Every training session you do produces reactive oxygen species. ROS, as they are usually called, are unstable molecules generated as a byproduct of how your mitochondria turn oxygen and substrate into ATP. At low levels, they are the signal that tells your body to adapt: build more mitochondria, upgrade antioxidant defences, get fitter. At high levels, accumulated across hours of training and weeks of high volume, they damage cell membranes, proteins, and DNA. The reason your HRV stays suppressed for three days after a hard race, the reason your legs never quite feel reset during a heavy training block, the reason endurance athletes carry measurably higher inflammatory markers than the general population: oxidative stress sits behind a meaningful portion of it.
This article walks through what oxidative stress actually is at the cellular level, why endurance athletes accumulate more of it than any other athlete type, the controversy over antioxidant supplementation, and what the research says about managing chronic oxidative load without blunting the adaptive signal that drives training response. For the underlying mitochondrial biology, see the Mitochondria Guide.
What is oxidative stress, and how does exercise cause it?
Oxidative stress is the imbalance between reactive oxygen species generated inside your cells and the antioxidant defences available to neutralise them. ROS are oxygen-containing molecules with unpaired electrons or otherwise high chemical reactivity. The main ones produced during exercise are superoxide (O2 radicals), hydrogen peroxide, and the highly damaging hydroxyl radical. They are not foreign chemicals or pollutants. They are normal byproducts of how your cells produce energy.
The primary source during endurance exercise is the mitochondrial electron transport chain itself. As electrons flow through the chain to produce ATP, roughly 1 to 2% of the oxygen passing through leaks out as superoxide. At rest, this is a small absolute amount. During hard endurance exercise, whole-body oxygen consumption rises from a resting 3.5 ml per kg per minute to 50 ml per kg per minute or more in trained athletes. The 1 to 2% leak rate stays similar, but the absolute amount of ROS generated rises 10 to 15-fold, according to the foundational review published in Physiological Reviews.
Two secondary sources also matter during exercise. NADPH oxidase enzymes in muscle cells and immune cells produce ROS as part of normal cell signalling and immune response. Xanthine oxidase produces additional ROS during ischemia-reperfusion (the cycle of reduced blood flow during eccentric muscle contraction followed by reperfusion). Both contribute to the total oxidative load, particularly in efforts that involve repeated muscle contractions, downhill running, or high-intensity intervals.
Your cells have evolved a sophisticated antioxidant defence system to handle this. Endogenous antioxidants include superoxide dismutase (which converts superoxide into hydrogen peroxide), catalase (which converts hydrogen peroxide into water), and glutathione peroxidase (which uses glutathione to neutralise hydrogen peroxide and lipid peroxides). When the system keeps up, ROS levels stay productive. When it does not, oxidative damage starts to accumulate.
Is oxidative stress good or bad for endurance athletes?
Both, depending on dose. This is the most important concept in the entire oxidative stress conversation and the one most labels skip. The principle is called hormesis: low doses of a stressor produce beneficial adaptive responses, while high doses produce damage. Exercise-induced ROS are a textbook example.
At low to moderate levels, ROS act as signalling molecules. They activate transcription factors including NRF2 (which upregulates antioxidant defence gene expression) and PGC-1 alpha (the master regulator of mitochondrial biogenesis). Through these pathways, the ROS produced during today's training session drive the adaptations that make tomorrow's training session easier: more mitochondria, stronger antioxidant defences, improved insulin sensitivity, better lipid handling. This is the cellular logic of "what does not kill you makes you stronger," and it is the reason training works at all.
Research published in Physiological Reviews established that exercise-induced ROS production is essential for normal training adaptation. The transcriptional response to acute ROS exposure is what drives mitochondrial biogenesis, antioxidant defence enzyme upregulation, and the broader cellular fitness gains athletes are training for.
At higher levels, the same molecules damage what they no longer signal. The mitochondrial membrane, which is rich in unsaturated fats vulnerable to oxidation, is the primary target. Lipid peroxidation (the chain reaction of ROS attacking membrane lipids) impairs the integrity of the mitochondrial membrane itself, reducing ATP output and creating a feedback loop where damaged mitochondria leak more ROS. Proteins get oxidised. DNA accumulates oxidative lesions. Inflammatory pathways activate. The athlete experiences this as suppressed HRV, slow recovery, persistent fatigue, and reduced training response.
