Vitamin C

What vitamin C does in the body

Unlike most mammals, humans cannot make ascorbic acid themselves, and it is not stored either. Its core task is enzymatic: it keeps iron atoms in the active center of certain enzymes reduced, including the prolyl and lysyl hydroxylases, without which collagen is not stably cross-linked. That is why in scurvy the gums bleed and wounds do not heal — an enzyme defect, not an antioxidant problem.

The second role concerns the immune cells: neutrophils, monocytes and lymphocytes actively accumulate vitamin C and contain concentrations at least 14 times higher than plasma. It does not follow that more vitamin C means more immune function in a person who is already saturated — and that is exactly where advertising comes in.

The cold question: three cases, three answers

The Cochrane review by Hemilä and Chalker is the reference. It analyzes only placebo-controlled studies with at least 0.2 g a day and separates what advertising merges into one promise: continuous intake, starting at symptoms, extreme strain.

Case one, continuous intake, endpoint frequency: 29 study comparisons with 11,306 participants. In the general population, 10,708 participants, the pooled risk ratio is 0.97 (0.94 to 1.00). Anyone who takes vitamin C daily over the winter does not catch colds less often — a clear, well-supported negative finding.

Case two, the same continuous intake, endpoint duration and severity: 31 comparisons covering 9,745 episodes. In adults, colds are shortened by 8 percent (3 to 12 percent), in children by 14 percent. A 2023 meta-analysis by the same authors evaluated severity: 15 comparisons from 10 randomized double-blind studies, result minus 15 percent (9 to 21 percent). In the 5 comparisons that contrasted mild and severe symptoms, the effect concerned the severe ones.

Case three is the most common in practice: you only reach for the effervescent tablet when your nose is running. For this, 7 study comparisons with 3,249 cold episodes are available, and they show no consistent effect. Hemilä later suspected that these studies had used too low a dose at 3 to 4 g a day — a hypothesis, not a replication.

The special case explains the rest. In 5 studies with 598 marathon runners, skiers and soldiers on subarctic exercises, the risk ratio falls to 0.48 (0.35 to 0.64), that is, a halving. The European legislator has adopted exactly this: the only EU claim linking vitamin C with physical exercise requires 200 mg in addition to the recommended daily intake. For recreational sport, the finding does not exist.

How much actually gets in

The dose–concentration curve is sigmoid: its steep section lies between 30 and 100 mg a day; the immune cells are already saturated at 100 mg, the plasma only at 1,000 mg. From a single dose of 500 mg, bioavailability falls and what is absorbed goes into the urine; above 400 mg a day, the authors saw no discernible benefit. The argument for spreading intake over the day is therefore correct — it speaks for small portions, not for a larger total amount.

Iron and sport: two limited promises

That vitamin C makes plant iron available is mechanistically solid: it reduces trivalent to divalent iron and forms a soluble complex that is absorbed even alongside phytate. A molar ratio of about 2:1 is needed, for example 20 mg of ascorbic acid to 3 mg of iron. The limitation lies in the time scale: Cook and Reddy measured absorption over a complete diet instead of from a test meal, with 12 participants over three periods of 5 days each. Between mean daily intakes of 51 to 247 mg, iron absorption did not differ significantly. The bell pepper with the lentil soup works; a capsule at some point in the day does not, measurably.

In sport, something similar applies with the opposite sign. That exercise generates oxidative stress and vitamin C dampens it can be shown with markers; that this is a good thing does not stand up to scrutiny. In a double-blind study, 14 men between 27 and 36 years of age trained for 8 weeks, 5 of them on 1 g of vitamin C daily. Their endurance performance developed less well (p equal to 0.014). The position stand of the International Society of Sports Nutrition classifies it this way: antioxidants can support recovery, but in supraphysiological doses can hinder training adaptations.

What the studies show

Cochrane review on the common cold, 2013

Systematic review of all placebo-controlled studies with at least 0.2 g a day. On frequency under continuous intake, 29 comparisons with 11,306 participants contributed; in the general population with 10,708 participants, the risk ratio was 0.97 (0.94 to 1.00), and the endpoint was missed. On duration, 31 comparisons with 9,745 episodes contributed. In the subgroup under extreme strain, 5 studies with 598 participants, the risk ratio was 0.48.

