Vitamin C

Best-in-class: Vitamin C (Ascorbic Acid)

Water-soluble antioxidant and essential cofactor for collagen synthesis, immune cells and neurotransmitter production.

How Vitamin C actually works

Beyond the antioxidant reputation, vitamin C's specific job is as an enzyme cofactor — it keeps iron and copper in the reduced state that certain enzymes need to function. The best-defined are prolyl and lysyl hydroxylase, which cross-link collagen; without vitamin C collagen is structurally unsound, which is scurvy. It's also the cofactor for dopamine beta-hydroxylase (dopamine to noradrenaline) and for carnitine synthesis. Absorption is actively transported and saturates: past roughly 200 mg in one sitting the fraction absorbed falls steeply and the rest is excreted, which is the honest limit on megadosing.

Where to get Vitamin C

The evidence for Vitamin C

Graded by what exists behind each claim.

✅ Clinically validated

• Regular supplementation modestly reduces the duration/severity of the common cold (meta-analysis), with larger effects in people under heavy physical stress.
• Essential for collagen cross-linking — corrects the connective-tissue failure of deficiency (scurvy).

📊 Correlative data

• Higher plasma vitamin C is associated with lower cardiovascular and all-cause mortality in cohort studies.

🧪 Theoretical / extrapolated benefits

• High-dose/IV vitamin C is being explored as a pro-oxidant adjunct in oncology — mechanistically interesting but not established standard care.

How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →

What Vitamin C actually does

Ascorbate is not primarily an antioxidant. It is a specific electron donor for a family of iron-containing enzymes, and that is where its non-negotiable jobs are. The 2-oxoglutarate-dependent dioxygenases — prolyl-4-hydroxylase and lysyl hydroxylase for collagen, the TET enzymes for DNA demethylation, the hypoxia-inducible factor prolyl hydroxylases — all use a non-heme iron that occasionally ends up oxidized and stuck. Ascorbate reduces it back to the ferrous state. Without that, the enzyme runs once and stops.

Scurvy is a collagen disease and its features follow from one reaction. Unhydroxylated procollagen cannot form a stable triple helix, so it is degraded rather than exported. Perifollicular hemorrhage, corkscrew hairs, bleeding gums, poor wound healing and capillary fragility are all consequences of connective tissue that was never assembled. It is the cleanest structure-to-symptom chain in nutrition.

Two other enzyme classes explain the effects people notice. Dopamine beta-hydroxylase needs ascorbate to make norepinephrine, and peptidylglycine alpha-amidating monooxygenase needs it to amidate peptide hormones. And two hydroxylations in the carnitine synthesis pathway need it, which is the biochemical basis for the fatigue of early deficiency.

The transporters are what make the pharmacokinetics interesting. Ascorbate enters cells through the sodium-dependent vitamin C transporters SVCT1 in intestine and kidney and SVCT2 everywhere else, both saturable and both downregulated by high ascorbate. Its oxidized form, dehydroascorbate, enters through the glucose transporters GLUT1 and GLUT3 and is reduced back inside — which is why glucose competes with vitamin C uptake and why neutrophils can concentrate it 50-fold.

And the antioxidant story has a direction problem that matters for anyone training. Exercise-induced reactive oxygen species are not damage to be mopped up; they are the signal that activates PGC-1alpha, NRF2 and mitochondrial biogenesis. An antioxidant taken to protect a training session is competing with the message that session is trying to send Li 2022.

Cell, rodent, human — and where it stops

The marker on this page saturates before the dose on the label starts, and the two best human trials show the marker moving in the wrong direction for the goal.

In cells and in classical human depletion studies. The enzyme dependencies are established biochemistry, and controlled depletion-repletion work in humans established the dose-concentration relationship: plasma ascorbate is a sigmoid function of intake that is steep between 30 and 100 mg a day and essentially flat above roughly 200 mg, where plasma approaches 70 to 80 micromol/L and renal clearance disposes of the rest Dosedel 2021.

That single fact undermines most of the shelf. A 1,000 mg tablet raises plasma ascorbate barely above what 200 mg achieves, because SVCT1 in the intestine saturates and the kidney excretes the excess. Bioavailability falls from near 100 percent at 200 mg to roughly half at 1,250 mg. The marker the product is bought to raise cannot be raised much further by mouth.

In the common cold, the outcome has been measured exhaustively and the answer is stable. Regular supplementation does not reduce cold incidence in the general population, shortens duration by a small and consistent margin, and does reduce incidence roughly by half in people under extreme physical stress such as marathon runners and soldiers in subarctic training Hemila 2013. That is a real effect in a narrow population and it is not the claim on the bottle.

In training, the outcome trials found harm rather than nothing. 1,000 mg vitamin C with 235 mg vitamin E during 11 weeks of endurance training blunted the training-induced rise in mitochondrial biogenesis markers Paulsen 2014; the same combination blunted the gain in total lean body mass in elderly men over 12 weeks of strength training Bjornsen 2016; and the newest work shows antioxidant supplementation suppressing the proteome response to sprint interval training preferentially in type 2 fibers Wyckelsma 2025.

The obstacle to transfer is a category error about what an antioxidant is for. The trials that raised plasma ascorbate succeeded at their surrogate and then damaged the adaptation the training existed to produce. That is not a null result; it is a surrogate pointing the opposite way from the outcome Li 2022.

Vitamin C — which form, and does it matter

Every form on the shelf delivers the same anion, and the question is what comes with it. Ascorbic acid is the free acid. Sodium ascorbate, calcium ascorbate and magnesium ascorbate are buffered salts that are gentler on the stomach and deliver their cation too — a gram of sodium ascorbate carries about 111 mg of sodium, which is worth knowing for anyone counting it. Ester-C is calcium ascorbate with a small amount of dehydroascorbate and threonate; the threonate is the marketing and the evidence for a distinct advantage is thin.

