Vitamin C

Mechanism: Vitamin C is a required cofactor for prolyl- and lysyl-hydroxylase, the enzymes that crosslink collagen. Without vitamin C, the body cannot build stable collagen — gums bleed, wounds fail to heal, capillaries leak.

Scurvy is a real disease that develops within weeks on a vitamin-C-free diet and reverses fully with 10 mg/day. The RDA (75 mg for women, 90 mg for men) is set well above this threshold. This is the single strongest piece of vitamin C evidence — clinical disease, demonstrated reversal, regulatory recognition.

Modern scurvy is rare but real — case reports cluster in alcohol use disorder, severe eating disorders, isolated elderly, and ultra-restrictive diets. If you eat any fruit or vegetables, you almost certainly do not need a supplement to prevent scurvy.

• Iowa State Penitentiary experimental scurvy studies

  positive

  Hodges, Hood, Sauberlich et al., 1971, American Journal of Clinical Nutrition · rct · n=6

  Volunteers on a vitamin-C-free metabolic diet developed clinical scurvy — bleeding gums, follicular hyperkeratosis, perifollicular hemorrhages — within roughly 4 weeks. 10 mg/day fully reversed and prevented disease. This is the experimental basis for setting RDAs and remains the strongest single demonstration that vitamin C is essential.

  Only 6 subjects. Modern ethics make replication impossible — but no replication is needed; the disease and its reversal are unambiguous.

• Vitamin C pharmacokinetics in healthy volunteers

  positive

  Levine et al., 1996, PNAS · pilot · n=7

  Inpatient pharmacokinetic study showing vitamin C plasma concentration follows sigmoid kinetics with the steep portion between 30 and 100 mg/day. Plasma saturates around 200 mg/day total intake. Doses above ~200 mg show diminishing returns; excess is excreted in urine.

  Used to argue the RDA was set too low; subsequent updates raised it to 75 mg (women) and 90 mg (men), still on the low end of the saturation curve.

Mechanism: Vitamin C reduces ferric iron to the absorbable ferrous form in the gut, increasing non-heme iron uptake by up to 3-fold. It also regenerates reduced vitamin E and supports glutathione recycling.

The iron-absorption effect is robust and clinically useful — taking 100 mg of vitamin C with iron-rich plant foods or an iron supplement meaningfully boosts absorption. The general 'antioxidant' framing is mechanistically real but does not translate into reliable disease-prevention outcomes in trials.

Iron-overload conditions (hereditary hemochromatosis, repeated transfusions) make this a feature to avoid, not seek. Standard advice: pair vitamin C with iron sources only if you actually need to raise iron status.

• Vitamin C pharmacokinetics in healthy volunteers

  positive

  Levine et al., 1996, PNAS · pilot · n=7

  Inpatient pharmacokinetic study showing vitamin C plasma concentration follows sigmoid kinetics with the steep portion between 30 and 100 mg/day. Plasma saturates around 200 mg/day total intake. Doses above ~200 mg show diminishing returns; excess is excreted in urine.

  Used to argue the RDA was set too low; subsequent updates raised it to 75 mg (women) and 90 mg (men), still on the low end of the saturation curve.

Mechanism: Vitamin C concentrates in white blood cells and supports neutrophil chemotaxis, phagocytosis, and apoptosis. Plasma levels drop during acute infection — whether that is cause or consequence remains debated.

Daily vitamin C does not prevent colds in the general population. It does shorten cold duration by ~8% in adults and ~14% in children when taken prophylactically — roughly half a day off a typical cold. In marathoners, skiers, and soldiers under heavy physical stress, it cuts cold incidence in half. Therapeutic dosing started after symptoms begin shows mixed results.

Strongest effect in heavy-physical-stress populations and possibly in low-baseline-vitamin-C individuals. Modest effect in the general population. Not a substitute for sleep, hydration, or vaccination.

• Cochrane review — vitamin C for the common cold

  mixed

  Hemilä & Chalker, 2013, Cochrane Database of Systematic Reviews · systematic review · n=11306

  Pooled 29 trials with 11,306 participants. Daily vitamin C did not prevent colds in the general population. It did shorten cold duration modestly — roughly 8% in adults and 14% in children. The 'vitamin C prevents colds' folk wisdom is wrong; the duration effect is real but minutes-to-a-day, not transformative.

