Vitamin C (ascorbic acid) is an essential water-soluble vitamin required for collagen synthesis, iron absorption, and antioxidant defence. Outright deficiency (scurvy) is now rare in high-income countries but still occurs in malnourished, heavily processed-diet, alcohol-dependent, or institutionalised populations. Plasma levels plateau near 100–200 µmol/L regardless of dose — mega-dosing produces more urinary excretion, not more benefit.
Get ~75–110 mg/day from food or a cheap supplement if you don't. Vitamin C does not prevent colds in the general population — daily use modestly reduces duration, and it is useful at onset only in specific contexts. High-dose (1–2 g) protocols have minimal incremental benefit and can cause GI upset and kidney stones in predisposed people.
Each row grades the claimed effect by strength of human evidence, not mechanism or marketing.
Essential nutrient with a clear RDA. Clinical guidance supports correcting deficiency and permits modest supplementation; there is no broad recommendation to mega-dose.
Frequently marketed for immunity, cold prevention, skin, longevity and even cancer, at doses of 1–10 g/day — most of which is excreted and none of which is well supported by outcome trials in replete adults.
The largest gap between perception and evidence is common-cold prevention: the general public believes it prevents colds, whereas RCT evidence shows only a modest duration effect and requires regular daily intake, not at-onset dosing.
Plasma vitamin C saturates near 200 µmol/L; increasing oral dose beyond ~400 mg/day produces diminishing returns and mostly increased urinary excretion.
Smokers have higher requirements (roughly +35 mg/day) due to increased oxidative stress.
Co-ingesting vitamin C with iron supplements or iron-rich plant foods is a genuinely useful trick for non-heme iron absorption.
Very high doses (>2 g/day) can cause GI upset, osmotic diarrhoea, and increased urinary oxalate — relevant for people prone to calcium-oxalate kidney stones.
Intravenous high-dose vitamin C studied in oncology is not interchangeable with oral high-dose supplementation — the pharmacokinetics are entirely different.
Mechanism is not outcome. Each mechanism is labelled by how far it has been validated in humans.
Vitamin C is required for prolyl and lysyl hydroxylase activity during collagen synthesis. This is why vitamin C deficiency produces the connective-tissue failure seen in scurvy.
Ascorbic acid reduces Fe3+ to Fe2+ in the gut and forms chelates that are more readily absorbed — a clinically useful interaction for non-heme iron.
Vitamin C donates electrons to neutralise reactive oxygen species and regenerates other antioxidants including vitamin E. In vivo benefit beyond replete status is less clear.
Vitamin C accumulates in neutrophils and lymphocytes and is consumed during immune activation. Correcting deficiency restores function; supplementing above replete levels has not reliably translated to clinical immune outcomes.
Very safe at typical doses. Water-soluble; excess is excreted. Adverse effects are concentrated at high doses.
This page is educational and not medical advice. Discuss high-dose vitamin C with a clinician if you have haemochromatosis, a history of kidney stones, G6PD deficiency, kidney disease, or are undergoing cancer treatment.
A small, curated set — not a literature dump. Each reference comes with a single-line takeaway.
Regular daily vitamin C (≥0.2 g/day) did not reduce incidence in the general population but shortened cold duration by ~8% in adults and ~14% in children. At-onset therapeutic use showed no consistent effect.
In ~7,600 women, 500 mg/day vitamin C did not reduce total cancer incidence or cancer mortality over ~9 years.
Long-term vitamin C supplementation (500 mg/day) did not reduce major cardiovascular events in middle-aged and older men.
Plasma and tissue vitamin C saturate near 200 mg/day oral intake; higher doses are progressively excreted in urine, establishing the physiological ceiling for oral supplementation.