What is oxidative stress, and how does it really affect ageing and energy?

"Oxidative stress" and "free radicals" appear on everything from face creams to fizzy vitamin tablets, usually with the implied promise that you can scrub them away. The reality is more interesting, and more honest. The term was coined in 1985 by the physiologist Helmut Sies, and the modern understanding is that oxidative stress is an imbalance between the production of reactive oxygen species and a biological system's ability to detoxify them. This page explains what that imbalance is, where mitochondria fit, and — crucially — where the science is genuinely uncertain.

This article sits inside our wider guide to cellular energy and healthy ageing, which is one branch of the bigger question, why am I always tired? If you have read about CoQ10 or antioxidants and wondered what they are actually defending against, oxidative stress is the concept underneath it all.

What exactly is oxidative stress, in plain language?

Oxidative stress is what happens when reactive molecules called free radicals outnumber the antioxidant defences meant to balance them. A free radical is simply a molecule with an unpaired electron, which makes it chemically "grabby" — it can react with and damage other molecules. Your body constantly makes them and constantly neutralises them; oxidative stress is the state where that balance tips.

Think of it as a household budget rather than a poison. Free radicals — more formally, reactive oxygen species (ROS) — are produced every second as a normal by-product of being alive. Against them stands a layered defence system: enzymes your body makes itself (like superoxide dismutase) plus antioxidants from food. According to a review in Oxidative Medicine and Cellular Longevity, oxidative stress is the imbalance between production and accumulation of reactive oxygen species and the ability of a biological system to detoxify these reactive products. The same review makes a point the marketing usually skips: when maintained at low or moderate levels, free radicals are of crucial importance to human health, acting as signalling molecules. The villain is chronic excess, not the molecules themselves. If the term itself is new, our glossary defines it briefly too.

Where do free radicals actually come from?

Most of your free radicals are made inside you, as an unavoidable side effect of turning food and oxygen into energy. The mitochondria — the tiny power plants inside cells that generate ATP, the molecule cells spend to do work — are the main internal source, because a small fraction of electrons "leak" during energy production and form reactive oxygen species.

The numbers are small but constant. Reviews of mitochondrial biology estimate that roughly 1 to 3% of the electrons passing through the mitochondrial electron transport chain leak out and produce superoxide, a primary free radical. That is the price of aerobic life: the same machinery that powers you also produces a steady trickle of reactive molecules. On top of this internal source, external factors add to the load — ultraviolet light, cigarette smoke, air pollution and certain toxins all generate reactive species, which is why a polluted, sun-heavy, urban Malaysian lifestyle is a reasonable thing to be mindful of without panicking about.

So the picture is not "free radicals invade from outside." The bulk are made internally, every moment, simply by living and producing energy. That is exactly why you cannot — and should not want to — eliminate them entirely.

How does oxidative stress link to ageing and energy?

The proposed link is intuitive: over a lifetime, free radicals can damage the fats, proteins and DNA inside cells — including the mitochondria that make energy — and this accumulated wear was long thought to drive ageing and a gradual drop in cellular vitality. Because mitochondria both produce reactive molecules and are damaged by them, the idea of a self-reinforcing decline is appealing.

This is the heart of the famous free-radical theory of ageing, first proposed by Denham Harman in the 1950s and later extended to point at mitochondria specifically. In its simplest telling, more free-radical damage equals faster ageing, and antioxidants should slow it down. It is a tidy story, and it is partly why antioxidants became a marketing fixture. The mechanism connecting oxidative stress to the felt experience of "less energy with age" runs through the mitochondria: if the energy-producing machinery accumulates damage, the same daily demands can feel heavier — which is the thread we follow in cellular energy and healthy ageing.

What this tends to mean in practice: oxidative stress is a real, measurable biological state, and supporting your body's own balance — through sleep, movement, not smoking, and a diet rich in whole plant foods — is sensible. But "sensible foundations" is a very different claim from "a pill that erases ageing," and the gap between them is where the next section lives.

Is the free-radical theory of ageing actually true?

Honestly, it is contested — far more than the supplement aisle implies. The classic free-radical theory is now widely regarded as an oversimplification, because much of its supporting evidence is correlative (free radicals are present where ageing happens) rather than proof that they cause ageing. Some of the strongest counter-evidence comes from the lab.

Two threads are worth knowing. First, animal experiments have produced awkward results for the simple theory. As Scientific American reported in 2009, researchers at McGill University disabled an antioxidant gene (sod-2) in worms and, instead of dying sooner, the worms lived about 30% longer despite higher oxidative stress — leading one researcher to argue that cell damage may be a product of ageing, not its cause. The same article noted bluntly that synthetic antioxidants have failed to show a clear longevity benefit in humans. Second, more recent reviews have rewritten the role of free radicals entirely: a 2020 update in Frontiers in Cell and Developmental Biology describes how low levels of mitochondrial stress may actually be protective and extend longevity, with reactive species acting as signals rather than pure damage — a concept called mitohormesis.

An association does not always mean causation. The simple fact that free radicals are present where ageing or disease occurs does not prove they cause it — a point a 2016 review in the British Journal of Pharmacology makes directly about the whole oxidative-stress theory of disease.

That same 2016 review in the British Journal of Pharmacology is worth sitting with: it points out that despite the theory's popularity, randomised clinical trials have failed to provide the evidence of efficacy required for drug approval, and in many conditions antioxidant supplements are ineffective or even harmful. None of this means oxidative stress is a myth — it is a genuine biological phenomenon. It means the neat "free radicals age you, antioxidants reverse it" narrative does not survive close contact with the evidence.

So do antioxidant supplements fix it?

For most people, no — taking high-dose antioxidant supplements has not been shown to prevent disease or slow ageing, and large doses can backfire. Major health agencies are unusually clear on this, which is striking given how heavily antioxidants are marketed. The contrast between antioxidant-rich foods and isolated antioxidant pills is the key nuance.

The US National Center for Complementary and Integrative Health (NCCIH) summarises decades of trials plainly: it states that large amounts of antioxidant supplements may interfere with important functions in the cell, including its own defence mechanisms, and that current evidence does not support antioxidant supplements for preventing disease. Tellingly, the same body notes that eating antioxidant-rich vegetables and fruit is linked with lower disease risk, while isolated supplement forms show minimal benefit — suggesting purified chemicals act differently from those eaten in whole foods.

What this tends to mean in practice: the cheapest and best-supported "antioxidant strategy" is not a bottle. It is the unglamorous foundation — a varied diet built around plants, not smoking, sensible sun habits, regular movement and adequate sleep — which supports your body's own balance without the risks of mega-dosing. Where a specific nutrient genuinely fits is an individual question, and we look at it more closely in antioxidants for energy and ageing: where they genuinely help versus where it is hype.

So the short version stays grounded: oxidative stress is real, it comes mostly from your own energy-making mitochondria, and a low level of it is normal and even useful. The dramatic "free radicals are ageing you and antioxidants reverse it" story is the part that does not hold up. Whether any of this is relevant to your energy depends on your age, your habits and your overall picture — exactly the kind of thing worth talking through rather than guessing from a label.

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