What Are Reactive Oxygen Species (ROS)?

Reactive oxygen species, or ROS, have a reputation as the villains of aging, the molecules that antioxidants are supposed to mop up. The real story is more interesting and more useful: ROS are a normal, necessary product of making energy, and the goal is not to eliminate them but to keep them in balance. Here is what they actually are, where they come from, and why mitochondrial health, not endless scavenging, is the part worth paying attention to.

What reactive oxygen species are

Reactive oxygen species are a family of small, chemically reactive molecules derived from oxygen. The main players are superoxide (a free radical with an unpaired electron), hydrogen peroxide (not itself a radical but reactive and able to travel), and the hydroxyl radical (the most aggressive and short-lived of the group). "Reactive" is the key word: they readily react with whatever is nearby, and depending on the level and location, that reactivity is either useful signaling or unwanted damage.

It helps to separate ROS from two related terms this site covers elsewhere. Free radicals are molecules with an unpaired electron, so superoxide and the hydroxyl radical are radicals while hydrogen peroxide technically is not, even though it is still a ROS. Oxidative stress is not a molecule at all; it is the state that develops when ROS production outpaces the body's ability to neutralize them. ROS are the actors; oxidative stress is the situation.

Where ROS come from

The largest everyday source of ROS is the mitochondria. When cells make energy, electrons are passed down a series of protein complexes called the electron transport chain, which drives the production of ATP, the cell's energy currency. The system is efficient but not perfect: a small fraction of electrons "leak" at certain points, especially at complexes I and III, and combine with oxygen to form superoxide. In other words, making energy and making ROS are two sides of the same reaction. Every one of your mitochondria is doing this right now.

Mitochondria are not the only source. Specialized enzymes called NADPH oxidases produce ROS deliberately, for example as part of the immune response, and structures like peroxisomes generate them too. But for the purposes of healthy aging and cellular energy, the mitochondrial source is the one that matters most, because it scales with how much energy you make and with how healthy your mitochondria are.

The part most people get wrong: ROS are not simply bad

For years the popular framing was that ROS are toxic and the solution is to flood the body with antioxidants. The biology is more nuanced. At low, controlled levels, ROS are signaling molecules. They tell cells when to adapt, when to divide, and when to ramp up their own defenses. This is central to how exercise works: the burst of ROS produced during a hard workout is one of the signals that tells muscle to build more mitochondria and get stronger, a beneficial stress response sometimes called mitohormesis.

That is why blindly megadosing antioxidants is not automatically a good idea. Some research suggests that very high antioxidant doses taken around training can blunt the body's adaptation to exercise, because they interfere with the very ROS signal the adaptation depends on. The target is balance, not zero.

When the balance tips: oxidative stress

Problems begin when ROS production overwhelms the cell's defenses. At high levels, ROS react with and damage lipids (including cardiolipin, a fat found in mitochondrial membranes), proteins, and even mitochondrial DNA, which sits close to the ROS source and has limited repair compared with the DNA in the cell's nucleus. This is the damage end of oxidative stress.

There is a self-reinforcing loop here that is worth understanding. Damaged mitochondria tend to be leakier and less efficient, so they produce relatively more ROS while making less energy. More ROS means more damage, which means more dysfunctional mitochondria. Left unchecked, this cycle is one reason cellular energy production can decline with age. Breaking the loop is less about scavenging individual ROS molecules and more about removing the damaged mitochondria that are over-producing them in the first place.

How the body keeps ROS in check

Cells have a built-in defense network. Enzymes such as superoxide dismutase convert superoxide into hydrogen peroxide, and catalase and glutathione peroxidase then convert hydrogen peroxide into water. Small molecules like glutathione, and the NADPH that regenerates it, act as a reservoir of reducing power. Dietary antioxidants contribute too, but the enzymatic system is the heavy lifter, and it is tuned by the same ROS signaling described above, which is one more reason wiping out all ROS is counterproductive.

The other defense is quality control. Mitophagy is the process that identifies worn, ROS-leaking mitochondria and recycles them, so the remaining pool stays healthier and cleaner-burning. Think of it as fixing the leaky engines rather than endlessly wiping up the oil.

Where urolithin A fits

Urolithin A does not work like a conventional antioxidant. It is not primarily about neutralizing ROS molecule by molecule. Instead, the research interest is in its ability to support mitophagy, clearing the damaged mitochondria that over-produce ROS. In a first-in-human study (Andreux and colleagues, 2019, Nature Metabolism), urolithin A was safe and well tolerated up to 1,000 mg per day and shifted mitochondrial gene-expression markers in a favorable direction. A 2025 randomized, placebo-controlled study in Nature Aging reported that roughly a month of supplementation supported the mitochondrial health of immune cells in midlife adults. Foundational preclinical work (Ryu and colleagues, 2016, Nature Medicine) first established the mitophagy mechanism in cell and animal models.

In honest terms, that makes urolithin A a mitochondrial-quality strategy rather than a ROS eraser. Human data on specific ROS or oxidative-stress markers is still early, and supporting mitochondrial health is a structure-and-function goal, not a treatment for any disease. But conceptually it sits upstream of the scavenging approach: healthier mitochondria produce ROS in a more controlled way to begin with.

Learn more

For the bigger picture of how this ingredient works, read our urolithin A guide, and compare standardized options in best urolithin A gummies. Related science explainers include what is oxidative stress and what are free radicals.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This guide is educational and is not a substitute for advice from a qualified healthcare provider.