The recommended intake of vitamin C is 90 mg a day for adult men and 75 mg for adult women. Plasma levels are close to saturated somewhere around 200 to 400 mg a day, and above that most of what you swallow is absorbed less efficiently and excreted in urine. That single fact explains most of the confusion around this vitamin. It is genuinely essential, deficiency is genuinely bad, and the dose-response curve flattens far earlier than the supplement aisle implies.
The version of this article that stood here called vitamin C “the single most important vitamin required for human health” and described commercial ascorbic acid as a product of “toxic pollutants.” Both statements were wrong. The corrections are below.
What vitamin C actually does
Vitamin C is a cofactor for a family of enzymes, and its best-characterized job is holding collagen together. Prolyl and lysyl hydroxylases require ascorbate to hydroxylate proline and lysine residues in procollagen, which stabilizes the triple helix. Without it, the collagen the body builds is structurally unsound, as the Linus Pauling Institute sets out in detail.
It also acts as a water-soluble antioxidant, is required for carnitine and catecholamine synthesis, and substantially increases absorption of non-heme iron from plant foods eaten in the same meal. That last effect is among the most useful things about it and gets almost no marketing attention.
Humans, other primates and guinea pigs cannot synthesize it, because the gene for the final enzyme is non-functional. True, and it does not make vitamin C more important than the other vitamins you also cannot synthesize.
The numbers: RDA, upper limit, and the smoker adjustment
The NIH Office of Dietary Supplements sets the RDA at 90 mg daily for men and 75 mg for women aged 19 and over, with a Tolerable Upper Intake Level of 2,000 mg from all sources combined.
People who smoke are advised to add 35 mg a day. The rationale is oxidative load: smokers deplete ascorbate faster and run lower blood concentrations at any given intake. In the CDC’s analysis of serum vitamin C in US adults, deficiency in 2017–2018 ran at 14.6 percent among smokers against 4.4 percent among non-smokers.
| Intake | What it corresponds to |
|---|---|
| About 10 mg/day | The floor below which the deficiency state develops |
| 75–90 mg/day | RDA for adult women and men |
| 110–125 mg/day | RDA plus the 35 mg smoker adjustment |
| 200–400 mg/day | Plasma approaches saturation; further intake adds little plasma |
| 2,000 mg/day | Tolerable Upper Intake Level for adults |
Why very high oral doses mostly end up in urine
The pharmacokinetics were settled by depletion-repletion studies at the NIH. In the 1996 PNAS study by Levine and colleagues, seven healthy men were hospitalized for four to six months and given seven daily doses from 30 mg to 2,500 mg while plasma and cell concentrations were tracked.
The curve turned out to be sigmoid, with the steep part between 30 and 100 mg a day. Neutrophils, monocytes and lymphocytes were saturated at 100 mg; plasma was fully saturated at 1,000 mg. Bioavailability was complete for a single 200 mg dose and fell off sharply above that, and at 500 mg and above most of what was absorbed came straight back out in urine. At 1,000 mg, urinary oxalate and urate excretion rose.
The practical consequence is that intestinal transport saturates. A 1,000 mg tablet does not deliver ten times the plasma of a 100 mg tablet. It delivers a modest increment and an expensive urine sample.
Deficiency and scurvy, which are not historical curiosities
Below roughly 10 mg a day, symptoms appear within about a month: fatigue, bleeding gums, perifollicular hemorrhages, corkscrew hairs, poor wound healing, joint pain. All of it traces back to defective collagen.
Scurvy has not disappeared. The CDC figures above put overall deficiency at 6.8 percent of US adults in 2017–2018, essentially unchanged from 7.0 percent in 2003–2006, clustered in smokers, people eating under 20 mg a day, and lower-income households. Among adults taking a vitamin C-containing supplement, deficiency was 0.5 percent.
That is the honest case for supplementation, and it is narrow: it is about the bottom of the distribution, not about pushing an already-adequate intake higher.
What the Cochrane review on colds actually found
This is the claim that sells the most vitamin C, and the evidence is more specific than either side usually reports. The document is Hemilä and Chalker’s Cochrane review of vitamin C for preventing and treating the common cold, last substantively updated in January 2013.
On incidence in the general community, 29 trial comparisons with 11,306 participants gave a risk ratio of 0.97 (95% CI 0.94 to 1.00). Effectively nothing. The exception was a subgroup: in five trials totaling 598 marathon runners, skiers and soldiers under short bouts of extreme physical stress, several in subarctic conditions, the pooled risk ratio was 0.48 (95% CI 0.35 to 0.64). The authors were explicit that this is not the general population.
On duration, across 31 comparisons and 9,745 episodes, regular supplementation was associated with an 8 percent shorter cold in adults (3 to 12 percent) and 14 percent in children (7 to 21 percent). Eight percent of a seven-day cold is about thirteen hours. In seven comparisons covering 3,249 episodes, starting vitamin C after symptoms began showed no consistent effect.
