Vitamin · C₆H₈O₆
Prevention does not hold up in the general population, daily use buys about half a day, and starting at the first symptom has never worked in trials.
Ascorbic acid; L-ascorbic acid; ascorbate; L-ascorbate. Buffered mineral salt forms sold as sodium ascorbate, calcium ascorbate and magnesium ascorbate; Ester-C is a branded calcium ascorbate preparation. Liposomal vitamin C and sustained-release vitamin C are delivery formats of the same molecule, not different compounds. Historical names: hexuronic acid (the name it was isolated under, before it was identified as the antiscorbutic factor) and the antiscorbutic vitamin. Dehydroascorbic acid (DHA) is the oxidised form the body interconverts with ascorbate, not a synonym for it.
Doses differ sharply by what they are for. For plasma and immune-cell repletion: Levine et al. 1996 concluded from steady-state kinetics that the RDA should be 200 mg/day, that white blood cells reach saturation at 100 mg/day, and that daily doses above 400 mg have no evident added value. Carr and Maggini 2017 propose 100 to 200 mg/day as the level that saturates plasma for infection prevention. For common cold trials: the Cochrane review's inclusion floor is 0.2 g/day, and 1 g/day is the most common regular-supplementation dose across the trial set. For treatment started at symptom onset: trials have tested 1 to 8 g/day. Hemilä 2017 argues the dose-response only becomes visible at 6 to 8 g/day, above the 3 to 4 g/day used in most of the null therapeutic trials, and Carr and Maggini likewise put treatment of an established infection at gram-level doses.
2,000 mg a day for adults 19 and older, the Tolerable Upper Intake Level set by the US Institute of Medicine in 2000 for vitamin C from food and supplements combined. It rests on osmotic diarrhea and other gut upset, which the panel judged likely in many people above 3 g a day. The limits for younger ages are 400 mg a day at 1 to 3 years, 650 mg at 4 to 8, 1,200 mg at 9 to 13 and 1,800 mg at 14 to 18, and the limit does not apply to vitamin C given under medical supervision. Separately, Levine et al. 1996 (NIH) concluded from their own kinetic and excretion data that safe doses are below 1000 mg/day, the dose at which oxalate and urate excretion rose in their volunteers, and in men, total intake at or above 1000 mg/day is associated with a 43% higher kidney stone risk (Ferraro et al. 2016), an association not seen in women.
Vitamin C stops you getting sick. Take a big dose at the first sniffle and you will fight the cold off, and take it every day through winter and you will not catch one in the first place. More is better, so make it a gram.
Across 29 placebo-controlled comparisons in over 11,000 people, taking vitamin C every day did not change how often people in the general population caught colds. What it did change was the illness itself: colds ran about 8% shorter in adults and 14% shorter in children, and 15% less severe across 15 trials, with the benefit falling on the severe symptoms rather than the mild ones. Starting vitamin C only once symptoms had already begun, which is how most people actually take it, showed no consistent effect across seven trials. Prevention does appear in one group: in five trials of marathon runners, skiers and soldiers on subarctic exercises, vitamin C halved cold incidence. And more is not straightforwardly better, because oral absorption saturates: white blood cells are full at about 100 mg a day, and past a single 500 mg dose the surplus is largely passed in urine.
The belief and the evidence are not answering the same question. Vitamin C does not keep a cold away; it makes the cold you get somewhat shorter and noticeably less rough, and only if you were already taking it before you fell ill.
In the five comparisons reporting symptom types separately, vitamin C shortened severe symptoms and left mild ones unchanged. All of them dosed daily, before the cold.
Only five comparisons split the symptoms this way, out of ten trials in the pooling. Severity was measured inconsistently, in some trials by counting days spent indoors, which mixes how ill someone felt with whether they could take time off work.
Plasma fills completely at 1000 mg a day and the white cells that fight infection at 100. Past a single 500 mg dose absorption falls off and the surplus leaves in urine.