The line between productive and damaging is not a single threshold. It is a chronic-load curve. A single hard session pushes you toward the damaging end briefly, then the antioxidant systems recover. A high-volume training block holds you closer to that end for longer. A poorly planned year of high mileage without adequate recovery flips the curve into chronic oxidative overload, where the damage outpaces repair across weeks and months.
Why do endurance athletes accumulate more oxidative stress than other athletes?
Endurance athletes accumulate more oxidative stress than power, strength, or team-sport athletes for two reasons: volume and duration. The two stack on top of each other.
Volume is the obvious one. A typical endurance training week of 10 to 20 hours produces more cumulative ROS exposure than any other athletic discipline. Strength athletes train 5 to 8 hours per week at high intensity. Team-sport athletes train 6 to 12 hours including skill work. Endurance athletes are in the 10 to 25-hour range routinely, and ultra-distance athletes push higher during peak preparation. The cumulative oxidative load is structurally larger.
Duration matters as much as volume. A 4-hour ride does not produce the same ROS profile as four separate 1-hour sessions, even at the same total intensity. Sustained sub-maximal effort holds the mitochondria at high oxygen flux for hours at a time, generates ongoing ROS exposure without the recovery breaks that broken-up sessions provide, and depletes circulating antioxidants progressively across the effort. Long efforts also generate exercise-induced gastrointestinal distress, hepcidin elevation, and inflammatory responses that compound the oxidative challenge. Ultra-distance events push these effects to extremes.
The practical result is that endurance athletes operate closer to the chronic-load threshold than any other athletic group. The recovery margins between sessions are narrower. The cumulative antioxidant demand is higher. The risk of tipping from productive ROS exposure into chronic oxidative overload is elevated by structural factors that no other sport faces in the same way.
Should endurance athletes take antioxidant supplements?
The honest answer is: it depends on which antioxidant, what dose, and when. Mega-doses of isolated vitamin antioxidants taken immediately post-exercise have been shown to blunt some of the adaptive responses that training is designed to produce. Lower-dose plant-based antioxidants taken as daily baseline support appear to behave differently. The blanket claim that "antioxidants are good" is wrong. So is the reactionary opposite.
The landmark studies on this are published by Ristow, M. and colleagues in the Proceedings of the National Academy of Sciences, and in the Journal of Physiology by Paulsen, G. ad colleagues. Ristow gave young men either 1,000 mg of vitamin C plus 400 IU of vitamin E daily during a 4-week exercise programme, or placebo. The placebo group showed the expected improvements in insulin sensitivity and the upregulation of antioxidant defence genes. The supplemented group did not. Paulsen replicated similar findings in endurance-trained subjects: high-dose isolated vitamin C and E supplementation blunted some markers of mitochondrial adaptation.
The mechanism is the hormesis principle in reverse. The acute ROS spike after a training session is the signal that drives adaptation. Drowning that signal in high-dose antioxidants suppresses the adaptive response. The same compound that supports daily baseline antioxidant status, when delivered at the wrong dose at the wrong time, blocks the cellular signal you trained to produce.
Paulsen, G. and colleagues, publishing in The Journal of Physiology, found that combined vitamin C and E supplementation at high doses (1,000 mg and 235 mg respectively) hampered cellular adaptations to endurance training in healthy young adults. The mitochondrial markers that should have responded to training responded less in the supplemented group. The signal being quenched by the supplements was the same signal that drove the adaptation.
The nuance most labels skip is that this finding is specific to high-dose isolated synthetic vitamins. Plant-based polyphenols at moderate doses behave differently in the published literature. They engage multiple antioxidant pathways at once (direct radical scavenging, NRF2 upregulation, anti-inflammatory effects) rather than completely quenching the acute ROS signal. They are also typically delivered in food-form matrices alongside cofactors that modulate their bioavailability. For the full breakdown of the polyphenol distinction and which compounds have endurance evidence, see the Plant-based antioxidants guide and the deeper look at How polyphenols support cellular energy.
