The Cochrane review of decades of trials settled vitamin C's cold claims: regular daily supplementation does not prevent colds in ordinary people, shortens them slightly (about 8% in adults, 14% in children), and starting it only when symptoms begin does little — with one striking exception: people under heavy physical stress (marathon runners, soldiers in training) halved their cold incidence. Deficiency, meanwhile, is quietly returning in restricted diets and heavy alcohol use, and responds completely to modest doses. Absorption saturates around 200 mg per sitting, making megadoses mostly expensive urine — with real kidney-stone and lab-interference costs at gram levels.
What vitamin C is
Ascorbic acid is a water-soluble antioxidant humans famously cannot synthesize — a broken gene separates us from most mammals — making dietary intake obligatory. Its day jobs: collagen cross-linking (vessels, skin, gums, bone), carnitine and neurotransmitter synthesis, iron absorption enhancement, and immune-cell antioxidant support. Deficiency is scurvy — the sailors' disease whose modern returns the FAQ maps — and full-blown deficiency needs only ~10 mg/day to prevent, which is why it's rare and why the interesting questions all concern doses far above need.
Evidence by use
The cold file — settled by Cochrane. Across dozens of placebo-controlled trials and 10,000+ participants: prevention in ordinary populations — no (incidence unchanged at ≥200 mg/day); duration — slightly (~8% shorter in adults, ~14% in children, with regular daily use before the cold arrives); treatment-on-symptoms — essentially nothing in most trials; and the genuine outlier — heavy acute physical stress (marathoners, skiers, soldiers on exercises), where incidence roughly halved. That last finding is real, replicated within its niche, and the honest kernel every winter's marketing inflates; the whole aisle is ranked in Do immune boosters work?
Deficiency and near-deficiency — the strong column: scurvy's rapid reversal, and the bleeding-gum and poor-healing gradients in low-intake groups.
Iron — a legitimate supporting role: vitamin C with meals meaningfully boosts non-heme (plant) iron absorption; relevant to vegetarian iron strategies.
The megadose frontier — cancer (oral trials negative; IV research is a separate, unresolved clinical question), heart disease, gout (small urate reductions) — nothing that survived controlled testing at decision grade.
Doses studied
RDA 75–90 mg (smokers +35); cold-duration trials used 200–1,000+ mg daily, with the absorption ceiling (FAQ) arguing the low end achieves what the high end pretends to. Food covers it comfortably: a guava, a pepper, or citrus-plus-anything clears 100 mg — one reason supplement trials struggle to beat well-fed placebos.
Safety
Water-soluble and forgiving at sensible doses. At grams: osmotic diarrhea and cramping (~2 g threshold), oxalate kidney stones — the meaningful risk, especially in prior stone-formers, since ascorbate metabolizes to oxalate — and lab interference: high-dose vitamin C can distort some glucose-meter readings and stool/urine tests (relevant to diabetes self-monitoring — mention it, pause before tests). Kidney disease lowers the ceiling; iron-overload conditions (hemochromatosis) should avoid enhancement doses; and "buffered"/liposomal formats change comfort more than outcomes.
Where it appears in supplements
Solo tablets (honest, cheap), effervescent gram-bombs (the ceiling's favorite customers), gummies (with sugar irony), and every immunity blend at amounts diet already supplies. As with the rest of the aisle: dose printed, brand tested, claims sized to Cochrane.
Bottom line
Eat the fruit and peppers; that's the whole prescription for most people. Supplement rationally where it earns: 100–200 mg daily through winter if the small duration effect appeals, deliberately if you're a marathoner-or-equivalent, with iron for plant-based iron strategies, and diagnostically wherever scurvy's forgotten face fits the story. Skip the grams — they buy diarrhea thresholds and stone chemistry with money the transporters already declined.
Frequently asked questions
Why did Linus Pauling get it so wrong?
Pauling — a genuine double-Nobel giant — extrapolated from thin early data and personal conviction to gram-dose universal claims, and his fame outran the trials that followed. The episode is the supplement world's founding cautionary tale: authority and mechanism are not outcomes, and the doses he championed were later shown to mostly exit unabsorbed. His cancer claims fared worse still in controlled tests.
How much vitamin C is actually absorbable?
Transporters saturate: near-complete absorption up to ~200 mg per dose, falling steeply after — at 1,000 mg, roughly half or less is taken up and plasma levels plateau regardless. Practical translation: food amounts and 100–200 mg doses achieve what 2,000 mg achieves, minus the diarrhea threshold (~2 g+) and stone chemistry. Divided small doses beat single megadoses if supplementing at all.
Who actually gets scurvy today?
It never left: severely restricted eaters (tea-and-toast elderly, extreme "carnivore" adherents, food-insecure households), heavy alcohol dependence, malabsorption, and some psychiatric and autistic restricted-diet presentations. The picture — fatigue, then bleeding gums, corkscrew body hairs, bruising, poor healing — reverses within days-to-weeks on modest vitamin C, one of medicine's most satisfying fixes.
Does vitamin C help skin or collagen if swallowed?
It is genuinely required for collagen synthesis — scurvy is collagen failure — so adequacy matters for skin and healing. Beyond adequacy, oral extra hasn't shown meaningful skin-aging benefits in trials; topical vitamin C serums have separate, modest cosmetic evidence. The collagen connection sells far past the biology of "enough is enough."
References
- NIH Office of Dietary Supplements. Vitamin C — Fact Sheet for Health Professionals (2021). https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/
- National Center for Complementary and Integrative Health (NCCIH). Diabetes and Dietary Supplements: What You Need To Know (2021). https://www.nccih.nih.gov/health/diabetes-and-dietary-supplements-what-you-need-to-know
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