  The duration effect requires daily prophylaxis, not dosing once you already feel sick. Therapeutic dosing started after symptom onset showed no consistent benefit in the same review.

  DOI

• Vitamin C in physical-stress populations (Hemilä subgroup)

  positive

  Hemilä & Chalker, 2013, Cochrane Database of Systematic Reviews (subgroup) · meta analysis · n=598

  Five trials in marathoners, skiers, and soldiers on subarctic exercises pooled to a 50% reduction in cold incidence (risk ratio 0.48). The exception that proves the rule: in everyone else, prophylactic vitamin C does not prevent colds.

  This is an extreme-physical-stress subgroup, not a general-population finding. Do not generalize the marathon-runner effect to office workers.

• Therapeutic dose-response for vitamin C and cold duration

  mixed

  Hemilä, 2017, Nutrients (analysis of two RCTs) · meta analysis

  Re-analysis of two earlier RCTs showed a dose-response relationship: 8 g taken at symptom onset shortened cold duration by ~19%, twice the effect of 4 g. The Cochrane review's 'no consistent therapeutic effect' conclusion may underweight high-dose-at-onset dosing, but the trials are old and unreplicated at scale.

  Two trials. Headline-grabbing but not yet confirmed in a large modern RCT. Cochrane treats the therapeutic-dose evidence as inconsistent.

Mechanism: Vitamin C is non-negotiable for collagen crosslinking and acts as an antioxidant in dermis. Topical vitamin C also has independent dermatologic evidence for photoaging.

Adequate vitamin C is required for collagen synthesis — full stop. Whether oral supplementation above the RDA improves skin in non-deficient adults is a different question with weaker evidence. Topical L-ascorbic acid has dermatology trials behind it; oral mega-doses do not.

Oral vitamin C above the RDA does not visibly transform skin in healthy adults. Topical formulations and adequate dietary collagen plus vitamin C cofactor are the more defensible skin strategy.

• Iowa State Penitentiary experimental scurvy studies

  positive

  Hodges, Hood, Sauberlich et al., 1971, American Journal of Clinical Nutrition · rct · n=6

  Volunteers on a vitamin-C-free metabolic diet developed clinical scurvy — bleeding gums, follicular hyperkeratosis, perifollicular hemorrhages — within roughly 4 weeks. 10 mg/day fully reversed and prevented disease. This is the experimental basis for setting RDAs and remains the strongest single demonstration that vitamin C is essential.

  Only 6 subjects. Modern ethics make replication impossible — but no replication is needed; the disease and its reversal are unambiguous.

Mechanism: Endothelial cells use vitamin C to support nitric oxide bioavailability. Mechanistic story is plausible. Clinical trials say it doesn't translate.

The Physicians' Health Study II tested 500 mg/day vitamin C against placebo in 14,641 men over 8 years. No reduction in heart attacks, strokes, or cardiovascular death. SU.VI.MAX added 7.5 years of antioxidant-cocktail data with no cardiovascular signal. Routine vitamin C supplementation does not prevent cardiovascular disease.

Observational links between higher vitamin C intake and lower cardiovascular risk most likely reflect produce-eating habits, not the vitamin in pill form.

• Physicians' Health Study II — vitamin C and cardiovascular events

  negative

  Sesso et al., 2008, JAMA · rct · n=14641

  Same 14,641-physician cohort, cardiovascular endpoint. 500 mg/day vitamin C produced no reduction in major cardiovascular events, MI, stroke, or all-cause mortality versus placebo over 8 years.

  The cardiovascular case for routine vitamin C supplementation is weak. Observational associations between dietary vitamin C and lower cardiovascular risk likely reflect confounding by overall produce intake, not a direct supplement effect.

• SU.VI.MAX — antioxidant cocktail and chronic disease

  mixed

  Hercberg et al., 2004, Archives of Internal Medicine · rct · n=12735

  12,735 adults randomized to a low-dose antioxidant cocktail (including 120 mg vitamin C) for 7.5 years. No effect on cardiovascular events. Modest reduction in total cancer incidence in men only, with no effect in women.

  The cocktail design makes it impossible to attribute any signal to vitamin C specifically.

Mechanism: Antioxidant rationale is mechanistically straightforward. The clinical trial data are not.

