Oxidative stress is an imbalance between reactive oxygen species and the antioxidant systems that neutralise them, which leaves proteins, fats and DNA damaged. Smoking, alcohol, pollution, sunlight, poor sleep and high blood sugar all push the balance. Food lowers it. Antioxidant supplements, in randomised trials, did not.
Every article on this subject ends the same way. Eat more antioxidants. It sounds obvious, and it is exactly what the randomised evidence stopped supporting almost twenty years ago.
The chemistry is real. Free radicals do damage tissue, and that damage does show up in heart disease, diabetes, neurodegeneration and skin ageing. What has changed is the conclusion drawn from it. Reactive oxygen species turn out to be signals as well as wreckage, and flooding the body with vitamins to erase them has produced some of the more uncomfortable results in nutrition research.
What is oxidative stress, exactly?
Burning fuel produces exhaust. Your mitochondria burn glucose and fat with oxygen to make ATP, and a small fraction of that oxygen escapes the chain partly reduced. The result is a family of molecules called reactive oxygen species: superoxide, hydrogen peroxide, the hydroxyl radical.
Many of them are free radicals, meaning they carry an unpaired electron and will take one from whatever they touch. A membrane lipid. A protein. A base in your DNA. Each theft creates a new radical, so one reaction becomes a chain.
Against that you run a defence: superoxide dismutase, catalase, glutathione peroxidase, plus dietary vitamins C and E and thousands of plant polyphenols. Some compounds sit inside that system rather than beside it, alpha lipoic acid among them, because they regenerate the other antioxidants instead of only scavenging radicals themselves. Oxidative stress is not the presence of radicals. It is the point where production outruns that defence, and the damage starts accumulating faster than repair clears it.
That imbalance is measurable in a laboratory. Oxidised DNA bases, oxidised lipids and carbonylated proteins all rise. It is also where oxidative stress meets inflammation, because damaged molecules act as danger signals and recruit immune cells, which then produce more oxidants themselves. The two processes feed each other, which is why an approach aimed at the inflammatory side of that loop is often discussed alongside antioxidant intake.
What is the main cause of oxidative stress?
There is no single cause, and that is the honest answer to the most asked question on this topic. Ordinary metabolism produces oxidants every second you are alive. What varies is how much extra load you add on top, and how well your defences are fed.
WHAT ADDS TO THE OXIDANT LOAD
stress
Tobacco smoke
Each puff delivers a large dose of free radicals directly to lung tissue, and depletes circulating vitamin C.
Alcohol
Ethanol metabolism generates acetaldehyde and oxidants in the liver, and drains glutathione.
Air pollution
Fine particulates carry transition metals that catalyse radical formation once inhaled.
Ultraviolet light
Sunlight generates oxidants directly in skin, which is why photoageing looks different from chronological ageing.
High blood sugar
Persistent hyperglycaemia drives glycation and mitochondrial superoxide production.
Short sleep
Curtailed sleep raises markers of oxidation and blunts the overnight repair window.
What does oxidative stress feel like, and can you be tested?
It does not feel like anything. There is no symptom that belongs to oxidative stress and to nothing else, which makes the question one of the most common searches on the subject and one of the least well answered.
Fatigue, brain fog, aching muscles, grey skin and slow healing all get attributed to it online. Each of those has a dozen more likely explanations, several of them treatable and one or two of them serious. If you feel persistently unwell, that warrants a GP appointment and blood tests, not an antioxidant.
Nor can you test for it in any routine way. Research laboratories measure 8-hydroxy-2-deoxyguanosine for DNA oxidation, F2-isoprostanes for lipid oxidation, and the ratio of reduced to oxidised glutathione. All three are research instruments. None is available on the NHS, none has a validated threshold that says treat this, and the commercial panels sold direct to consumers have no reference range you could act on. A direct-to-consumer telomere test hits the same wall: ten laboratories measured the same blinded DNA and agreed on the ranking while disagreeing on the number.
The practical answer is uncomfortable and correct. You cannot know your oxidative stress level, so the sensible target is the exposures that raise it, which you can name without a test.
Free radicals are also messengers, not only damage
The picture of radicals as pure vandalism dates from the 1950s and has not survived. Sies and Jones set out the modern view: reactive oxygen species act as signalling molecules across normal physiology, with hydrogen peroxide behaving much like a hormone inside the cell.
They control how cells sense oxygen, how they decide to divide, how immune cells kill bacteria, and how muscle responds to a hard session. Remove them completely and you do not get a healthier cell. You get a deaf one.