When is the best time to take antioxidant support?
The timing recommendation that flows from the research above is straightforward. Avoid mega-dose isolated antioxidants in the 30 to 90 minutes immediately after training, when the acute ROS signal is driving adaptation. Daily baseline antioxidant support, whether from food or from a foundation supplement at moderate doses, can be taken at most other times without interfering with the signal. Morning intake with breakfast is the simplest pattern for most athletes.
The practical implications follow from there. A whole-food diet rich in colourful fruits, vegetables, olive oil, green tea, and dark chocolate provides a steady stream of polyphenols throughout the day. A daily polyphenol-based supplement taken in the morning fits the same pattern. A 1,000 mg vitamin C tablet swallowed alongside the post-race recovery drink fits the worst pattern. The amount of vitamin C is not the issue. The timing and the isolation of the compound are.
For chronically high-load periods (race build-ups, training camps, back-to-back ultras), the baseline antioxidant support matters more, not less. The challenge is to deliver it in a way that supports recovery between sessions without blunting the signal during them. This is the formulation problem most antioxidant supplements solve in the wrong direction.
How endurance athletes manage chronic oxidative load in practice
Practical management of oxidative load sits on four pillars: sleep, varied diet, training periodisation, and targeted supplementation at appropriate doses.
Sleep is the largest single lever. The body produces glutathione (its master antioxidant) and melatonin (a potent direct antioxidant) primarily during sleep. Chronic sleep restriction measurably increases oxidative stress markers and impairs antioxidant defence. For an athlete already operating near the chronic-load threshold, sleeping 6 hours instead of 8 is one of the fastest ways to tip into oxidative overload regardless of nutrition or supplementation.
A varied whole-food diet provides the antioxidant cofactor inputs the endogenous defence system relies on. Coloured fruits and vegetables, green tea, dark chocolate, olive oil, herbs and spices, nuts and seeds all contribute polyphenols and supporting nutrients. The mistake is to skip the food-level antioxidant strategy in favour of a single high-dose supplement. The food layer is the foundation. Supplements are not a substitute.
Training periodisation matters because antioxidant recovery is time-dependent. A 10-day recovery week every fourth week of training gives the antioxidant defence system time to reset. A taper in the final 1 to 3 weeks before an A-race does the same. Athletes who train hard every day for months without programmed recovery weeks tend to accumulate oxidative load that no nutrition strategy can overcome.
Targeted supplementation at moderate doses fits inside this framework. The OLEUS Daily Shot was built around exactly this principle: 50 mg of Oleuropein from 250 mg of olive leaf extract for mitochondrial membrane support, 15 mg of Vitamin C from Acerola extract delivered alongside its natural cofactor matrix, plus the full B-vitamin complex, Magnesium, and Iron that the antioxidant defence enzymes depend on. The doses are deliberate. This is not a megadose strategy.
The trap most antioxidant supplements fall into is the one the research has been clear on for over a decade. High-dose isolated vitamins post-training blunt the signal training is supposed to produce. We formulated the Daily Shot to provide chronic baseline support without that cost: lower doses, plant-based compounds, taken as a morning foundation rather than a post-workout megadose.
OLEUS Performance Lab
What does the research show?
The OLEUS formula was evaluated in a placebo-controlled trial with 28 cyclists from a Switzerland-based World Tour professional cycling team, across a multi-day endurance protocol. The riders taking the formula showed +25% sustained power output over the test period compared to placebo. The trial measured the integrated formula in race-relevant conditions, with the primary outcome being sustained performance under prolonged endurance load.
More than 5,000 endurance athletes across Belgium, the Netherlands, Switzerland, and beyond now use the OLEUS system as part of their daily nutrition.
Frequently asked questions
Does this mean I should not take any antioxidants?
No. The research is specific to high-dose isolated synthetic vitamins (notably vitamin C above 1,000 mg per day and vitamin E above 400 IU per day) taken immediately post-exercise. Moderate-dose plant-based polyphenols, antioxidant-rich whole foods, and lower-dose foundation supplements taken as daily baseline support do not appear to blunt training adaptations in the published literature. The takeaway is "the right antioxidants at the right dose at the right time," not "no antioxidants."
