What are free radicals?
Free radicals are unavoidable by-products of metabolism. Understanding them begins with basic chemistry.
Electrons in stable molecules are usually paired. A free radical is an atom, molecule or ion with an unpaired electron, making it reactive. It may take an electron from a nearby membrane lipid, protein or DNA molecule, causing oxidation and cellular damage. When free radical production exceeds the body's capacity to control it, oxidative stress develops, contributing to aging, inflammation and chronic disease.
How are free radicals measured?
Free radicals do not have one dedicated measurement unit. Because many are short-lived—sometimes reacting within a millionth of a second—research and clinical assessment use two broad approaches. 1. Concentration in research: Electron paramagnetic resonance (EPR) spectroscopy detects radicals and measures their concentration, commonly expressed in micromoles per liter ($\mu\text{mol/L}$) or molarity ($\text{M}$). 2. Indirect clinical markers: Rather than capture the radicals themselves, tests measure the products left by oxidative damage, for example:
MDA (malondialdehyde): A product of lipid oxidation, commonly measured in $\mu\text{mol/L}$.
8-OHdG: A marker of oxidative DNA damage measured in urine, expressed here in $\text{ng/mg Creatinine}$.
Do thinking, stress and exercise generate free radicals?
1. Does intense mental activity generate free radicals?
Yes. The brain accounts for about 2% of body weight but uses nearly 20% of the body's oxygen and energy. Demanding mental work increases the energy requirements of neurons and their mitochondria. During respiratory-chain activity, around 1–2% of electrons may leak and react with oxygen, generating reactive oxygen species (ROS).
How is it assessed? Researchers use neuroimaging, such as PET, to observe glucose and oxygen metabolism in specific brain regions. Blood and urine tests can also measure oxidative-damage markers such as 8-OHdG.
2. Does stress produce free radicals?
Yes. Chronic psychological stress can sustain cortisol and catecholamine release, keeping the body in a prolonged fight-or-flight state. This increases mitochondrial energy demand while suppressing antioxidant defenses such as SOD and glutathione peroxidase. Stress can therefore increase radical production while weakening the systems that control it.
How is it assessed? The d-ROMs test measures reactive oxygen metabolites through a blood hydroperoxide reaction, reporting results in the proprietary unit U.CARR.
Normal value: 250–300 U.CARR.
Stress and inflammation: Values above 400 U.CARR are presented here as indicating severe oxidative damage.
3. Does exercise generate free radicals?
Yes, but the response can be adaptive. During intense exercise, skeletal muscle oxygen consumption may increase 100 to 200 times, raising mitochondrial demand and free radical production. This contributes to microscopic muscle damage and soreness. The longer-term response involves adaptation:
Short term: Exercise causes a temporary rise in free radicals and oxidative stress.
Long term: This transient challenge activates signaling that stimulates the body's production of antioxidant enzymes such as SOD.
The key is balance. Regular, moderate exercise reduces chronic oxidative stress over time. Excessive training without sufficient rest can overwhelm antioxidant defenses and lead to persistent tissue damage.
Which habits generate particularly high free radical exposure?
Beyond normal physiological and psychological processes, external exposures and everyday habits can add substantially to the burden:
Smoking: A puff of cigarette smoke can introduce up to $10^{15}$ free radicals, rapidly depleting vitamin C and other antioxidant reserves.
Late nights and insufficient sleep: Sleep provides time for repair and clearance processes, while melatonin contributes antioxidant activity. Losing sleep increases the workload while limiting recovery.
High-temperature frying and grilling: Heating fats beyond their smoke point or repeatedly reheating them promotes lipid peroxidation and reactive compounds that can damage intestinal and vascular endothelial cells after consumption.
Excessive ultraviolet exposure: UV light drives chemical reactions and free radical generation in the skin, contributing to inflammation, pigmentation and collagen breakdown.