Vitamin C in Endurance Sport: Benefits and Risks

Few nutrients are as tightly linked to the wish for health and performance as vitamin C. It is regarded as a cold shield and as an antioxidant that is supposed to protect against the rigours of training. Both sound logical — and both lead you astray if you pick the wrong dose. This article separates benefit from risk: what vitamin C really does in endurance sport, where it demonstrably helps, and why too much of it can even take away your training adaptation.

What vitamin C does in the body

Vitamin C, chemically ascorbic acid, is a water-soluble vitamin and a central building block for connective tissue. It is a cofactor for collagen synthesis and plays a role in wound healing, immune defence and protecting cells from oxidative stress. Unlike fat-soluble vitamin E, it acts in the watery environment of cell plasma and blood.

The body cannot make vitamin C itself — humans depend on dietary intake. The stores are limited: if daily intake stays in the low double-digit milligram range, the body's pools empty within a few weeks. At the same time, from around 200 milligrams of oral intake per day, tissue stores are saturated in most people. Anything beyond that is excreted through the kidneys, not stored.

This matters for endurance athletes because intense training drives cellular metabolism and raises the demand on antioxidant defence systems. The widespread conclusion "more training, so more vitamin C" is only partly right — as the following sections show.

Vitamin C and the immune system

The idea of vitamin C as a cold remedy goes back to the chemist Linus Pauling, who promoted high doses in the 1970s. The Cochrane review of more than 30 studies paints a nuanced picture: in the general population, ongoing vitamin C supplementation barely reduces the number of colds — but it shortens the duration by around eight percent in adults.

More interesting is the view of exceptional situations. In people under extreme physical stress — marathon runners, ultramarathon athletes or soldiers in severe cold — a daily intake of 200 to 1,000 milligrams halved the cold risk versus placebo in some analyses. The reason: these extreme loads measurably depress immune function for a while. Endurance sport at this dose is therefore a genuine special case — though only at very high volumes.

From a training perspective that means: a solid vitamin C supply through fruit and vegetables is worthwhile, but a blanket mega-dose approach as "cold insurance" is not. Anyone who is a little more susceptible for a few days after a hard session or a race benefits more from good sleep, adequate energy intake and carbohydrates than from high-dose vitamin C.

The best-supported benefit: iron absorption

Where vitamin C genuinely shines in endurance sport is iron absorption — and that is a central issue for runners, cyclists and triathletes. Iron deficiency is the most common deficiency in endurance sport, and even more frequent in female athletes than in men. What matters is the distinction between two forms of iron.

Haem iron from meat and fish is absorbed well by the body regardless of accompanying nutrients. Non-haem iron from plant sources — wholegrains, legumes, nuts, green leafy vegetables — is harder to dissolve and strongly dependent on the meal. This is exactly where vitamin C comes in: it reduces trivalent iron (Fe³⁺) to divalent iron (Fe²⁺) and keeps it in a soluble form that the gut absorbs more easily.

The effects are substantial. Just 50 milligrams of vitamin C with a meal can increase non-haem iron uptake two- to threefold. Conversely, coffee, tea, milk and calcium markedly inhibit absorption. In practice this means: oats with berries and a little lemon juice, a lentil stew with peppers or a glass of orange juice with wholemeal bread do more for your iron level than any isolated pill. If you take iron tablets for a deficiency, take them with vitamin C or a glass of fruit juice — never with coffee or tea.

The risk: high doses and training adaptation

Here the downside begins. Endurance training does not work through the load alone, but through the body's response to it. Hard sessions produce oxidative stress and free radicals — and precisely these signals are part of the adaptation stimulus. They activate pathways that lead to new mitochondria, more capillaries and improved endurance performance.

Very high antioxidant doses can suppress this stimulus. Several controlled studies showed that combining vitamin C (1,000 milligrams) and vitamin E weakened training-induced adaptations: the gain in endurance performance and the mitochondrial adaptations were smaller than in the control group without supplements. Free radicals are therefore not only damage but also messengers — anyone who neutralises them wholesale partly erases the training response.

This does not mean vitamin C is dangerous — it means the dose decides. In a normal diet the effect is negligible. Only isolated high-dose intake from around 1,000 milligrams per day flips the benefit into its opposite. For athletes, then, the simple rule applies: antioxidants from food yes, high-dose powder before training rather not.

