Oxidative Stress: How Cells Protect Against Invisible Rust

Your cells protect themselves against oxidative damage mainly with their own enzymes, and they do it around the clock. The chemistry they hold in check is the same one behind a rusty bike chain, a browning apple slice and rancid butter: oxidation. Inside a cell it runs all the time, and since 1985 there has been a scientific term for the moment it tips out of balance. That term is oxidative stress. By early 2015, the PubMed literature database held around 138,000 entries on it. Oxygen-derived molecules are not harmful across the board. At 1 to 10 nanomoles per liter, hydrogen peroxide works as a signaling molecule in the body. Above roughly 100 nanomoles per liter, damage to molecules begins. Laboratories use exactly this dose dependence: they expose cell cultures to measured amounts of hydrogen peroxide and count how many cells hold up. A result from such a culture is a signal from the lab dish, not a finding in people.

What does oxidative stress mean in a cell?

Oxidative stress is a disturbance in the balance between oxidizing and reducing processes in the body, tipped toward the oxidizing side. Helmut Sies coined the term in 1985 and revisited it in 2015 in the journal Redox Biology. His review gives one figure that shows how large the field has grown: around 138,000 PubMed entries at the start of 2015.

The chemistry is the same as for metal left out in the rain. One molecule gives up electrons, and another takes them on. With iron, the result is visible rust. In a cell, the result is altered fats, proteins and genetic material. You cannot see any of it, but a great deal of it can be measured.

Sies uses the same paper to warn against a common shortcut in how the term gets used. In his own words:

“This can lead to indiscriminate use of the term, oxidative stress, without clear relation to redox chemistry.”

Sies H, Oxidative stress: a concept in redox biology and medicine, Redox Biology 4 (2015), pages 180–183, full text.

For everyday life, that means two things. The process is real and has been described for decades. And the term on its own says nothing yet about what is happening in a particular tissue right now.

Why does a free radical start a chain reaction?

A free radical is a molecule with a single, unpaired electron in its outer shell. That state is energetically unfavorable. The radical pulls the missing electron out of a neighboring molecule, which in turn becomes a radical itself. The result is a chain that only ends when two radicals pair off with each other or a protective system steps in.

Radicals form in the body constantly, and not by accident. In a 2020 review, Helmut Sies and Dean Jones list more than 40 enzymes that produce reactive oxygen species on purpose, among them the NADPH oxidases and the respiratory chain in the mitochondria. Two of these molecules carry most of the signaling work: hydrogen peroxide and the superoxide anion.

“Elevated formation of the different ROS leads to molecular damage, denoted as ‘oxidative distress’.”

Sies H, Jones DP, Reactive oxygen species (ROS) as pleiotropic physiological signalling agents, Nature Reviews Molecular Cell Biology 21 (2020), pages 363–383.

The amount is what counts, not the mere presence. Reactive oxygen species belong to a healthy body, which would lose a signaling pathway without them.

When does oxidation help, and when does it harm?

Oxidative eustress is the scientific name for a level of oxidation that the body needs to run its own processes. The opposite term is oxidative distress, which means the amount at which molecules start to suffer damage. Both words come from the same research group that coined the original term.

The line between the two is drawn with surprising precision. In a 2017 paper in Redox Biology, Helmut Sies gives a physiological range of 1 to 10 nanomoles per liter for hydrogen peroxide. Above roughly 100 nanomoles per liter, biomolecules begin to take damage. Signal and damage are therefore separated by a factor of about 10 to 100.

Three points sum up the current understanding:

• Reactive oxygen species act as messengers and as damaging agents, depending on their concentration.
• Protection against damage runs along three routes: prevention, interception and repair.
• Central switches such as Nrf2/Keap1 and NF-κB control how a cell responds to rising oxidation.

A simple rule follows from this: oxidation becomes a problem only in excess. That is why, in this field, the question of dose matters more than the question of substance.

How does your body protect itself from oxidative damage?

The antioxidant defense network is the full set of enzymes and molecules that intercept reactive oxygen species, convert them or repair the damage they have done. It includes superoxide dismutase, catalase and the glutathione and thioredoxin systems. These systems work permanently, not only when the body is under strain.

Anyone who concludes from this that the main thing is to take antioxidant supplements is off track, according to current redox biology. Helmut Sies puts it this way:

“The major role in antioxidant defense is fulfilled by antioxidant enzymes, not by small-molecule antioxidant compounds.”

Sies H, Redox Biology 4 (2015), pages 180–183.

In 2024, Barry Halliwell published a review of how antioxidants act, in which he examines the successes and failures of such approaches one by one. Two points from it are useful for non-specialists. Much of what counts as a healthy lifestyle may work partly through antioxidant mechanisms. And in several places, precise measurements from the living organism are still missing, for example on the concentrations of reactive sulfur compounds.