Linus Pauling's 1970s claim that vitamin C prevents and treats cancer launched the supplement industry. The Mayo Clinic trial (10 g/day oral, advanced colorectal cancer) showed zero benefit. The Physicians' Health Study II (500 mg/day, 14,641 men, 8 years) showed zero reduction in any cancer. Pauling was wrong.

IV pharmacological-dose vitamin C in oncology remains under research as adjunctive therapy in specific cancer types, but as of 2026 no randomized evidence supports it as a stand-alone treatment. Never use as a substitute for standard cancer care.

• Physicians' Health Study II — vitamin C and cancer

  negative

  Gaziano et al., 2009, JAMA · rct · n=14641

  Large 8-year randomized trial in 14,641 male physicians on 500 mg/day vitamin C versus placebo. No reduction in total cancer, prostate cancer, or site-specific cancers. The largest, longest, cleanest test of the cancer-prevention hypothesis came back null.

  Long follow-up extension (Wang et al. 2014) reproduced the null finding.

• Mayo Clinic high-dose oral vitamin C in advanced cancer

  negative

  Moertel et al., 1985, New England Journal of Medicine · rct · n=100

  Double-blind randomized trial of 100 advanced colorectal cancer patients on 10 g/day oral vitamin C versus placebo. No tumor response, no survival benefit, no slowing of progression. The trial that closed the door on Linus Pauling's oral-vitamin-C-for-cancer claim.

  Pauling's defenders argued the route mattered — IV ascorbate reaches plasma concentrations 70–100x higher than maximally tolerated oral dosing. That argument is partially valid; subsequent IV trials still failed to show survival benefit in randomized settings.

Mechanism: Vitamin C levels collapse in sepsis. Early hypothesis: replenish to support endothelial and immune function during the cytokine storm.

The Marik 2017 retrospective cohort sparked the 'vitamin C for sepsis' wave. Two large randomized trials closed it: CITRIS-ALI (167 patients, no organ-failure benefit) and LOVIT (863 patients, possibly worse outcomes on vitamin C than placebo). High-dose IV vitamin C is not a sepsis treatment.

This is critical-care research, not consumer supplementation. Listed here only so the page reflects the full evidence base — including the failures.

• CITRIS-ALI — IV vitamin C in sepsis-ARDS

  negative

  Fowler et al., 2019, JAMA · rct · n=167

  167 sepsis-ARDS patients randomized to high-dose IV vitamin C versus placebo. No improvement in organ-failure score or inflammatory biomarkers. A secondary 28-day mortality signal generated headlines but was not the primary endpoint and did not survive subsequent confirmatory trials.

  Single secondary-endpoint mortality finding is fragile. Subsequent larger trials (LOVIT) reversed the apparent benefit signal.

• LOVIT — IV vitamin C in adult ICU sepsis

  negative

  Lamontagne et al., 2022, New England Journal of Medicine · rct · n=863

  863 ICU sepsis patients randomized to IV vitamin C versus placebo. 44.5% in the vitamin C group died or had persistent organ dysfunction at day 28 versus 38.5% on placebo — vitamin C performed worse than placebo. The largest, cleanest sepsis trial reversed the early enthusiasm.

  Suggests possible harm, not just absence of benefit. Closes the door on routine high-dose IV vitamin C in sepsis.

• Ascorbic acidWell absorbed up to ~200 mg per dose; saturable transporters
  Best for: General supplementation, scurvy prevention, iron-absorption pairing
  The cheapest and most studied form. Acidic, can be hard on sensitive stomachs at high doses. Above ~500 mg per dose, fractional absorption drops sharply and excess spills into urine.
• Sodium ascorbateComparable to ascorbic acid
  Best for: Buffered vitamin C for adults sensitive to ascorbic acid's GI effects
  Adds ~131 mg sodium per 1,000 mg of sodium ascorbate. Worth checking the label if you watch sodium intake.
• Calcium ascorbateComparable to ascorbic acid
  Best for: Buffered form, often marketed as gentler on the stomach
  Adds calcium (~90 mg per 1,000 mg of calcium ascorbate). Often the buffered form in 'non-acidic' vitamin C marketing.
• Ester-C® (calcium ascorbate with metabolites)Ester-C®Similar to standard ascorbate forms in head-to-head bioavailability data
  Best for: Marketed as longer-lasting and gentler on the stomach
  Manufacturer claims of 24-hour retention or superior absorption versus plain ascorbic acid are not well supported in independent trials. The 'gentler' framing has more evidence than the 'better-absorbed' framing.
• Liposomal vitamin CModestly higher plasma peak than equivalent oral ascorbic acid in some studies
  Best for: Higher-dose oral delivery without saturating gut transporters as quickly
  The liposomal-delivery story is plausible — phospholipid encapsulation may bypass the saturable transporter. The pharmacokinetic differences in published studies are real but smaller than marketing implies. Premium price tag, modest premium effect.
• Intravenous vitamin CBypasses the gut saturation ceiling — plasma levels 70–100x higher than maximally tolerated oral
  Best for: Research and integrative oncology — not consumer supplementation
  Reaches truly pharmacological concentrations. Tested in sepsis (LOVIT, CITRIS-ALI — both failed) and in oncology (mixed and largely unconvincing trial data). Belongs in clinical research settings, not in routine wellness.

