Vitamin C for Colds, Iron Absorption, and Kidney Stone Risk

Linus Pauling's 1970 claim that gram-dose vitamin C would prevent the common cold has outlived the evidence against it by more than fifty years. The largest body of trial data, a Cochrane review covering more than 11,000 participants, found that routine vitamin C supplementation does not reduce cold incidence in the general population.¹ The same review identified a narrower benefit for people under severe physical stress. In five trials of marathon runners, skiers, and soldiers training in subarctic conditions, regular vitamin C supplementation cut cold risk roughly in half.¹

The response changes with physical stress, which is a useful lens for the rest of what vitamin C does. Vitamin C is a cofactor with specific jobs, a documented upper limit, and a supplement dose that stops paying off well before most bottles suggest.

01Vitamin C's cofactor work starts with collagen

Vitamin C is a required cofactor for prolyl and lysyl hydroxylase, the enzymes that stabilize the collagen triple helix. Without it, the body still assembles collagen chains, but they are structurally weak, which is the mechanism behind scurvy.⁴ Collagen synthesis timing covers the tendon-and-ligament application in detail, including the roughly 48 mg amount used in a small biomarker study of vitamin-C-enriched gelatin.⁵ That study does not establish an optimal vitamin C dose for clinical tendon or ligament outcomes.

Vitamin C also participates in carnitine synthesis, norepinephrine synthesis, and the regeneration of vitamin E after it neutralizes a free radical.⁴ Regenerating vitamin E is part of why vitamin C sits inside the same antioxidant network as selenium and other micronutrient cofactors, rather than functioning as a standalone defense system.

02How much vitamin C you need and who needs more

The adult RDA is 90 mg per day for men and 75 mg per day for women, with an additional 35 mg per day for smokers, whose oxidative stress and metabolic turnover of vitamin C both run higher.⁴ Those numbers are easy to clear with food. Half a cup of raw red pepper has about 95 mg, a medium orange has about 70 mg, and half a cup of sliced strawberries has about 49 mg.⁴ Regular servings of fruit and vegetables can therefore cover the RDA without a supplement.

The population that actually has a decision to make is narrower than supplement marketing implies:

03Why plasma vitamin C concentrations flatten above 200 mg

The clearest argument against high-dose vitamin C supplementation is the early plasma-concentration plateau. Levine and colleagues hospitalized healthy volunteers on a controlled diet and measured steady-state plasma vitamin C across doses from 30 to 2,500 mg per day. Plasma concentration followed sigmoid kinetics: a steep rise between 30 and 100 mg, then a flattening curve as dose increased further. The study found complete bioavailability at 200 mg and declining fractional bioavailability at larger doses. Higher doses can still increase the absolute amount absorbed, but they add little to steady-state plasma concentration as renal excretion rises.² The study evaluated the historical 60 mg RDA reference. Current adult RDAs are 75 mg for women and 90 mg for men.⁴

That kinetic ceiling means a 1,000 mg tablet produces a much smaller incremental increase in steady-state plasma concentration than the dose increase alone would suggest. The additional absorbed vitamin C is increasingly cleared in urine rather than retained in the circulating pool.²

04High-dose vitamin C and kidney stone risk

The tolerable upper intake level for vitamin C is 2,000 mg per day for adults. High doses can cause gastrointestinal symptoms such as diarrhea and nausea, with risk increasing above the upper limit.⁴ A prospective cohort of 23,355 Swedish men with no prior kidney stone history found that ascorbic-acid supplement use at typical doses around 1,000 mg was associated with roughly double the kidney stone incidence of nonuse.³

Increased urinary oxalate is a proposed pathway because vitamin C is partially metabolized to oxalate, but the cohort study cannot establish that pathway as the cause of its association.³⁴ The absolute risk in the study remained low in population terms. People with renal disorders or hyperoxaluria have a stronger reason to review a daily gram-plus vitamin C habit with a clinician.⁴

05Vitamin C food pairings improve non-heme iron absorption

The most practically useful role vitamin C plays for active people has nothing to do with colds. Iron absorption depends heavily on what sits next to iron-rich food on the plate. Vitamin C keeps more non-heme iron in an absorbable form, which helps with the iron found in lentils, spinach, tofu, and fortified grains.⁴ A meal that combines a non-heme iron source with a vitamin C-rich food creates a better absorption opportunity than the same iron source eaten beside coffee, tea, or a large calcium dose.⁶

This matters most for the exact population already managing a narrow iron margin: menstruating athletes, endurance athletes with elevated hepcidin after training, and anyone building meals around plant-based iron.⁶⁷ Iron repletion protocols for endurance athletes cover the hepcidin timing side of that problem. Vitamin C is the food-pairing side, and it does not require a supplement to work. A pepper, citrus, tomatoes, or berries alongside the iron-rich food handles it.

06How to use vitamin C at the doses that matter

1. Cover the RDA through food when possible. A varied intake of vitamin-C-rich fruits and vegetables can cover the adult RDA.⁴
2. Pair vitamin C foods with non-heme iron sources, especially during a deliberate ferritin repletion window.
3. Recognize that a small biomarker study used roughly 48 mg of vitamin C with gelatin before loading. That amount is not an established clinical protocol or a required cofactor dose.⁵
4. Do not expect routine supplementation to prevent colds in general-population trials. Evidence for reduced incidence is stronger during brief severe physical stress or cold exposure, while effects may differ in people with marginal vitamin C status.¹⁴
5. Review daily gram-plus dosing when you have renal disorders or hyperoxaluria, given the association with roughly doubled kidney stone incidence in a cohort of male supplement users.³⁴
6. Smokers should add the extra 35 mg per day the RDA already accounts for, ideally from food.

Vitamin C's reputation outran its evidence decades ago. What survived the scrutiny is smaller than the megadose story and more useful than the myth: a cofactor that makes collagen stable, makes non-heme iron absorbable, and helps under real physical stress, at doses far below what most supplement labels sell.