Vitamin C: What Colds, Iron Absorption and Kidney Stones Really Have to Do With It

Vitamin C is one of the best-selling dietary supplements of all – and also one of the most misunderstood. The research paints a more nuanced picture than the vitamin's reputation as a cold-fighter suggests: some effects are well supported, others considerably weaker than commonly assumed.

Vitamin C: a cofactor the body cannot make on its own

Ascorbic acid is water-soluble. Unlike most mammals, humans cannot synthesise vitamin C themselves, because the body lacks an enzyme needed for its biosynthesis. It must therefore come from food – regularly, since the body can only store limited amounts.

In the body, vitamin C acts as a cofactor for enzymes involved in collagen formation and as a water-soluble antioxidant that neutralises reactive oxygen compounds. These physiological functions are also the basis for the function claims scientifically assessed by the European Food Safety Authority (EFSA) regarding connective tissue, the immune system and tiredness – function descriptions, not promises of healing.

Colds: vitamin C shortens their duration but does not reliably prevent them

A Cochrane review (an RCT meta-analysis) evaluated trials using regular daily intake of at least 200 mg vitamin C. In adults, cold duration was reduced by an average of 8%, in children by 14%; at 1 to 2 g daily, the reduction in children reached 18%. However, the frequency of colds in the general population barely changed.

A more recent meta-analysis (2023) by the same research group found an additional reduction in symptom severity of around 15%. When vitamin C is taken only after symptoms begin rather than preventively, the authors note that the evidence is considerably thinner. In physically demanding conditions – such as marathon running – the preventive effect was stronger in some studies than in the general population.

Iron absorption: the effect is far larger from a single meal than across a full day

Vitamin C converts ferric iron from plant-based food into the more readily absorbed ferrous form and can partly offset inhibitors such as phytates and tannins. In a controlled study with 12 participants, iron absorption from a single meal increased by up to 100% when the vitamin C content was raised from 50 to 250 mg.

When the complete daily diet was varied over several days instead (51 to 247 mg vitamin C per day), the same research group found no significant difference in iron absorption. For people with iron deficiency, the combination remains a simple, well-studied measure nonetheless – more effective with a single iron-rich meal than spread across the day.

Kidney stones: an increased risk has so far mainly shown up in men

Part of the vitamin C the body absorbs is broken down into oxalate and excreted via the kidneys – a substance involved in the formation of calcium oxalate kidney stones. A prospective cohort study of more than 45,000 men found that those who regularly took vitamin C supplements had roughly double the risk of kidney stones compared with men who did not.

In women, a prospective cohort study of 85,557 participants found no such association: even at an intake of 1,500 mg vitamin C daily or more, stone risk did not differ from that at low intake. Why the sexes differ here has not been fully clarified. For people known to be prone to kidney stones, the recommendation is to use high-dose supplements cautiously.

Food sources: where vitamin C occurs naturally

Anyone wanting to cover their needs through diet will find vitamin C at varying concentrations in many vegetables and fruits. The following reference values (per 100 g) vary depending on variety, ripeness and preparation:

Food	Vitamin C (approx. per 100 g)
Sea buckthorn, raw	approx. 450 mg
Red bell pepper	approx. 120 mg
Parsley	approx. 160 mg
Broccoli, cooked	approx. 65 mg
Kiwi fruit	approx. 90 mg
Orange	approx. 50 mg
Potato, cooked	approx. 12 mg

Because vitamin C is sensitive to heat and light, prolonged cooking or storage reduces its content. Raw vegetables and brief steaming preserve the concentration best.

Forms of vitamin C: what sets supplements apart

Vitamin C is sold mainly as plain ascorbic acid, but also in buffered forms – such as calcium or sodium ascorbate – where the acid is bound to a mineral salt. Buffered forms are generally gentler on the stomach, since they are less acidic. Current research shows little difference in biological effectiveness between the forms; what ultimately matters is the total amount of ascorbic acid absorbed.

Dosage and upper limits: why the figures vary between authorities

Source	Figure
EU reference value (NRV)	80 mg/day
DACH reference values (adults)	approx. 95–110 mg/day, depending on sex
BfR recommendation for supplements	max. 250 mg per single dose
EFSA (earlier assessment, 2004)	up to 1,000 mg/day without increased risk of gastrointestinal effects
EFSA (current position, 2024)	no binding upper limit currently derived (data judged insufficient)
US health authorities	2,000 mg/day as the upper limit for adults

A pronounced deficiency – below roughly 10 mg per day for several weeks – leads to scurvy, with bleeding gums, fatigue and poor wound healing. This has become rare in countries with varied diets but can still occur with a very limited diet, in older age, or with heavy alcohol use. Because of increased oxidative stress, smokers have a roughly 35 mg higher daily requirement.

Side effects: when high doses cause gastrointestinal discomfort

At very high single doses, above the gut's individual absorption capacity, undigested vitamin C can act osmotically and trigger diarrhoea, bloating or nausea. This effect can usually be avoided by reducing the dose or splitting it into several smaller intakes over the day.

Interactions and particular caution during cancer therapy, pregnancy and breastfeeding

Anyone undergoing cancer treatment with chemotherapy or radiotherapy should not take high-dose vitamin C supplements without consulting a doctor first. It remains unclear whether vitamin C might also protect tumour cells during treatment, rather than only sparing healthy tissue. In one study, combining vitamin C with other antioxidants also weakened the cholesterol-lowering effect of a statin-niacin combination.

For pregnancy and breastfeeding: the usual dietary intake is considered safe. Less data exists for very high-dose supplements, which is why medical advice is recommended before starting supplementation. For children, too, doses above the age-appropriate reference values should only be used after consulting a paediatrician.

One important exception concerns people with a deficiency of the enzyme glucose-6-phosphate dehydrogenase (G6PD deficiency), a mostly inherited metabolic feature of red blood cells. In case reports, very high intakes – mostly intravenous doses in the gram range, less often oral – have been associated with haemolysis (accelerated breakdown of red blood cells) in this group. Anyone who knows they have G6PD deficiency should therefore only use high-dose vitamin C supplements after medical evaluation.

Conclusion: well-supported basic functions, more modest additional effects

For vitamin C, the evidence moves at two different speeds. The fundamental physiological functions are well supported – collagen formation, antioxidant activity, contribution to iron absorption from a meal. The claims that loom larger in public perception need to be judged more cautiously: reliable prevention of colds has not been shown in the general population, and for kidney stones, at least in men taking high doses, a risk signal has emerged that needs further research to interpret.

Anyone eating a balanced diet rich in vegetables and fruit will usually cover their needs without a supplement.

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This article provides general information about vitamin C and is not a substitute for professional medical advice, diagnosis or treatment. If you have health concerns, please consult a doctor or pharmacist.

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