Training
Antioxidants and adaptation
The oxidative stress produced by hard exercise looks like damage, and the instinct is to mop it up. It is also the signal that tells the muscle to adapt, and there is trial evidence that mopping it up costs you some of the adaptation.
What most people believe
Exercise creates free radicals, free radicals are bad, so antioxidant supplements help recovery and are at worst harmless.
What the evidence actually says
The signal that looks like damage
Contracting muscle produces reactive oxygen species. For a long time these were understood purely as wear — something to be neutralised — and the supplement industry was built on that reading.
It turned out they are also messengers. The rise in oxidative stress after a hard session is part of how the cell knows it was worked: it switches on the genes that build mitochondria and improve antioxidant defence. The stress is the instruction.
Which sets up the awkward result. Flood the system with high-dose antioxidants at the wrong moment and you suppress the instruction along with the stress. Several controlled trials have found exactly that — training adaptations blunted in the supplemented group relative to placebo, in people doing identical training.
What the doses look like
The trials that found blunting used doses well above what food delivers — the kind printed on a sports supplement rather than found in fruit. The table gives the figures used in the best-known of them.
This is one of the clearer illustrations of a principle that runs through this whole site: a nutrient is not a substance with a direction. The same compound at dietary amounts and at pharmacological amounts is doing two different things, and the second is not simply more of the first.
Why food does not have this problem
No study has shown blunted adaptation from eating fruit and vegetables, and the reason is dose and delivery. A large serving of berries provides an order of magnitude less vitamin C than the capsules in these trials, arriving slowly, alongside hundreds of other compounds.
So the practical conclusion is not "avoid antioxidants". It is the reverse of what the supplement aisle suggests: get them from food freely, and be sceptical of the tub.
When suppressing the signal is the right call
There is a real exception, and it follows from the mechanism. If the goal is not to adapt but to recover as fast as possible — three matches in a week, a stage race, a tournament — then damping the response may be a reasonable trade.
That is a narrow, tactical use during competition, not a training-phase habit. During a build, the adaptation is the entire point and interfering with it is paying money to train less effectively.
The numbers, and how firmly they are held
| What it describes | Figure | Evidence |
|---|---|---|
| Vitamin C dose that blunted adaptation Daily, in trial conditions | 1,000 mg | Probable |
| Vitamin E dose in the same trials | 235 mg | Probable |
| Antioxidants from food No trial has shown blunted adaptation from eating them | no evidence of harm | Probable |
Marked probable rather than established because the trials are relatively few and not all agree. The direction of the finding is consistent; its size is not settled.
Established means the position stands agree and you can act on it. Probable means the weight of evidence points one way with real dissent. Contested means it is genuinely unsettled, and anyone selling you certainty about it is selling something.
What follows is easier when you know what you are already eating.
What to actually do
Eat the fruit, skip the capsule
Food doses have never shown this effect. The problem appears at supplement doses, which are an order of magnitude higher.
If you do take them, keep them away from training
The interference is with the post-exercise signalling window. Timing them away from sessions is the least-bad version of taking them.
Separate competition from build
Damping the response can be sensible in a congested fixture list and is counterproductive during a training block.
Sources
- Vitamin C and E supplementation blunts endurance training adaptations
- Antioxidant supplementation and exercise adaptation — a review
This page is education, not medical advice. It does not diagnose anything and it is not a substitute for a clinician who knows your history. Sports nutrition in particular is a field where the evidence moves, and where a figure that suits a trained twenty-five-year-old may be wrong for you.
Every nutrient named above, measured — in the app.