This curve was mapped in seven volunteers living in hospital on a near-zero vitamin C diet for four to six months, which buys precision at the cost of a very small and unusual sample. It describes concentrations in blood and cells, not illnesses avoided. The work is thirty years old.
Across 24 comparisons, at least 0.2 grams a day left the chance of catching a cold essentially unchanged, a 3% difference whose range includes no difference at all.
The trials pooled here run from the 1960s onward, defined a cold in different ways, and mostly counted them by asking people. The two groups that agree worked from largely the same trial set, so this is one body of evidence read twice rather than two independent verdicts.
In healthy volunteers with no stone history, urinary oxalate went from 25 to 39 mg over three days, and calcium oxalate crystallised more readily in every group.
Nobody counted a stone here, and three days is nowhere near long enough to. Everyone took the vitamin and everyone was measured before and after, with no placebo group to compare against. The 2 gram group rose less than the 1 gram group, so dose and excretion do not line up cleanly.
Pooling five trials in marathon runners, skiers and soldiers, 598 people in all, daily vitamin C roughly halved the chance of a cold. In the wider community, no change.
Five small trials and 598 people between them, in settings almost nobody lives in: a subarctic military exercise, a 90 kilometre race, an Alpine ski camp. None of them tested ordinary gym-goers. The underlying trials date from the 1960s to the early 1990s and counted colds largely by asking people.
Across 31 comparisons covering 9,745 cold episodes, people already taking it daily recovered about 8% sooner. A later pooling put the drop in severity at 15%.
Earlier versions of this review called half a day of doubtful clinical usefulness. The pooled trials span six decades, with doses from 0.2 grams a day upward and no shared definition of a cold. One researcher has authored the review across its updates and argued this finding publicly for years.
Whether daily vitamin C shortens colds turns on which evidence counts. Every trial showing a benefit dosed people before the cold began, and a stricter overview of 8 primary studies found little or no effect.
In 48 adults picked for low blood levels, 13 weeks of 500 or 1000 mg raised glutathione inside infection-fighting white cells by 18%. Every one had a deficit to close.
Everyone in the supplement trial took the vitamin first and the placebo second, so an order effect cannot be separated from the treatment. Glutathione inside a lymphocyte is a laboratory reading, not an illness avoided. One author of the mechanism review is employed by a vitamin C manufacturer.
Across 40,536 men and 156,735 women followed 11 years, 1000 mg a day or more went with a 43% higher rate of first stones in men. Vitamin C from food tracked with nothing.
People who take a gram of vitamin C a day may differ from those who do not in ways a diet questionnaire misses, so this is a pattern rather than a cause. Hardly anyone ate more than 700 milligrams from food, which leaves the food comparison untested at the doses where the supplement finding appears.
Seven comparisons covering 3,249 cold episodes found no consistent effect on duration or severity, including a trial that began dosing within four hours of symptoms.
The strongest single trial here lost 251 of its 400 volunteers before any diary came back, and how ill someone felt could plausibly decide whether they filled one in. Its comparison group still took 0.03 grams of vitamin C rather than nothing. None of these trials went past 3 grams a day.
Vitamin C started at the first symptom failed in seven trials, all stopping at 3 to 4 grams a day. Two older trials hint an effect only emerges at 6 to 8 grams, and that dose has never been tested directly.
Over 11 weeks of endurance training, markers of muscle building new mitochondria rose on placebo and not on 1000 mg of vitamin C. Both groups gained the same 8% in oxygen uptake.
Vitamin C and vitamin E were given together, so neither can be credited or blamed on its own. The marker changes carry enormous spread: the placebo group's rise averaged 59% with a standard deviation of 97%. Reviewers add that the blunting is inconsistent and rarely reached in ordinary training.
No findings tagged to that goal.
What the evidence says about combining Vitamin C with each of these. The marker is how well established the interaction is; a caution is about the hazard, which is a different question.
6 open questions
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