Vitamin C, tart cherry and the adaptation you might be blunting

Everyone knows exercise produces free radicals, and everyone knows free radicals are bad. Both halves of that are roughly true and the conclusion drawn from them is wrong.

Reactive oxygen species are not just damage. They are signalling molecules. The transient rise in oxidants during a hard session is one of the ways the muscle cell learns that the session happened — it activates the pathways that build new mitochondria, upregulate your own antioxidant enzymes and improve insulin sensitivity. Flatten that signal with a large dose of an antioxidant supplement and you can flatten some of the response to training along with it.

This is one of the more counter-intuitive findings in exercise physiology, and it is also one where the evidence is genuinely more nuanced than the headline. Here is what has actually been shown, and where it stops.

The signal, not the damage

During contraction, muscle produces superoxide and hydrogen peroxide from several sources. At low to moderate concentrations these oxidants activate redox-sensitive transcription pathways — MAPK signalling, NF-κB, and importantly PGC-1α, which is the master regulator of mitochondrial biogenesis. They also switch on NRF2, which increases your own production of superoxide dismutase, catalase and glutathione peroxidase.

That last point is the one people miss. Training makes you better at handling oxidants. A trained cyclist has a stronger endogenous antioxidant system than an untrained one, and that improvement is itself driven by the exposure. Supply the defence from outside in large amounts and the cell has less reason to build its own.

What the studies actually found

Three pieces of work carry most of the weight here, and they say slightly different things.

The 2008 vitamin C trial. Untrained men trained for eight weeks; half took 1 g of vitamin C daily. The supplemented group improved their aerobic capacity less than the placebo group. In parallel rat work, vitamin C reduced the exercise-induced increase in mitochondrial biogenesis markers and in endogenous antioxidant enzymes. This is the study that started the argument, and its performance outcome is the strongest single result in the field.

The 2009 insulin sensitivity study. Four weeks of training improved insulin sensitivity in men taking placebo. In men taking 1000 mg vitamin C and 400 IU vitamin E, it did not — and the training-induced increase in their own antioxidant defences was blunted too. The health outcome is the striking part: this is not only a performance question.

The 2014 eleven-week endurance trial. Here is where honesty matters. Trained participants took 1000 mg C and 235 mg E through eleven weeks of endurance training. The markers of mitochondrial biogenesis — COX4, cytochrome c, PGC-1α — rose less in the supplemented group. But maximal oxygen uptake and running performance improved similarly in both groups. The cellular signal was clearly blunted; the eleven-week performance outcome was not.

That is the real state of the evidence. The mechanism is well demonstrated. The bridge from blunted markers to a slower rider is plausible, partially supported, and not proven in every study. Reviews of the whole literature land on "the weight of evidence suggests interference, and the effect is not universal". Anyone telling you it is settled in either direction is overselling.

The dose is the whole argument

An orange has around 70 mg of vitamin C. A varied diet with a decent quantity of vegetables might supply 150–300 mg across a day, absorbed slowly and distributed among everything else in the food. The supplement studies used 1000 mg in one hit, often timed close to the session. Those are different interventions, and treating them as the same thing is the most common error in this discussion.

The same logic applies to polyphenols. Blueberries, tea, coffee and cocoa are part of a normal diet and nobody has demonstrated that any of them impairs training adaptation. A concentrated extract taken at pharmacological dose within an hour of every hard session is a different proposition, and it has not been studied enough to reassure you.

Where tart cherry sits

Tart cherry is the most-studied of the fruit polyphenol products, and the evidence for it is real but narrower than the marketing implies. Following a marathon, cherry juice was associated with faster recovery of isometric strength and lower inflammatory markers. A later meta-analysis supported a modest benefit for the recovery of muscle function, with weaker and less consistent effects on soreness and on blood markers of damage.

Two caveats. The effect is on recovery of function in the next day or two, which is a different question from whether chronic use dulls long-term adaptation — and that second question has essentially not been tested for cherry. And the studies mostly used eccentric or muscle-damaging protocols, which cycling largely is not. On a bike you do very little eccentric work, so the damage tart cherry is good at addressing is damage you may not be generating.

The rule that falls out of all this

Food-level antioxidants always. High-dose supplements only around competition. Never through a build block.

That sentence is doing a lot of work, so here is what it means across a season.

The underlying principle is worth stating separately, because it applies well beyond antioxidants. Adaptation is a response to a disturbance. Anything that reduces the disturbance reduces the response. This is the same reason an anti-inflammatory taken after every session is a bad habit, and the same reason a training block that never leaves you tired never makes you fitter. Recovery is training makes the constructive version of that argument, and base miles and building an aerobic engine is mostly a long essay about protecting the mitochondrial signal you are trying to blunt here.

What to do instead

• Eat plants, in quantity, at meals. Five to seven portions of fruit and vegetables a day is a nutritional baseline, not an antioxidant strategy. It does not need optimising.
• Get protein and carbohydrate right after hard sessions. That is where the recovery leverage genuinely is. Do you need protein after every ride and recovery drinks or a meal cover the useful version.
• Reserve concentrated products for congestion. Three race days in four is a recovery problem. A Tuesday interval session in February is not.
• Correct deficiencies as deficiencies. If a blood test says you are short of something, treating it is medicine, and none of the above applies. That is a separate conversation with someone qualified to have it.
• Do not confuse nitrate with antioxidants. Beetroot juice and nitrates operates on a different pathway and does not carry this concern.

The cleanest way to hold all of this is to remember what you are actually buying with your training hours. You are not buying fatigue and you are not buying kilometres. You are buying a set of cellular signals that tell the muscle to build something new. It seems a waste to spend the morning generating them and the afternoon washing them away.