutrophils use to destroy pathogens. Exercise induces a temporary increase in ROS production that, paradoxically, triggers adaptive increases in antioxidant enzyme expression — a phenomenon termed hormesis, where a mild stressor produces beneficial adaptations. This is why a sedentary person has higher oxidative stress markers than a regularly exercising one, despite the latter producing more ROS per unit time. The body’s antioxidant system adapts to regular challenge; without challenge, the system remains underutilised and oxidative burden accumulates.
The Antioxidant Defence System: Enzymatic and Non-Enzymatic Components
The body has evolved a multi-layered antioxidant system designed to neutralise ROS before they can damage cellular structures. The enzymatic antioxidants — superoxide dismutase (SOD), catalase, and glutathione peroxidase — form the primary defence. SOD converts superoxide to hydrogen peroxide, which catalase then converts to water. Glutathione peroxidase performs a similar function but uses glutathione as the electron donor, converting hydrogen peroxide to water while oxidising glutathione in the process.
The glutathione system is particularly critical. Glutathione exists in both reduced (GSH) and oxidised (GSSG) forms, and the ratio of GSH to GSSG is a key determinant of cellular redox state. A low GSH/GSSG ratio indicates oxidative stress. Notably, glutathione levels decline with age — by some estimates by 30-40% between ages 20 and 65 — which may partially explain why age-related diseases are uniformly associated with oxidative damage markers. The implications for chronic disease are significant: as antioxidant capacity declines, ROS accumulation accelerates, damaging mitochondrial DNA, membrane lipids, and cellular proteins in a self-perpetuating cycle that compounds with each passing year.
Mitochondrial DNA: The Primary Target of Oxidative Damage
Mitochondrial DNA (mtDNA) is uniquely vulnerable to oxidative damage because it lacks the protective histones that shield nuclear DNA, is located in close proximity to the electron transport chain where ROS are generated, and has a limited capacity for DNA repair compared to nuclear DNA. Accumulating mtDNA mutations impair the function of electron transport chain complexes, reducing ATP production efficiency and generating even more ROS — creating a self-amplifying cycle of mitochondrial dysfunction.
This vicious cycle is considered a central mechanism of ageing and age-related disease. The mitochondrial theory of ageing proposes that the cumulative damage to mtDNA from oxidative stress is the primary driver of cellular ageing. This hypothesis is supported by observations that animals with genetically engineered enhanced antioxidant defences show extended lifespans in some studies, though the relationship remains debated. The accumulating evidence suggests that supporting mitochondrial function through targeted supplementation and lifestyle intervention may be more effective than attempting to neutralise ROS after they have formed.
Lifestyle Drivers of Oxidative Stress
Diet is among the most modifiable factors influencing oxidative stress levels. Polycyclic aromatic hydrocarbons (PAHs) generated by high-temperature cooking — particularly frying, grilling, and charring — are potent generators of oxidative stress. Advanced glycation end products (AGEs), formed when proteins or fats combine with sugars during cooking at high temperatures, activate the receptor for advanced glycation end products (RAGE) and trigger NF-kappaB-mediated inflammatory responses that generate further ROS.
Chronic alcohol consumption depletes glutathione stores in the liver and lungs while simultaneously generating large amounts of acetaldehyde, a highly reactive ROS precursor. Smoking introduces thousands of chemical compounds, including quinones and semiquinones, which generate superoxide and hydrogen peroxide in lung tissue. Environmental pollutants including particulate matter (PM2.5) similarly activate inflammatory pathways that increase oxidative burden throughout the body. The cumulative effect of these exposures, combined with age-related decline in antioxidant capacity, explains why oxidative stress markers track closely with chronic disease burden in population studies.
Polyphenols and the Dietary Strategy for Oxidative Stress Reduction
Dietary polyphenols represent the most practical nutritional lever for reducing oxidative stress in otherwise healthy individuals. Found in brightly coloured fruits, vegetables, tea, coffee, and dark chocolate, polyphenols function as antioxidants in the body by upregulating the Nrf2 pathway — the master regulator of antioxidant gene expression. Unlike direct antioxidant supplementation, which can interfere with the beneficial ROS signalling produced during exercise, polyphenol-induced Nrf2 activation enhances the body’s own antioxidant enzymes without suppressing essential ROS-mediated signalling. This is likely why whole-food polyphenol sources have shown more consistent health benefits than isolated antioxidant supplements in clinical trials.
The most practical dietary strategy is variety and colour: a minimum of five portions of fruits and vegetables daily, with emphasis on deeply pigmented varieties — berries, leafy greens, tomatoes, citrus, and onions — provides a broad spectrum of polyphenols sufficient to meaningfully support antioxidant capacity. Supplements cannot substitute for this foundation, but they can fill gaps in individuals whose dietary patterns consistently fall short of this target.
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