How do I know if my oxidative load is too high?
Direct measurement of oxidative stress markers (F2-isoprostanes, malondialdehyde, oxidised LDL) is possible in research settings but not standard in clinical practice. The proxy signals an athlete can track are more practical: HRV that stays suppressed for more than 48 hours after a hard effort, sleep quality that declines without an obvious cause, persistent low-grade fatigue across multiple days, slower recovery between sessions, increased perceived exertion at familiar paces, and increased frequency of upper respiratory infections. Any sustained pattern of these warrants a closer look at training load, sleep, and nutrition.
Can I just eat more fruits and vegetables instead of supplementing?
For most athletes, mostly yes. A varied whole-food diet rich in polyphenols is the foundation of any antioxidant strategy and should be the first layer. Supplementation comes in when the dose required to hit a specific cellular mechanism is impractical to get from food consistently (50 to 100 mg of standardised Oleuropein, for example, would require unrealistic volumes of olive oil or olives), or when training volume creates structural gaps the diet does not close. Food first, supplement second.
Should I take antioxidants before or after exercise?
The simplest pattern is morning, with breakfast, as part of a daily baseline. This avoids the post-exercise window where high-dose isolated antioxidants have shown signal-blunting effects. For race day specifically, the Pre-Activity Shot is designed for the 45 to 60 minutes before the start, which is well outside the post-exercise window and inside the acute pre-effort priming window. For daily training, morning intake of the Daily Shot covers the same use case for cumulative baseline.
Does sleep really matter that much for oxidative stress recovery?
Yes. Sleep is when the body produces most of its glutathione (the master antioxidant) and melatonin (which is itself a potent direct antioxidant). Chronic sleep restriction measurably elevates oxidative stress markers and impairs antioxidant defence enzyme activity. For an athlete already operating near the chronic oxidative load threshold from high training volume, dropping from 8 hours of sleep to 6 hours can do more damage than any nutritional decision. Sleep is the largest single lever for managing oxidative load.
The bottom line
Oxidative stress is both the signal that makes endurance training work and the damage that limits how much endurance training you can absorb. The line between productive and damaging depends on dose, duration, sleep, and recovery. The fix is not megadose antioxidants. It is a varied diet, programmed recovery, protected sleep, and a daily foundation that supports baseline antioxidant capacity without blunting the adaptive signal. The OLEUS Daily Shot was built for that role.
Sources
-
Powers, S.K., Jackson, M.J. (2008). Exercise-induced oxidative stress: cellular mechanisms and impact on muscle force production. Physiological Reviews, 88(4), 1243-1276.
-
Ristow, M., Zarse, K., Oberbach, A., Klöting, N., Birringer, M., Kiehntopf, M., Stumvoll, M., Kahn, C.R., Blüher, M. (2009). Antioxidants prevent health-promoting effects of physical exercise in humans. Proceedings of the National Academy of Sciences, 106(21), 8665-8670.
-
Paulsen, G., Cumming, K.T., Holden, G., Hallén, J., Rønnestad, B.R., Sveen, O., Skaug, A., Paur, I., Bastani, N.E., Østgaard, H.N., Buer, C., Midttun, M., Freuchen, F., Wiig, H., Ulseth, E.T., Garthe, I., Blomhoff, R., Benestad, H.B., Raastad, T. (2014). Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans. The Journal of Physiology, 592(8), 1887-1901.
-
Nikolaidis, M.G., Kerksick, C.M., Lamprecht, M., McAnulty, S.R. (2012). Redox biology of exercise. Cellular and Molecular Life Sciences, 69(1), 169-187.
-
Pingitore, A., Lima, G.P., Mastorci, F., Quinones, A., Iervasi, G., Vassalle, C. (2015). Exercise and oxidative stress: potential effects of antioxidant dietary strategies in sports. Nutrition, 31(7-8), 916-922.
-
Margaritis, I., Rousseau, A.S. (2008). Does physical exercise modify antioxidant requirements? Nutrition Research Reviews, 21(1), 3-12.
-
OLEUS placebo-controlled trial, 28 cyclists, Switzerland-based World Tour team, multi-day endurance protocol. Data on file, OLEUS Performance Lab.