Levine and colleagues, dose–concentration curve, 1996

Seven healthy volunteers were admitted as inpatients for 4 to 6 months, put on less than 5 mg a day and then taken through seven daily doses from 30 to 2,500 mg. The primary endpoint was the relationship between dose and steady-state concentration in plasma and cells. Complete plasma saturation occurred at 1,000 mg, the immune cells were already saturated at 100 mg, and at 1,000 mg a day oxalate and urate excretion were increased.

LOVIT, intravenous vitamin C in sepsis, 2022

Randomized placebo-controlled study in 872 adults in intensive care receiving vasopressors for sepsis: 50 mg per kilogram of body weight intravenously every 6 hours for up to 96 hours. The primary endpoint was death or persistent organ dysfunction on day 28: 44.5 percent with vitamin C versus 38.5 percent with placebo, risk ratio 1.21 (1.04 to 1.40), p equal to 0.01 — reached to the disadvantage of vitamin C. The situation is one of intensive care medicine, but it refutes the assumption that more is always unproblematic with a water-soluble vitamin.

Where the data stop

The only clear effect on frequency rests on 5 studies with 598 participants under marathon, skiing and subarctic military conditions; for recreational sport it does not exist. Between continuous intake, which works consistently, and starting at symptoms, which does not, there is an unexplained contradiction. For skin and joints, the EU claims speak of normal collagen formation, that is, of the normal state with adequate supply; figures on an additional benefit are missing. Liposomal formulations achieved higher peak concentrations in 9 of 10 studies in a scoping review (factor 1.2 to 5.4), but none measured excretion.

Status, approval and legal

Vitamin C is an essential nutrient and is permitted in food supplements as well as in fortified foods. The reference values of the German Nutrition Society (DGE) are 110 mg a day for men and 95 mg for women, 155 and 135 mg respectively for smokers. According to the German National Nutrition Survey II, the mean intake of adults was around 100 mg a day; the recommended intake is therefore reached. The German Federal Institute for Risk Assessment (BfR) proposes 250 mg per recommended daily portion as the maximum amount for food supplements. Authorized EU claims concern, among other things, the immune system, iron absorption and collagen formation. Vitamin C is not on the World Anti-Doping Agency’s Prohibited List.

Safety

The US Institute of Medicine sets a Tolerable Upper Intake Level of 2,000 mg a day for adults; EFSA has not derived such a value but considers an additional 1,000 mg a day not to be associated with gastrointestinal effects. The DGE names 3 to 4 g a day as the range from which temporary diarrhea can occur. More relevant than tolerability is oxalate metabolism: in the analysis of 156,735 women and 40,536 men over a median of 11.3 to 11.7 years with 6,245 stone events, a total intake of 1,000 mg a day or more was associated with a hazard ratio of 1.43 (1.15 to 1.79) for kidney stones in men; not in women, and vitamin C from food in neither sex. According to the DGE, people prone to stones, with impaired kidney function or with an iron utilization disorder should not exceed the limit of 1 g; in hemochromatosis, vitamin C can increase iron overload. It can also weaken the HDL increase under niacin plus simvastatin. During ongoing chemotherapy, the question belongs with the treating oncology team.

Sources

• Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev 2013
• Hemilä H, Chalker E. Vitamin C reduces the severity of common colds: a meta-analysis. BMC Public Health 2023
• Levine M et al. Vitamin C pharmacokinetics in healthy volunteers. Proc Natl Acad Sci USA 1996
• Cook JD, Reddy MB. Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet. Am J Clin Nutr 2001
• Gomez-Cabrera MC et al. Oral administration of vitamin C decreases muscle mitochondrial biogenesis. Am J Clin Nutr 2008
• Lamontagne F et al. Intravenous Vitamin C in Adults with Sepsis in the Intensive Care Unit (LOVIT). N Engl J Med 2022
• Ferraro PM et al. Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones. Am J Kidney Dis 2016
• Carr AC. Do Liposomal Vitamin C Formulations Have Improved Bioavailability? Basic Clin Pharmacol Toxicol 2025
• German Nutrition Society (DGE): reference values for vitamin C
• German Federal Institute for Risk Assessment (BfR): proposed maximum levels for vitamin C in foods including food supplements