• !Vitamin C does not prevent cancer. The Physicians' Health Study II tested 500 mg/day in 14,641 men over 8 years and found no reduction in any cancer. The Mayo Clinic trial of 10 g/day in advanced cancer was likewise null. Linus Pauling's vitamin-C-cures-cancer claim launched the supplement industry but did not survive controlled testing.
• !Vitamin C does not prevent colds in the general population. The Cochrane review of 11,306 participants is unambiguous on incidence. The duration effect is real but small — about 8% in adults, half a day off a typical cold.
• !High-dose vitamin C is not a sepsis treatment. The LOVIT trial (863 patients) found vitamin C performed worse than placebo at 28 days. The CITRIS-ALI trial was likewise negative on its primary endpoint.
• !Hereditary hemochromatosis or other iron-overload conditions are a real contraindication for high-dose supplementation. Vitamin C dramatically increases non-heme iron absorption.
• !Multi-gram daily dosing increases urinary oxalate and is associated with kidney stones in men. If you have a history of calcium-oxalate stones, keep doses at ≤500 mg/day.

• Will vitamin C stop me from getting sick?▾

  In the general population, no — daily vitamin C does not prevent colds, full stop. The Cochrane review of 11,306 participants is clear on this. What it does do is shorten cold duration modestly when taken daily as prophylaxis: about 8% in adults, 14% in children — minutes to a half-day off a typical cold. In marathoners, skiers, and soldiers under heavy short-term physical stress, vitamin C cuts cold incidence in half. If you are not in that group, the prevention story doesn't apply to you.
• How much vitamin C do I actually need?▾

  The RDA is 75 mg/day for women and 90 mg/day for men. Smokers add 35 mg/day. Plasma saturation occurs around 200 mg/day total intake — beyond that, fractional absorption drops and excess is lost in urine. A medium orange has roughly 70 mg, a cup of strawberries about 85 mg, half a red bell pepper about 95 mg. Most people who eat any produce hit the threshold without trying. The upper limit before GI side effects becomes likely is around 1,000–2,000 mg in a single dose.
• Is liposomal vitamin C worth the extra money?▾

  Modestly. Liposomal formulations can produce slightly higher plasma peaks than equivalent doses of plain ascorbic acid because they bypass saturable gut transporters. The pharmacokinetic difference is real but smaller than marketing claims suggest. For most people taking 200–500 mg/day, plain ascorbic acid or a buffered form (sodium or calcium ascorbate) does the job at a fraction of the price. Liposomal is most defensible when high oral doses are the specific goal.
• Can vitamin C cause kidney stones?▾

  At high chronic doses, yes. Vitamin C is metabolized partly to oxalate, and multi-gram daily intake (>1,000 mg/day) is associated with calcium-oxalate stone formation, particularly in men with a stone history. Standard doses up to 500 mg/day are not a meaningful stone risk for most people. If you have had calcium-oxalate stones before, keep supplementation modest and stay well hydrated.
• Does vitamin C help skin and collagen?▾

  Adequate vitamin C is non-negotiable for collagen synthesis — the body literally cannot crosslink collagen without it. Whether oral supplementation above the RDA visibly improves skin in healthy adults is a different question, with much weaker evidence. Topical L-ascorbic acid serums have dermatology trials behind them for photoaging and pigmentation; oral mega-dosing does not. The defensible skin strategy is adequate dietary or supplemental vitamin C as the cofactor, paired with a topical formulation if photoaging is the concern.