Micronutrients
Vitamin C and immunity: what it really does
Vitamin C is the nutrient most associated with immunity in the public mind, and the gap between that reputation and the trial data is instructive.
What it does
Vitamin C is required to synthesize collagen, which is why severe deficiency causes scurvy — poor wound healing, bleeding gums, and connective tissue breakdown. It also functions as an antioxidant and supports several immune cell functions, and it markedly improves absorption of non-heme iron from plant foods.[1]
How much you need
| Group | RDA | Upper limit |
|---|---|---|
| Men 19+ | 90 mg/day | 2,000 mg/day |
| Women 19+ | 75 mg/day | 2,000 mg/day |
| Smokers | +35 mg/day | 2,000 mg/day |
A single medium orange supplies roughly 70 mg; a cup of raw red pepper supplies more than a day's requirement.[1]
Does it prevent colds?
Regular supplementation does not appear to reduce the likelihood of catching a cold in the general population. It has been associated with a modest reduction in cold duration, and the picture differs for people under heavy physical stress such as marathon runners, where supplementation has shown a larger effect on incidence.[1]
Why megadoses do little
Absorption efficiency falls sharply as intake rises, and the excess — being water-soluble — is excreted. Beyond roughly 400 mg/day, plasma levels plateau regardless of how much more you take.[1]
Frequently asked questions
Does vitamin C prevent colds?
Regular vitamin C supplementation does not appear to reduce how often people in the general population catch colds, though it has been linked to a modest reduction in duration. People under heavy physical stress, such as marathon runners, show larger effects.
How much vitamin C do I need per day?
The RDA is 90 mg per day for adult men and 75 mg for adult women, with an additional 35 mg for smokers. The tolerable upper limit is 2,000 mg per day.
Do high-dose vitamin C supplements work better?
Not meaningfully. Absorption efficiency falls as intake rises and the excess is excreted in urine, so plasma levels plateau beyond roughly 400 mg per day.
Keep reading: Iron deficiency in athletes · Micronutrients vs. macronutrients · Vitamin D explained