How much vitamin C endurance athletes need

Official recommendations in the EU are 95 to 110 milligrams per day for adults. This amount prevents deficiency diseases such as scurvy but does not necessarily cover the increased needs of people training intensely. For endurance athletes, 100 to 200 milligrams a day is a sensible target — the level at which tissue stores are saturated in most people.

The good part: this amount is easily reached through normal nutrition. One pepper, a handful of berries, one kiwi or one orange each supply a substantial share of the daily requirement. Fruit and vegetables also deliver vitamin C in a package with secondary plant compounds that isolated supplements cannot replace. The following overview shows what matters in practice:

• Requirement: 100–200 mg per day for endurance athletes — stores are saturated from around 200 mg oral.
• Timing: Combine with iron-rich meals, never with coffee or tea.
• Sources: Peppers, citrus fruit, berries, kiwi, broccoli — fresh and gently prepared.
• Caution: High-dose supplements from 1,000 mg daily can blunt adaptation.
• Loss: Vitamin C is heat- and oxygen-sensitive — long storage and long cooking reduce the content.

A word on shelf life: vitamin C is sensitive to heat, light, oxygen and long storage. Freshly harvested and briefly cooked vegetables contain far more than produce stored for weeks or boiled for hours. In practice this is less about culinary art than kitchen logic: steam briefly rather than simmer long, choose fresh over canned.

Common mistakes

• Mega-doses as "cold protection". The hope of fending off every infection with 2,000 mg a day does not hold up to the evidence — and may even blunt adaptation.
• Vitamin C only as an isolated pill. The synergy with plant compounds is missing; fruit and vegetables are the better source.
• Antioxidants right before training. If you want to set training stimuli deliberately, do not take high doses immediately before the session.
• Pairing iron intake badly. Swallowing iron tablets with coffee or tea costs most of the effect — vitamin C boosts it.
• Boiling vegetables to death. Long cooking times destroy much of the sensitive vitamin.

How Peakora thinks nutrition and load together

Nutrients never act in isolation, but in the context of your training load. Peakora links your sessions through training stress (TSS) and the fitness curve (CTL), so you can see in which weeks your body needs especially much recovery. The recovery monitor tracks your form (TSB) — in high-load phases it is worth looking at a solid nutrient supply, while in easy weeks the demand is lower.

For iron in particular the link is important: how to spot and fix deficiencies is explained in Iron deficiency in endurance sport, and the overview of supplements you need is in Supplements in endurance sport. What the key lab values mean is covered in the Peakora lexicon. How to periodise your overall fuelling is summarised in the guide to marathon training.

Frequently asked questions about vitamin C in endurance sport

Does vitamin C protect endurance athletes from colds?

No, at normal doses it does not reduce the number of colds in the general population. In people under extreme physical stress — marathon runners, ultrarunners, soldiers in the cold — a daily intake of 200 to 1,000 milligrams shortens the duration by around eight percent in adults.

Does vitamin C help you absorb iron better?

Yes, this is the best-supported benefit. Vitamin C is one of the strongest enhancers of iron absorption: just 50 milligrams with a meal can increase the uptake of plant-based (non-haem) iron two- to threefold. For athletes with iron deficiency, combining it with citrus, peppers or berries is therefore especially worthwhile.

Can high doses of vitamin C harm your training?

Yes. Very high doses — from around 1,000 milligrams daily — can blunt training adaptation, because as an antioxidant they dampen the oxidative signals that endurance training is meant to trigger. In studies, high-dose vitamin C and E supplementation produced weaker gains in endurance and mitochondria.

How much vitamin C do endurance athletes need per day?

For athletes, 100 to 200 milligrams per day is a sensible target — the level at which tissue stores are saturated. The EU recommends 95 to 110 milligrams daily; for endurance athletes, a moderate 150 to 200 milligrams is a good guide, attainable through fruit and vegetables.

Do you lose much vitamin C through sweat?

Only marginally. Vitamin C is water-soluble, but only a small part of daily turnover is lost through sweat. The higher demand in intense training is driven mainly by metabolism and cellular recovery, not by sweat loss.