Vitamin C: High-Dose Supplementation — What It Does and Doesn't Do
Vitamin C prevents scurvy and supports collagen synthesis, but megadose claims (immune boosting, cancer treatment, cold prevention) have a mixed evidence profile. This guide explains what the research shows.
Quick Answer
Vitamin C is essential for collagen synthesis, immune function, and antioxidant defence. Most adults get adequate amounts from diet.
Key Takeaways
- 01---
- 02| Dose | % Absorbed | Plasma Level |
- 03|---|---|---|
- 04| 200 mg | ~100% | ~12–15 µmol/L |
- 05| 500 mg | ~73% | Marginally higher |
Quick Answer
Vitamin C is essential for collagen synthesis, immune function, and antioxidant defence. Most adults get adequate amounts from diet. High-dose supplementation (>1 g/day) reduces cold duration by about 8% in general populations — a modest effect. Megadose claims for cancer, immune "boosting," or as an antiviral are not adequately supported by clinical trial data.
Dietary vs Supplemental Needs
The body saturates vitamin C absorption at approximately 200 mg/day from food. Above this, absorption efficiency drops sharply — only about 50% of a 500 mg dose and roughly 12% of a 1,250 mg dose is absorbed. Excess is excreted rapidly in urine.
| Dose | % Absorbed | Plasma Level |
|---|---|---|
| 200 mg | ~100% | ~12–15 µmol/L |
| 500 mg | ~73% | Marginally higher |
| 1,250 mg | ~46% | Near ceiling |
| 12,000 mg | ~16% | Ceiling reached |
For most people, 200 mg from a varied diet achieves near-maximal plasma levels. Supplementing beyond this primarily affects urine vitamin C levels.
The Common Cold: What the Data Shows
A 2013 Cochrane review (21 trials, n=10,000+) is the definitive reference:
- Prevention in general population: Regular supplementation does not reduce cold incidence.
- Reduction in duration: Regular supplementation reduces cold duration by ~8% in adults (about half a day) and ~14% in children.
- Prevention in extreme conditions: Athletes, soldiers, and people under extreme physical stress may see reduced incidence with supplementation.
- Therapeutic use (taking at onset): Only one large trial exists; limited effect.
Antioxidant Effect vs Pro-Oxidant Risk
At physiological concentrations, vitamin C is a primary antioxidant. At very high plasma concentrations (achievable only via IV, not oral), it can act as a pro-oxidant in some contexts. This duality is why high-dose IV vitamin C research in cancer is complicated to interpret.
Vitamin C and Collagen Synthesis
Vitamin C is directly required as a cofactor for prolyl and lysyl hydroxylases, the enzymes that crosslink collagen. This is not a supplementation benefit claim — it reflects the essential role vitamin C plays in the mechanism. Adequate vitamin C is necessary; excess does not produce extra collagen.
Dosage Reference
| Context | Recommended Dose |
|---|---|
| RDA (adults) | 75–90 mg/day (women/men) |
| Smokers (increased oxidative stress) | +35 mg/day above RDA |
| General supplementation | 200–500 mg/day |
| Upper tolerable limit | 2,000 mg/day |
| Cold treatment | 1,000 mg/day while symptomatic (modest evidence) |
Safety Notes
- Kidney stones: Vitamin C is metabolised partly to oxalate. People with a history of oxalate kidney stones should limit supplemental vitamin C to 500 mg/day.
- Haemochromatosis: Vitamin C enhances iron absorption; avoid high doses in haemochromatosis.
- GI upset (diarrhoea) is the primary dose-limiting side effect and typically resolves at doses below 2 g/day.
- Drug interactions: May affect warfarin levels at very high doses; monitor INR.
Practical Next Steps
- Adequate diet (5 servings of fruit and vegetables) typically provides 150–200 mg/day.
- If supplementing, 200–500 mg/day is a reasonable pragmatic dose.
- During colds, 1 g/day reduces duration modestly; evidence does not support benefit from higher doses.
- Megadose protocols (>10 g/day) have minimal evidence for any indication outside clinical IV use in specific contexts.
FAQ
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