This is why the field moved from oxidative stress to redox biology, and why a small transient rise in oxidants can be beneficial. The term is hormesis: a dose that would harm in bulk triggers an adaptive response in small amounts. Exercise is the cleanest example. It raises oxidant production sharply, and the body answers by building more of its own antioxidant enzymes.
What happened when antioxidants were tested properly
If oxidative damage causes disease, supplementing antioxidants should reduce disease. That hypothesis has been tested more thoroughly than almost any other in nutrition, and the result is not the one the supplement aisle implies.
Bjelakovic and colleagues pooled 68 randomised trials covering 232 606 people who had taken beta-carotene, vitamin A, vitamin C, vitamin E or selenium against placebo. Across the 47 trials at low risk of bias, involving 180 938 participants, the supplements increased all-cause mortality.
Relative risk of death with antioxidant supplements against placebo, across 47 low-bias randomised trials and 180 938 participants. Vitamin A reached 1.16 and beta-carotene 1.07. Vitamin C and selenium showed no significant effect.
Bjelakovic et al., JAMA, 2007. DOI: 10.1001/jama.297.8.842
A 5 per cent relative increase is small in absolute terms, and the meta-analysis has been argued over ever since. What nobody has produced is the opposite result: a large, well-run trial in which antioxidant pills lowered mortality. Two decades on, that trial still does not exist.
Read that alongside the beta-carotene story and a pattern appears. The compounds that look protective in people who eat them in food behave differently when isolated, purified and given at ten times the dietary dose. It is also why the USDA withdrew its league table of antioxidant values in 2012, on the grounds that a figure measured in a cuvette predicted nothing about a person.
The training result that changed how researchers think
The most instructive experiments were not about disease at all. They were about people getting fitter.
Ristow and colleagues gave 39 healthy young men either 1 000 mg of vitamin C with 400 IU of vitamin E, or nothing, alongside four weeks of training. Insulin sensitivity improved in the men who trained without the vitamins. In the supplemented group it did not. The genes that carry the adaptation, including PGC-1 alpha and the cell’s own superoxide dismutase and glutathione peroxidase, were switched on only in the unsupplemented men.
Paulsen and colleagues repeated the idea over eleven weeks in 54 men and women running three to four sessions a week. Maximal oxygen uptake rose by 8 per cent in both groups, so the supplement did not slow performance. But the mitochondrial markers rose only in the placebo group. COX4 climbed 59 per cent there and fell slightly on vitamins.
The signal your muscle uses to build new mitochondria is the oxidant burst you produce while training. Mop it up with a megadose and the message never arrives. This is the same reason antioxidant loading is now questioned around tendon and connective tissue repair, where controlled loading and the local response it provokes matter more than any capsule, and where what you take alongside rehabilitation deserves the same scrutiny.
How can you reduce oxidative stress?
Everything that works is either free or edible. Nothing on this list is a supplement, and the order reflects the size of the evidence behind each one.
WHERE THE EVIDENCE ACTUALLY POINTS
Stop smoking
The single largest oxidant exposure most people ever have, and the only one you can remove completely. NHS stop smoking services roughly triple the quit rate.
Eat the plants, not the extracts
Aune and colleagues pooled 95 studies: risk of death kept falling up to 800 g of fruit and vegetables a day, which is around ten portions.
Train regularly, and let it hurt a little
Exercise raises oxidants acutely and builds your own antioxidant enzymes chronically. This is the adaptation the vitamin trials blocked.
Sleep the full night
Repair and clearance run overnight. Short sleep raises oxidation markers and is the easiest driver to underestimate.
Cut alcohol and cover up in strong sun
Both are direct, dose-dependent oxidant loads on the liver and the skin respectively.
Treat the underlying condition
Uncontrolled diabetes, untreated sleep apnoea and chronic infection generate more oxidative load than any diet can offset.
Why does the plant work when the pill does not? Probably because a tomato is not a dose of lycopene. It is several hundred compounds in small quantities, arriving with fibre, in a matrix that releases them slowly, in a meal, at a concentration your cells evolved alongside. A 500 mg tablet is none of those things.
| Approach | What the evidence shows | Verdict |
|---|---|---|
| Fruit and vegetables, 800 g a day | Dose-response fall in cardiovascular disease and all-cause mortality across 95 studies | Strongest single move |
| Regular exercise | Raises endogenous antioxidant enzymes, improves insulin sensitivity | Works, and free |
| Stopping smoking | Removes the largest avoidable oxidant exposure | Non-negotiable |
| High-dose vitamin E or beta-carotene | Increased all-cause mortality in low-bias randomised trials | Avoid |
| High-dose vitamin C plus E around training | Blunted mitochondrial and insulin adaptations in two trials | Counterproductive |
| Consumer oxidative stress test kits | No validated threshold, no NHS equivalent | Not actionable |
How long does it take to recover from oxidative stress?
