Your body makes damaging molecules every minute of every day, just by being alive. Breathing, digesting food and turning what you eat into energy all produce free radicals as a by-product. That is normal. The problem starts when those molecules outpace your defences.
That imbalance has a name: oxidative stress. It sits underneath a lot of what we think of as “just getting older”, from tired skin to slower recovery to the low-grade inflammation researchers now link to chronic disease.
This guide explains what oxidative stress actually is, the signs that yours may be running high, what drives it, and the role your body’s own antioxidant system plays in keeping it in check.
Table of Contents
What is oxidative stress, in plain English?
Oxidative stress is the imbalance between free radicals and the antioxidants that neutralise them. When free radicals build up faster than your body can mop them up, they start damaging cells, proteins, fats and even your DNA.
Free radicals are unstable molecules, often called reactive oxygen species (ROS). They are missing an electron, so they grab one from the nearest healthy molecule. That sets off a chain reaction of damage, a bit like rust spreading across metal.
Antioxidants are the counterbalance. They donate an electron to a free radical without becoming unstable themselves, which stops the chain reaction. Some come from your diet, such as vitamin C and vitamin E. Others your body makes itself, and the most important of these is glutathione.
In a 2020 review in Antioxidants, the biochemist Helmut Sies, who first defined the term in 1985, described oxidative stress as a disruption of “redox signalling and control”. In everyday language: the system that should stay balanced tips the wrong way.
| Free radicals (ROS) | Antioxidants |
|---|---|
| Unstable molecules missing an electron | Stable molecules that can donate an electron |
| Steal electrons from healthy cells, proteins and DNA | Neutralise free radicals and stop the chain reaction |
| Made by normal metabolism, plus smoking, alcohol, pollution | From diet (vitamins C, E) and made in the body (glutathione) |
| Useful in small bursts (immunity, exercise adaptation) | Decline with age, leaving the balance harder to hold |
Signs of oxidative stress
Oxidative stress is not a condition you can feel directly, and there is no single symptom that confirms it. It shows up indirectly, through the toll it takes on cells over time.
Commonly reported signs that researchers associate with higher oxidative load include:
- Persistent fatigue that rest does not fix
- Slower recovery after exercise or illness
- Skin that looks duller, drier or more lined than your age suggests
- Brain fog and trouble concentrating
- More frequent minor infections
- Aches and stiffness that linger
None of these proves you have high oxidative stress on its own. They overlap with plenty of other causes, from poor sleep to thyroid issues. If symptoms persist, see your GP rather than assuming the cause.
What matters is the bigger picture. A 2018 paper in Clinical Interventions in Aging (Liguori and colleagues) linked sustained oxidative stress to many age-related conditions, including cardiovascular disease, cognitive decline and reduced muscle function.
What causes oxidative stress?
Some free radical production is unavoidable and even useful. Your immune cells use ROS to kill bacteria, and short bursts during exercise help your muscles adapt. Trouble comes from the steady, chronic overload.
The main drivers researchers point to are:
- Ageing. Your natural antioxidant defences, including glutathione, decline with age, so the same free radical load lands harder at 55 than it did at 25.
- Diet. A diet high in ultra-processed food, sugar and fried fats, and low in vegetables, gives you more free radicals and fewer antioxidants.
- Alcohol. Processing alcohol burns through glutathione, which is why heavy drinking is strongly linked to oxidative damage in the liver.
- Smoking and air pollution. Both deliver a heavy dose of free radicals directly.
- Chronic stress and poor sleep. Both raise inflammation and oxidative load.
- Over-training without recovery. Helpful in moderation, harmful in excess.
The pattern is clear: oxidative stress is mostly a lifestyle and ageing story, not a mystery. That also means much of it responds to the things within your control.
Oxidative stress and ageing: the connection
The link between oxidative stress and ageing is one of the most studied ideas in biology. The “free radical theory of ageing”, first proposed by Denham Harman in 1956, suggests that the accumulation of free radical damage over a lifetime is a key driver of how we age.
The modern view is more nuanced. Free radicals are not simply villains, and flooding the body with high-dose antioxidant supplements has not reliably extended lifespan in trials. The goal is balance, not elimination.
Where the evidence is strongest is the overlap between oxidative stress and chronic, low-grade inflammation. The two feed each other, and together they sit behind much of age-related decline. We cover that loop in more detail in our guide to inflammaging and how chronic inflammation links to ageing.
This is also why vitamin D, which has its own role in cellular ageing, keeps appearing in this research. Our article on vitamin D and ageing looks at a Harvard study on that angle.
