Antioxidants in Endurance Sport: Benefit or Backfire

Antioxidants are the oldest miracle belief in sports nutrition: vitamin C against infections, vitamin E against soreness, an “antioxidant booster” after every hard block. The only problem is that the evidence says the opposite. Reactive oxygen species are not waste to be swept away, they are the signal that tells the body to adapt. Blunt them with high-dose pills and you train against yourself. This article explains what oxidative stress actually does in training, where supplements backfire and which foods reliably deliver.

What oxidative stress really is

Every use of oxygen produces reactive oxygen species (ROS): superoxide, hydrogen peroxide, nitric oxide derivatives. A small fraction of them damages cell membranes, proteins and DNA, which is why the body runs its own clearance system of superoxide dismutase, catalase and glutathione peroxidase. The measurable part of that damage is what we call oxidative stress.

The decisive point for endurance athletes is different: ROS are also messengers. They activate the PGC-1α and NRF2 pathways, and those two switches are what increase mitochondrial density, capillarisation and the enzymatic capacity for fat oxidation. In short, the oxidative stimulus is the reason a training block produces more endurance performance at all. A session without that stimulus is a session the body answers with fatigue but not with adaptation.

So the same mechanism helps on race day and hurts in training. A marathon at 30 degrees with high oxygen uptake produces many times the ROS of an easy base session. On race day, scavenging makes sense because you want to express performance, not adapt. In everyday training, scavenging is counterproductive.

Why vitamin C and E pills blunt adaptation

The best-known study on this is by Paulsen and colleagues (2014): cyclists completed eleven weeks of endurance training while one group took 1000 mg of vitamin C plus 235 mg of vitamin E daily. The supplement group showed markedly fewer markers of mitochondrial biogenesis in muscle tissue than the placebo group. Similar work from other research groups on runners found the expected performance gains simply absent under high-dose vitamin C and E.

The pattern repeats across almost all controlled trials: high-dose single nutrients in isolated form dampen the training-induced signalling response. At moderate doses from food — a glass of orange juice, a portion of peppers, a handful of berries — the effect is not detectable. The difference is not the substance but the dose and the matrix: food delivers antioxidants in micromolar amounts with hundreds of accompanying compounds, pills deliver one compound at a dose that does not occur in normal metabolism.

A second point concerns immune defence. High-dose vitamin C shortens colds slightly but does not prevent them — the old rule that endurance athletes should take 1000 mg daily through winter as protection does not survive scrutiny. If you get frequent infections after hard blocks, check sleep, training load and energy availability first, not the pill box. More on that in the article on the immune system in endurance training.

Which antioxidants actually deliver

Sort the literature by compound and remarkably little survives — and what does survive comes almost entirely from food.

• Polyphenols from berries and cherries: anthocyanins from tart cherries, blueberries and pomegranate show modestly reduced inflammatory markers (CRP, IL-6) and less soreness after eccentric loading across several trials. The effect is small — in the range of five to ten per cent for subjective muscle soreness — but consistent. Practical dose: 200 to 300 millilitres of tart cherry juice or a portion of berries after the session.
• Nitrate from beetroot and leafy greens: strictly speaking not an antioxidant, though sold in the same aisle. 400 to 500 milligrams of nitrate — roughly 500 millilitres of beetroot juice — improves movement economy by two to three per cent. The effect is largest at moderate intensity and in recreational athletes, not in elites.
• Vitamin C from fruit and vegetables: relevant for collagen synthesis and iron absorption — 75 to 100 milligrams per day covers it, which is one bell pepper. As a high-dose supplement for performance it does nothing.
• Vitamin E from nuts and seeds: sensible in the diet as a fat companion; as a supplement for performance or soreness prevention, controlled trials show no effect.
• Zinc and selenium: both are cofactors of antioxidant enzymes. Deficiency hurts, overdose hurts too — more than 25 milligrams of zinc per day long term interferes with copper absorption. Supplement only on a confirmed deficiency. Details in the article on zinc in endurance sport.
• N-acetylcysteine and glutathione precursors: no robust performance evidence in sports practice; oral glutathione is largely broken down in the gut anyway. Not money you need to spend.

The pattern is unambiguous: what comes through food is unproblematic and occasionally useful. What comes as an isolated high-dose compound costs money and can dampen the training effect. Twenty euros a month for an antioxidant complex buys less benefit than a bag of frozen berries.

The dose question: where the line sits

There is no official threshold above which antioxidants block adaptation — the studies use different doses. But the literature suggests three bands that work well in practice.

Band 1 — eating normally (safe): five portions of fruit and vegetables a day, nuts, olive oil, spices. That is roughly 150 to 300 milligrams of vitamin C, 10 to 20 milligrams of vitamin E and several hundred milligrams of polyphenols per day. In this band no negative effect on training adaptation has been shown; if anything, fibre and polyphenols favour gut microbiome and inflammatory status.

Band 2 — targeted foods (situationally useful): tart cherry juice, berries, beetroot juice, turmeric, ginger — around hard blocks and races. This is about a few per cent of recovery, not performance gains. Placement matters: after the load, with enough distance to the next key session. More in the article on post-workout nutrition.