How do labs make oxidative damage measurable with hydrogen peroxide?

The hydrogen peroxide model is a standard experiment in cell biology. Cell cultures are exposed to a precisely measured amount of H₂O₂. The appeal of the model is how clean it is: the dose is known, the timing is known, and the number of surviving cells can be counted afterward.

Almost every lab follows the same sequence:

1. Cells of a defined line grow in a plate with several wells.
2. Some of the wells receive hydrogen peroxide, while others serve as an unexposed control.
3. After a set time, the number of living cells is determined and compared with the control.
4. The experiment is repeated several times, so that scatter can be told apart from a real effect.

Hydrogen peroxide suits this purpose because the body makes it anyway and because its effect depends so strongly on concentration. In the nanomolar range it is a signaling molecule, and at the concentrations used in a lab experiment it is a stressor. The same substance answers two different questions, depending on how high the dose is.

What can a cell study tell you about humans?

In vitro literally means “in glass” and refers to experiments on cells outside a living organism. The ladder of evidence in research has two more rungs above it: the animal model and the controlled study in humans. Every result from the hydrogen peroxide model stands on the first rung.

Three questions help you place any cell study in this field:

• How many independent labs have produced the result? A single lab is a start, not a confirmation.
• Who supplied the material that was tested? The answer belongs next to the result.
• What does the dish leave out? A cell culture says nothing about how long a person is exposed, at what distance, or what their everyday life looks like.

A result from cell culture is therefore a signal and not proof in humans. How the body organizes its own nightly maintenance is the subject of our article How Sleep Protects Your Cells: Nighttime Repair.

Why does Qi Blanco follow redox research?

Qi Blanco is a jewelry brand, and one of the subjects we keep coming back to is how the body protects its cells. That is why we follow redox biology closely, from the first definition of the term in 1985 to the current reviews on antioxidant enzymes. We keep track of what has been measured and what is still open.

Readers who want to go further into water and order will find more in our articles Does Water Have Memory? What Physics Has Actually Measured and Is Structured Water Real? What Measurements Show. The research we cite on our own subjects is collected on our studies page.

Frequently asked questions about oxidative stress

Is oxidation in the body always harmful?

Oxidation is a normal part of metabolism. At 1 to 10 nanomoles per liter, hydrogen peroxide carries signals between cells. Only above roughly 100 nanomoles per liter does damage to biomolecules take over.

How can you tell whether your cells are under oxidative strain?

There is no single everyday marker for it. Researchers measure breakdown products of oxidized fats and proteins in blood or urine, along with the state of the glutathione and thioredoxin systems. These measurements belong in a lab, not in a self-assessment.

Do antioxidant supplements help?

The main work of the defense is done by the body's own enzymes, not by small-molecule substances taken in from outside. Helmut Sies stated this in 2015, and Barry Halliwell examined the successes and failures of such approaches one by one in 2024. Current research does not support a blanket answer.

Can a cell culture result be applied to people?

A cell culture result shows what happens in a dish under a known dose of hydrogen peroxide. It is the first rung on the ladder of evidence, followed by animal models and controlled studies in humans. On its own, it does not tell you what happens in your body.

Key takeaways on oxidative stress

• Since 1985, oxidative stress has meant an imbalance tipped toward oxidizing processes; by early 2015, PubMed listed around 138,000 entries on it.
• A free radical carries an unpaired electron and sets off a chain reaction until a protective system ends it.
• More than 40 enzymes produce reactive oxygen species on purpose; for hydrogen peroxide, signal and damage are separated by a factor of about 10 to 100.
• Enzymes, not small-molecule supplements, carry the main load of antioxidant defense.
• The hydrogen peroxide model makes oxidative damage countable in the lab, because dose and timing are known.
• A result from cell culture is a signal from the dish; on its own, it says nothing about people.

Sources and further reading

Every source listed here can be reached through its digital object identifier. The order follows the year of publication.

• Dröge W, Free Radicals in the Physiological Control of Cell Function, Physiological Reviews, 2002. Original paper
• Sies H, Oxidative stress: a concept in redox biology and medicine, Redox Biology, 2015. Full text
• Sies H, Berndt C, Jones DP, Oxidative Stress, Annual Review of Biochemistry, 2017. Journal article
• Sies H, Hydrogen peroxide as a central redox signaling molecule in physiological oxidative stress: Oxidative eustress, Redox Biology, 2017. Paper
• Sies H, Jones DP, Reactive oxygen species (ROS) as pleiotropic physiological signalling agents, Nature Reviews Molecular Cell Biology, 2020. Review
• Halliwell B, Understanding mechanisms of antioxidant action in health and disease, Nature Reviews Molecular Cell Biology, 2024. Review