Different layers move at different speeds, which is why no single number answers this.
Oxidised lipids in your blood turn over in hours to days, and change with a single meal. Circulating antioxidant capacity shifts within weeks of eating differently. Enzymatic defences, the ones exercise trains, take four to twelve weeks to rise measurably, which is roughly the length of the training studies above.
Accumulated damage is another matter. Oxidised proteins that cross-link, damaged mitochondrial DNA and photoageing in skin do not reverse on any timescale you can plan around. The realistic goal is to stop adding to the pile, not to empty it.
Which brings the whole subject back to something unglamorous. Sleep, greens, movement, no cigarettes, less alcohol. The chemistry is genuinely complicated. What to do about it turns out not to be.
Frequently asked questions
How can I reduce oxidative stress?
Stop smoking, eat a lot of plants, train regularly, sleep a full night and keep alcohol low. Aune and colleagues found risk of death still falling at 800 g of fruit and vegetables a day. High-dose antioxidant supplements are the one approach the randomised trials do not support.
How do you know if you have oxidative stress?
You cannot, in any routine way. The laboratory markers, 8-hydroxy-2-deoxyguanosine, F2-isoprostanes and the glutathione ratio, are research assays with no clinical threshold and no NHS test. Judge your exposures instead: tobacco, alcohol, sun, short sleep, uncontrolled blood sugar.
What does oxidative stress feel like?
It has no symptoms of its own. Fatigue, brain fog and slow healing are commonly blamed on it, but each has more likely explanations, including anaemia, thyroid disease, depression and sleep apnoea. Persistent symptoms belong with your GP rather than a supplement.
What is the main cause of oxidative stress?
Normal metabolism, which you cannot avoid, plus whatever you add on top. Tobacco smoke is the largest avoidable source, followed by alcohol, air pollution, ultraviolet light, persistently high blood sugar and short sleep. Most people carry several of these at the same time.
What vitamin deficiency causes oxidative stress?
Low vitamin C, vitamin E, selenium or zinc all weaken antioxidant defence, and low protein intake limits glutathione. Correcting a real deficiency helps. Taking large doses on top of an adequate diet is a different action, and it is the one that failed in trials.
How long does it take to recover from oxidative stress?
Blood markers of lipid oxidation change within days of eating differently. Enzymatic defences built by exercise take four to twelve weeks. Accumulated damage such as photoageing and cross-linked proteins does not reverse, which makes stopping the exposure the part that matters.
Are antioxidant supplements bad for you?
Across 47 low-bias randomised trials in 180 938 people, antioxidant supplements raised all-cause mortality slightly, with vitamin A and beta-carotene carrying most of the effect. High-dose vitamin C and E also blunted training adaptations. Correcting a diagnosed deficiency is a separate question, and one for your GP.
Sources
- Pizzino G., Irrera N., Cucinotta M., et al. Oxidative stress: harms and benefits for human health. Oxidative Medicine and Cellular Longevity, 2017. DOI: 10.1155/2017/8416763
- Sies H., Jones D.P. Reactive oxygen species (ROS) as pleiotropic physiological signalling agents. Nature Reviews Molecular Cell Biology, 2020. DOI: 10.1038/s41580-020-0230-3
- Bjelakovic G., Nikolova D., Gluud L.L., Simonetti R.G., Gluud C. Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis. JAMA, 2007. DOI: 10.1001/jama.297.8.842
- Ristow M., Zarse K., Oberbach A., et al. Antioxidants prevent health-promoting effects of physical exercise in humans. Proceedings of the National Academy of Sciences, 2009. DOI: 10.1073/pnas.0903485106
- Paulsen G., Cumming K.T., Holden G., et al. Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial. The Journal of Physiology, 2014. DOI: 10.1113/jphysiol.2013.267419
- Aune D., Giovannucci E., Boffetta P., et al. Fruit and vegetable intake and the risk of cardiovascular disease, total cancer and all-cause mortality: a systematic review and dose-response meta-analysis of prospective studies. International Journal of Epidemiology, 2017. DOI: 10.1093/ije/dyw319
This article is general information, not medical advice. Persistent fatigue or other unexplained symptoms should be assessed by your GP. Speak to a pharmacist before starting any supplement, particularly if you take prescribed medication.
