Vitamin C
vitamin C · ascorbic acid · ascorbate
The classic antioxidant vitamin, promoted for immunity, skin and longevity.
TypeSupplement / dietary
The grade answers: What does the human evidence support for: Antioxidant, immune & deficiency correction?
Grade
C
Limited
The grade rates evidence quality — it is not advice to take or buy.
- Class
- Water-soluble vitamin
- Primary use
- Antioxidant, immune & deficiency correction
- Evidence strength
- medium
- Last reviewed
- 2026-07-01
Bottom line
Essential, and correcting deficiency matters — but megadoses don't prevent colds in most people or extend life, and the body simply excretes the excess. Useful, not a super-supplement.
What the evidence says
Vitamin C is essential (humans can't synthesise it) and true deficiency (scurvy) is serious, so adequacy clearly matters. Beyond that, the evidence deflates the hype: routine supplementation doesn't prevent colds in the general population (it may slightly shorten them, and helps in extreme physical stress), large antioxidant-supplement trials haven't shown longevity benefit, and the body tightly regulates blood levels — excess is largely excreted. Topical vitamin C has a separate, better cosmetic rationale for skin. Grade C: essential nutrient, unremarkable as a megadose longevity pill.
Key studies
- [1]
Vitamin C for preventing and treating the common cold · meta-analysis
No prevention in general population; minor duration effect.
Open on PubMed ↗ - [2]
Antioxidant supplements and mortality · review
No longevity benefit from antioxidant megadosing.
Open on PubMed ↗ - [3]
Mechanism
A cofactor for collagen synthesis and several enzymes and a water-soluble antioxidant; blood levels are homeostatically capped, limiting the effect of very high oral doses.
Safety
Very safe; high oral doses mainly cause GI upset/diarrhoea and, in predisposed people, raise kidney-stone risk. Excess is excreted rather than stored.
Dosage context
Requirements are ~75–120 mg/day, easily met by diet; gram-level 'immune' doses exceed absorption and are largely wasted. Topical formulations are a different, skin-specific use.
Examples of application
- Easily met from diet; supplements above absorption are largely excreted.
- Gram-level 'immune' doses mostly don't prevent colds.
- Topical vitamin C is a separate, skin-specific use.
From the field
Vitamin C is the original 'megadose it for everything' supplement, and the evidence mostly says your kidneys just filter out the excess. Essential in the diet, unremarkable as a longevity pill. Grade C.