Your body’s defence: glutathione, the master antioxidant
You are not defenceless against free radicals. Your cells run a built-in antioxidant network, and at its centre sits glutathione, often called the body’s master antioxidant.
Glutathione is a small protein your cells make from three amino acids: cysteine, glutamate and glycine. It is recycled and reused constantly, and it works inside the cell where much of the damage happens. It also helps regenerate other antioxidants like vitamins C and E.
The catch is cysteine. It is the limiting ingredient, meaning your body can only make as much glutathione as it has cysteine available. Diet, ageing, alcohol and illness can all leave you short.
This is where N-acetylcysteine (NAC) comes in. NAC is a stable, well-absorbed form of cysteine that your body can use to top up glutathione production. Research, including a 2021 review in the Journal of Nutrition and Metabolism (Schwalfenberg), describes NAC’s value as a glutathione precursor, particularly in situations of higher oxidative demand. NAC is not suitable if you have cystinuria or take nitrate medications. Consult your doctor before taking it.
If you want to understand how the two relate, our explainer on NAC and glutathione breaks it down, and NAC vs glutathione compares supplementing each directly.
How to reduce oxidative stress
There is no single switch, but the evidence points consistently to a handful of habits that lower oxidative load and support your antioxidant defences.
- Eat more plants. Colourful vegetables, berries, herbs and pulses deliver a broad mix of dietary antioxidants. Variety beats any single “superfood”.
- Go easy on alcohol. Cutting back spares glutathione for other jobs.
- Don’t smoke, and limit pollution exposure where you can.
- Move regularly, but recover properly. Aim for consistency over punishing sessions.
- Prioritise sleep and manage stress. Both are upstream of oxidative balance.
- Support glutathione if you are older or under higher load. Dietary cysteine and, where appropriate, a NAC supplement can help maintain production.
| Antioxidant | Where to find it |
|---|---|
| Vitamin C | Peppers, citrus, broccoli, blackcurrants |
| Vitamin E | Almonds, sunflower seeds, olive oil |
| Polyphenols | Berries, green tea, dark chocolate, herbs |
| Carotenoids | Carrots, sweet potato, leafy greens, tomatoes |
| Glutathione support (cysteine, sulphur) | Eggs, garlic, onions, cruciferous vegetables |
You can also support glutathione through food and lifestyle alone. Our guide on how to boost glutathione naturally covers the diet-first approach.
For those who want to top up cysteine directly, our NAC+ 600mg supplement provides a clean, vegan-certified source. As with any supplement, it supports your body’s own systems rather than replacing a balanced diet.
FAQ
What is oxidative stress in simple terms?
Oxidative stress is when damaging molecules called free radicals build up in your body faster than antioxidants can neutralise them. The imbalance can damage cells, proteins and DNA over time, and it is linked to ageing and chronic disease.
What are the symptoms of oxidative stress?
There is no single symptom. People with higher oxidative load often report persistent fatigue, slow recovery, brain fog, duller skin and more frequent minor infections. These overlap with many other causes, so see your GP if symptoms persist.
What causes oxidative stress?
The main drivers are ageing, a diet high in processed food and low in vegetables, alcohol, smoking, air pollution, chronic stress, poor sleep and over-training. Some free radical production is normal and even useful; the problem is chronic overload.
Does oxidative stress cause ageing?
Oxidative stress is closely linked to ageing and to age-related conditions, and it works alongside chronic inflammation. The current view is that balance matters more than eliminating free radicals entirely, since they also play useful roles in the body.
Can NAC help with oxidative stress?
NAC supplies cysteine, the limiting ingredient your body needs to make glutathione, its main internal antioxidant. Research describes NAC as a glutathione precursor that may help support antioxidant defences, especially when oxidative demand is high. It is not suitable if you have cystinuria or take nitrate medications, so consult your doctor first.
This article is for general information and is not medical advice. Food supplements are not a substitute for a varied, balanced diet and healthy lifestyle. If you are pregnant, taking medication or managing a health condition, speak to your GP or pharmacist before starting a new supplement.
References
- Sies H. Oxidative stress: concept and some practical aspects. Antioxidants. 2020. link
- Liguori I, et al. Oxidative stress, aging, and diseases. Clinical Interventions in Aging. 2018. link
- Pizzino G, et al. Oxidative stress: harms and benefits for human health. Oxidative Medicine and Cellular Longevity. 2017. link
- Schwalfenberg GK. N-acetylcysteine: a review of clinical usefulness. Journal of Nutrition and Metabolism. 2021. link
