Vitamin C is well studied and often overestimated. The mechanism is clear, the saturation curve has been measured in a tightly controlled study, and the cold trials have been pooled several times. Once you know those three things, most everyday questions become easier to answer : how much makes sense, whether it helps once you feel ill, and why taking more does not necessarily improve training outcomes.
What vitamin C is
A gene that stopped workingVitamin C is ascorbic acid, a small, water-soluble molecule. Most animals make it themselves from glucose. Humans cannot : the gene for the last enzyme in that pathway, L-gulono-gamma-lactone oxidase, is still present in our genome as a non-functional pseudogene. Other primates and guinea pigs cannot make it either. For us that makes vitamin C a true vitamin in the classic sense : the body needs it and has to take it in.
The body maintains only a limited vitamin C pool, regulated by absorption, tissue uptake and excretion via the kidneys, and with rising intake these cap the levels that can be reached. This is the first reason why the idea of stocking up does not work the way it does for fat-soluble vitamins.
How it works
One property, many jobsAlmost everything vitamin C does comes down to one chemical property : it readily donates electrons. That makes it a helper for enzymes that need a partner to stay active, a so-called cofactor. Four of these jobs are the best understood.
Collagen
Without vitamin C, stable collagen cannot be built. That is exactly what fails in scurvy : gums, skin and vessel walls lose their structural hold.
Carnitine
Carnitine shuttles fatty acids into the mitochondria. To make it, the body needs vitamin C as a helper for the enzymes involved.
Noradrenaline
The production of the messenger noradrenaline also depends on an enzyme that uses vitamin C as a cofactor.
Iron
Vitamin C lifts part of the inhibition by which phytate from grains and legumes slows the absorption of plant (non-haem) iron.
The same property makes vitamin C an antioxidant : it neutralises reactive molecules by donating electrons. This is the part that sells supplements, and it is the part that disappoints most in trials. A 2003 review from the group that measured the saturation curve summarised the state of research at the time : intervention studies with vitamin C had shown no change in markers of oxidation or clinical benefit. Later studies did lower individual oxidation markers, but a change in a biomarker does not by itself demonstrate a clinical benefit. That extra vitamin C gives people with adequate status a health advantage through its antioxidant effect has not been shown.
How much the body takes up
An S-shaped curveThe key study is small but unusually rigorous. Seven healthy men lived on a research ward for four to six months, were first depleted of vitamin C and then received defined daily doses from 30 to 2,500 milligrams. The blood level follows an S-shaped curve : it rises steeply between 30 and 100 milligrams a day, flattens from 200 milligrams and is completely saturated at 1,000 milligrams.
| Daily amount | What happened |
|---|---|
| 100 mg | Immune cells (neutrophils, monocytes, lymphocytes) saturated, at concentrations at least 14 times higher than in plasma |
| 200 mg | First dose beyond the steep part of the curve ; as a single dose, fully absorbed |
| 500 mg | At this single dose and above, the proportion absorbed falls and the absorbed surplus is excreted |
| 1.000 mg | Plasma completely saturated ; oxalate and urate excretion in urine rise |
The authors drew their own conclusion from the data : daily doses above 400 milligrams have no evident value. That is their interpretation of a study on blood and cell levels, not a tested threshold for every possible effect, but it is rarely quoted. By mouth, the level can only be raised to a limited extent. In another study, 1.25 grams by mouth produced a peak plasma concentration of about 135 micromoles per litre, while the same amount by infusion reached around 885. Absorption and excretion both work against ever higher levels.
What the official values say
Recommended intakes, not blood-level targetsThe German Nutrition Society (DGE) recommends 110 milligrams a day for men and 95 milligrams for women. The European Food Safety Authority arrives at the same figures. Both are derived from a blood level of about 50 micromoles per litre, the basis for estimating need, not a personal target. People who smoke turn over about 40 percent more vitamin C ; for them the DGE recommends 155 milligrams for men and 135 milligrams for women.
| Group | Per day | Source |
|---|---|---|
| Men | 110 mg | DGE, EFSA |
| Women | 95 mg | DGE, EFSA |
| Men who smoke | 155 mg | DGE |
| Women who smoke | 135 mg | DGE |
| Upper limit | 2.000 mg | US Food and Nutrition Board, total daily intake ; EFSA saw no adequate data to set one, which does not mean unlimited safety |
In everyday terms this is not much. Half a cup of raw red pepper provides about 95 milligrams, a medium orange about 70, a kiwi about 64, half a cup of cooked broccoli about 51. Two to three such vitamin C-rich portions a day can meet the reference intakes ; content varies with variety, storage and cooking. In a small study with 15 young men who started with below-average levels, one gold kiwifruit a day on top of their usual diet raised plasma levels above 50 micromoles per litre, and two brought them close to saturation.
How much vitamin C do you need a day?
