Vitamin C: High-Dose Supplementation — What It Does and Doesn't Do

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.

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

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

1. Adequate diet (5 servings of fruit and vegetables) typically provides 150–200 mg/day.
2. If supplementing, 200–500 mg/day is a reasonable pragmatic dose.
3. During colds, 1 g/day reduces duration modestly; evidence does not support benefit from higher doses.
4. Megadose protocols (>10 g/day) have minimal evidence for any indication outside clinical IV use in specific contexts.