So: no meaningful effect on how often adults got colds, a small effect on how long they lasted when taken daily beforehand, nothing once the cold had started, and one unusual subgroup where the effect was large. Anyone quoting only one of those four results is selling something. How nutrition relates to normal immune function is worth reading separately from the cold literature.
Where the high-dose hypothesis failed, and what the evidence cannot tell you
Linus Pauling’s proposal that gram-level vitamin C would change long-term health outcomes was tested properly and repeatedly. It did not hold up.
In the Physicians’ Health Study II, published in JAMA in 2008, 14,641 male physicians with a mean age of 64 took 500 mg of vitamin C daily or placebo for a mean of eight years. The hazard ratio for major cardiovascular events was 0.99 (95% CI 0.89 to 1.11).
Bjelakovic and colleagues’ Cochrane review of antioxidant supplements and mortality pooled 78 randomized trials and 296,707 participants. Vitamin C was neutral on all-cause mortality (RR 1.02, 95% CI 0.98 to 1.07). Beta-carotene and vitamin E, in the low-bias trials, came out slightly worse than placebo.
The intravenous route has been tested at doses no oral supplement can reach. In the LOVIT trial in the New England Journal of Medicine in 2022, adults with sepsis in intensive care given high-dose intravenous vitamin C had a higher risk of death or persistent organ dysfunction at 28 days than those given placebo. That is a hospital setting with nothing to say about a daily supplement, but it closes the door on the idea that more is reliably better.
None of that evidence is airtight. The Cochrane cold trials vary widely in dose, duration and how a “cold” was defined, and the extreme-physical-stress subgroup rests on 598 people across five studies, the kind of finding that replicates poorly. The large trials above enrolled well-nourished people who were already replete: they tested whether adding more to sufficiency helps, and answered no. Whether correcting a real deficiency helps is a different question with a much better answer.
The safety picture at the top end is not blank. Doses above 2,000 mg commonly cause diarrhea, nausea and cramping. Oxalate excretion rises at gram-level doses, which matters for anyone with calcium oxalate stones or hyperoxaluria. People with hemochromatosis or iron overload should be cautious, since vitamin C increases iron absorption.
What the previous version of this post claimed
Three claims stood here and should not have. The first was that ascorbic acid is “the single most important vitamin required for human health.” There is no basis for ranking vitamins that way — every essential nutrient is essential, and running out of any of them causes a deficiency state.
The second was that commercial vitamin C is made using “toxic pollutants” including acetone, perchloric acid and benzene, implying residues end up in the finished product. Solvents are used and removed in many food and pharmaceutical processes; finished-product purity is governed by specifications and testing. The framing was scare-mongering. The narrower, genuine question of whether nutrient form matters is covered in natural versus synthetic nutrients, and the whole-food vitamin C question in the article on camu camu.
The third was an ORAC value quoted for a discontinued product. USDA withdrew its ORAC database in 2012 on the grounds that the values had no demonstrated relevance to human health. We should not have been citing them then, and will not now.
What to actually do
Food covers the RDA easily. Per the NIH figures, one medium orange has about 70 mg, half a cup of raw red bell pepper about 95 mg, one kiwifruit about 64 mg, half a cup of cooked broccoli about 51 mg. Two or three of those in a day and you are done.
If supplementing, a modest dose is the rational one: 100 to 250 mg puts plasma near the top of its usable range at low cost. Vitamin C is water-soluble and heat-labile, so long boiling loses more of it than steaming or eating things raw.
Worth knowing: a scoop of any blended greens powder, ours included, delivers far less vitamin C than the doses used in the trials above. Label math is the only way to know what a given product actually provides.
Common questions
Does taking vitamin C when I feel a cold coming on help?
The Cochrane review looked at exactly this. Across seven comparisons and 3,249 episodes, therapeutic vitamin C started after symptom onset showed no consistent effect on duration or severity. The small duration effect that review found was only seen with regular daily supplementation started beforehand.
Is 1,000 mg a day safe?
It sits below the 2,000 mg upper limit, so for most healthy adults it is unlikely to cause harm beyond occasional gastrointestinal upset. It is also largely wasted, since absorption efficiency drops well below that dose. Anyone with kidney stone history, hyperoxaluria, iron overload or kidney disease should talk to a clinician before taking gram-level doses.
Do I need more vitamin C if I smoke?
The RDA adds 35 mg a day for smokers, making it 125 mg for men and 110 mg for women. NHANES showed deficiency roughly three times more common in smokers. Regular secondhand smoke exposure raises requirements too.
Does vitamin C help the body make collagen?
It is a required cofactor for the enzymes that hydroxylate collagen, so adequate intake is a precondition for normal collagen synthesis. Adequate is not the same as more being better, and there is no evidence that intakes above the RDA drive extra collagen production. That question, and the supplement claims around it, are covered in the article on collagen supplements.
The bottom line
Vitamin C is essential, deficiency still exists in the US, and the intake that fixes it is small and cheap. The evidence for anything beyond adequacy is thin, and the largest high-dose trials came back null.
Greens Plus builds blends around whole-food ingredients rather than single isolated nutrients, and our labels show what each serving contains. You can see the range at greensplus.com.
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