The German Nutrition Society recommends 110 mg a day for men and 95 mg for women, and 155 and 135 mg for people who smoke. Two to three vitamin C-rich portions of fruit and vegetables, such as peppers, kiwi or broccoli, meet these values.
Does vitamin C still help once a cold has started?
In the 2013 Cochrane review, starting only once symptoms appeared showed no consistent effect on duration or severity. Taken regularly, colds in adults lasted 8 percent shorter on average, but did not become rarer in the general population.
Does vitamin C improve VO2max or strength?
With adequate status, no performance gain is shown : a meta-analysis found no significant effect on VO2max, endurance, strength or lean mass. High doses of around 1,000 mg combined with vitamin E blunted some training adaptations at the cellular level in several trials.
Above what amount does more vitamin C stop helping?
In a controlled study, immune cells were saturated at 100 mg a day and plasma at 1,000 mg. From single doses of 500 mg upwards a smaller proportion is absorbed, and the authors saw no evident value above 400 mg a day.
Does it make sense to combine vitamin C with zinc?
For the combination there are only two small pilot trials with 94 people and manufacturer involvement ; whether it does more than either alone was not tested. Zinc on its own has separate, though uncertain, evidence, which our zinc article puts in context.
That explains the physiology. Now the questions that matter day to day.
From physiology to practiceEverything above explains how the body handles vitamin C. It does not yet say whether it is worth taking once a cold starts, whether athletes gain anything from more, whether zinc belongs alongside it, and where the risks begin. Clinical trials help answer these questions, although the certainty of the evidence varies, and one of the best-known sources on the topic does not hold up on every point.
The rest is for clients.
Up to this point the article covers the mechanism. What follows moves from the mechanism to your own values, and that part we keep for the people we work with.
- Colds : take it regularly or only once your throat scratches, and the one group in which the risk was halved
- Training : what vitamin C does for VO2max, endurance and strength, and why 1,000 mg can blunt adaptation
- Zinc and cold duration : what the lozenge trials show, how uncertain the data are and where the limits lie
- Safety : kidney stones in men, distorted glucose sensors and who needs to be careful with high doses
- Rhonda Patrick's FoundMyFitness topic page, sources checked (as of 23 September 2026) : what holds and what does not
There is no self-service sign-up. Access comes with working together ; the first conversation is free and non-binding.
Put it in context
How your diet, training and blood test results fit together is what I put in context in the Longevity Check-up. Whether and in what amount you take a supplement is a decision for you and your doctor.
Sources (studies and reviews)
- Nishikimi M et al. (1994). Cloning and chromosomal mapping of the human nonfunctional gene for L-gulono-gamma-lactone oxidase, the enzyme for L-ascorbic acid biosynthesis missing in man. J Biol Chem 269(18):13685-8. PMID 8175804.
- Carr AC et al. (2017). Vitamin C and Immune Function. Nutrients 9(11). PMID 29099763.
- Padayatty SJ et al. (2003). Vitamin C as an antioxidant : evaluation of its role in disease prevention. J Am Coll Nutr 22(1):18-35. PMID 12569111.
- Hallberg L et al. (1989). Iron absorption in man : ascorbic acid and dose-dependent inhibition by phytate. Am J Clin Nutr 49(1):140-4. PMID 2911999.
- Levine M et al. (1996). Vitamin C pharmacokinetics in healthy volunteers : evidence for a recommended dietary allowance. Proc Natl Acad Sci U S A 93(8):3704-9. PMID 8623000.
- Levine M et al. (2001). A new recommended dietary allowance of vitamin C for healthy young women. Proc Natl Acad Sci U S A 98(17):9842-6. PMID 11504949.
- Padayatty SJ et al. (2004). Vitamin C pharmacokinetics : implications for oral and intravenous use. Ann Intern Med 140(7):533-7. PMID 15068981.
- German Nutrition Society (DGE) (2015). New Reference Values for Vitamin C Intake. Ann Nutr Metab 67(1):13-20. PMID 26227083.
- Carr AC et al. (2012). Bioavailability of vitamin C from kiwifruit in non-smoking males : determination of 'healthy' and 'optimal' intakes. J Nutr Sci 1:e14. PMID 25191543.
- Offizielle Stellungnahme : EFSA NDA Panel (2013). Scientific opinion on dietary reference values for vitamin C. EFSA Journal 11(11):3418. doi:10.2903/j.efsa.2013.3418
This article is educational and does not replace medical advice. StoaVita provides longevity and performance coaching, not medical treatment, gives no dosing recommendations and sells no supplements. Anyone with kidney stones, iron overload, a known G6PD deficiency or regular medication should discuss vitamin C supplements with their doctor first. Felix Baier is neither a physician nor a German-licensed Heilpraktiker. Anyone considering zinc supplements above 25 milligrams a day or wearing a glucose sensor should also discuss this with their doctor beforehand.