Vitamin C: Dosage, Benefits, Food Sources and Upper Limit

An essential water-soluble antioxidant and enzyme cofactor, required for collagen hydroxylation, carnitine synthesis and noradrenaline synthesis, and it regenerates vitamin E. It also converts dietary non-haem iron to the more absorbable ferrous form, which is the basis of the standard advice to take iron with orange juice.

• Common cold: regular supplementation does not reduce how often you catch one. A meta-analysis found it reduced severity by about 15%, and duration slightly.
• Iron absorption: substantially improves absorption of non-haem iron when taken in the same meal. This is its most reliable practical use.
• Collagen synthesis: required, which is why it appears in skin and joint formulas. Extra beyond sufficiency has not been shown to add more.
• Deficiency: scurvy remains the only condition it unambiguously treats.

Hemilä's Cochrane work established that regular supplementation does not lower incidence but shortens duration; a 2023 meta-analysis quantified a 15% reduction in severity. Large prospective cohorts in men and women found no significant association between vitamin C intake and kidney stone formation. The NIH Office of Dietary Supplements fact sheet is the standard reference for intake levels.

The RDA is 75 mg/day for women and 90 mg/day for men, 35 mg higher for smokers. Absorption efficiency drops above roughly 400 mg in one dose, so splitting is more useful than escalating. The tolerable upper intake level is 2,000 mg/day.

Diarrhoea, nausea and abdominal cramping above about 2,000 mg/day, from unabsorbed ascorbate drawing water into the bowel. High intakes raise urinary oxalate, which is a consideration for anyone who has formed calcium oxalate stones, though large prospective cohorts in men and women did not find an association with stone risk at ordinary intakes.

Increases iron absorption, which is useful in deficiency and unhelpful in haemochromatosis. May reduce the effect of some chemotherapy agents; oncology patients should ask before supplementing. High doses can interfere with certain blood glucose meters and stool occult blood tests.