Band 3 — high-dose supplements (problematic): 1000 milligrams of vitamin C and more, 400 IU of vitamin E and more, daily and permanently. This is the band where the adaptation blockade is documented — and it is exactly the band sports nutrition marketing pushes hardest. Exception: a short high-dose course during a medically supervised infection. Not as training support.

Antioxidants on race day — the exception

On race day the logic flips. You do not want adaptation, you want performance. Here it is legitimate to keep oxidative stress low and support recovery after the finish. In practice that does not mean swallowing a vitamin C tablet in the morning, it means:

• Before the race: normal food, no new antioxidant supplement. A nitrate juice three hours before the start is the single exception with solid evidence — and even that belongs in training first.
• During the race: carbohydrate and sodium are the topics, not antioxidants. A drink with vitamin C is fine for taste, nothing more.
• After the finish: a meal with real fruit and vegetables, plus protein and carbohydrate. Adding 300 millilitres of tart cherry juice does no harm and supports recovery over the following days.

The real recovery lever is a different one, though: sleep, energy intake and training control. If you sleep four hours after a marathon and eat 1500 kilocalories short for two days, the antioxidant question is the wrong building site. The article on recovery in endurance sport puts the priorities in order.

In practice: one day that covers both

Instead of a supplement stack you need a pattern that separates TRAINING from recovery. A sample day for an athlete running 70 kilometres per week:

• Breakfast: oats with berries and yoghurt, plus a glass of water. Polyphenols and vitamin C from food, no supplement.
• Hard session in the morning: threshold intervals, 4 × 8 minutes. No antioxidant supplement beforehand — the oxidative stimulus is the point of the session.
• Lunch within 60 minutes of the session: potatoes or rice, chicken or lentils, a large salad with peppers and tomatoes. Protein for repair, carbohydrate for glycogen, vitamin C for iron absorption from the lentils.
• Afternoon: 200 to 300 millilitres of tart cherry juice if yesterday's session still sits in the legs. That is the only targeted antioxidant component of the day.
• Dinner: seasonal vegetables, olive oil, fish or tofu, nuts. Vitamin E as a natural fat companion.
• Before sleep: nothing green from a capsule, no pills. Sleep itself is the strongest antioxidant measure you have.

This is not a strict plan, it is a structure: plenty of plant food, no isolated high doses, targeted foods after hard loading. The difference to a supplement stack is not the effect but the price and the side-effect profile.

Common mistakes

• Antioxidants as insurance against overtraining. The marker that signals overtraining is not oxidation but a shifted load balance. Taking pills instead of cutting volume masks the problem. Warning signs are in the article on overtraining symptoms.
• Vitamin C right before the race. 1000 milligrams on race morning does nothing for performance and may disturb the signalling pathways you needed for the final adaptation.
• High-dose green tea extract. Concentrated EGCG capsules have caused liver injury in isolated cases — not an issue in tea form, but a real one at 800 milligrams and above as a capsule.
• Confusing “antioxidant” with nitrate. Beetroot juice is not a detox, it delivers nitrate for vasodilation. Different mechanism, different dose, different timing.
• Changing everything at once. Introducing new supplements in race week is the classic error. Anything you have not tested at least three times in training does not travel to the race.

How Peakora handles it

Peakora is not a nutritionist, but training control and nutrition building blocks interlock. The fueling calculator gives carbohydrate, sodium and fluid per hour for your estimated race time — the variables that actually matter on race day. The recipes in the Elite tier are built around recovery with real food rather than supplement lists. And because the engine tracks load through TSS, CTL and TSB, you can see when a hard week was too much — before you start compensating with pills.

For the fundamentals behind this, see the articles on 80/20 training and supplements in endurance sport.

Frequently asked questions about antioxidants

Are antioxidant supplements useful for endurance athletes?

As food yes, as high-dose pills mostly no. Oxidative stress is part of the training stimulus: blunting it with 1000 mg of vitamin C and 400 IU of vitamin E per day blocks part of the adaptation to endurance training. Food sources such as berries, vegetables and nuts deliver antioxidants in sensible amounts plus fibre and polyphenols.

Do antioxidants block the training effect?

High-dose single compounds can. Studies using 1000 mg vitamin C plus 400 IU vitamin E show an attenuated rise in mitochondria and endurance performance, because reactive oxygen species are missing as signalling molecules. At normal dietary doses this effect has not been demonstrated.

When should you take antioxidants?

If at all, after the session and not directly before it. High doses taken before or during training dampen the signalling effect of the stimulus. After the session, ideally through a meal with fruit and vegetables, is where a regeneration effect is most plausible.

Do tart cherry juice or beetroot help recovery?

There is reasonable evidence: tart cherry juice can slightly reduce soreness and inflammatory markers after eccentric loading, and nitrate from beetroot improves economy in endurance exercise by two to three per cent. Both are foods, not high-dose supplements, and neither belongs in the immediate build-up to an A race.