The saturable transporter is the reason 'high-dose oral' is mostly wishful. Absorption is SVCT1-mediated and saturable, so fractional absorption falls steeply with dose while the absolute amount absorbed plateaus. Splitting a gram into two 500 mg doses genuinely raises total absorption; taking 3 grams at once mostly produces osmotic diarrhea and expensive urine.

The clock is short and the pool is small. Plasma ascorbate has an elimination half-life of roughly 10 to 20 hours at saturating intake and longer at low intake, because renal reabsorption through SVCT1 is itself saturable: below a plasma threshold of about 70 micromol/L the kidney reclaims almost all of it, and above that it spills. Total body pool at saturation is around 1,500 mg and the fractional turnover is roughly 3 percent a day, which is why scurvy takes 1 to 3 months of near-zero intake to appear and why a missed day means nothing Dosedel 2021.

Liposomal preparations are the one form claim with a real mechanism and thin evidence. Encapsulation is proposed to bypass the saturable transporter by delivering ascorbate through a different route, and small studies report higher peak plasma concentrations than equivalent unencapsulated doses. No trial has shown that the higher concentration produces a clinical outcome that the ordinary dose does not.

Intravenous is a different drug and should be described that way. Bypassing the gut entirely produces plasma concentrations in the millimolar range, two orders of magnitude above anything oral dosing can reach, at which ascorbate becomes a pro-oxidant generating extracellular hydrogen peroxide. That is the pharmacology behind oncology trials, it has nothing to do with a capsule, and the terminal elimination is by rapid renal clearance in either case.

What would have to be true, and how you would know it was not

1. Plasma ascorbate rises to a ceiling and then stops, and the ceiling arrives sooner than the label suggests. Predict vitamin C measured as plasma ascorbic acid reaches roughly 70 to 80 micromol/L on 200 to 400 mg a day and does not go materially higher on 2,000 mg Dosedel 2021. Retest at 4 weeks. If plasma is still low on 500 mg, suspect smoking, which raises turnover substantially, or malabsorption.

2. Predict a shorter cold, not fewer colds. On regular supplementation, predict roughly an 8 percent reduction in adult cold duration and no reduction in the number of colds, and predict that starting it at the onset of symptoms does nothing at all Hemila 2013. Count episodes and days over one winter; the distinction between incidence and duration is the whole result.

3. The prediction that cuts against the product, and it is specific to anybody training. Predict that 1,000 mg daily taken through a training block reduces the adaptation the block was for — smaller gains in lean mass, smaller mitochondrial marker responses Bjornsen 2016 Paulsen 2014 Wyckelsma 2025. A well-powered trial showing that high-dose vitamin C IMPROVES training adaptation would falsify this page.

4. Predict urinary oxalate rises, and treat it as the real safety endpoint. Ascorbate is metabolized to oxalate, and interest in high-dose supplementation tracks with concern about oxalate nephrolithiasis Kemble 2025. In anybody with a stone history, predict a measurable rise in 24-hour urinary oxalate on grams per day, and use that rather than a symptom as the stopping rule.

5. Predict iron absorption goes up, and know whether that is what you want. Ascorbate reduces ferric to ferrous iron and roughly triples the absorption of non-heme iron from the same meal. Predict ferritin and transferrin saturation to drift upward over months of gram doses taken with meals — useful in iron deficiency, unhelpful in hemochromatosis.

What nobody has tested yet

Nobody has established whether the training interference applies at ordinary doses. The trials that found blunted adaptation used 1,000 mg of vitamin C with vitamin E. Whether 200 mg, or vitamin C alone, or dosing away from the training session avoids the effect has not been tested, and that is the single most useful experiment anyone could run for the athletic reader Li 2022.

The liposomal claim has never been taken to an endpoint. Higher plasma concentrations from encapsulated preparations have been reported; no randomized trial has shown that a person achieving them gets anything a 500 mg tablet does not. Given the price difference, that is a conspicuous gap.

The oxalate dose-response is not characterized. The conversion of ascorbate to oxalate varies between individuals and with gut microbiota, notably with Oxalobacter formigenes colonization, and no study has established the intake at which stone risk rises in an unselected population Kemble 2025.

And the extreme-stress finding has never been explained. Halving cold incidence in marathon runners and subarctic soldiers, while doing nothing in the general population, is a large and reproducible subgroup effect with no accepted mechanism Hemila 2013. Whether it reflects transiently raised turnover, neutrophil ascorbate depletion or something else is unresolved.

Vitamin C — its own safety story, not its category's

The dose-limiting effect is osmotic and self-correcting. Unabsorbed ascorbate holds water in the bowel, so loose stools, cramping and flatulence are how most people find their ceiling. Buffered salts are genuinely gentler. This is a nuisance rather than a hazard and it is the body reporting that the transporter is saturated.

The kidney is where the real risk lives. Oxalate is a terminal metabolite of ascorbate, and high-dose supplementation raises urinary oxalate excretion. In people with existing stone disease, chronic kidney disease or a high-oxalate diet, this is a specific and avoidable risk, and acute oxalate nephropathy has been reported after very large intravenous and oral exposures Kemble 2025.

Two hematological cautions that are not theoretical. In glucose-6-phosphate dehydrogenase deficiency, very high doses can precipitate hemolysis. And because ascorbate enhances non-heme iron absorption, it is contraindicated in hereditary hemochromatosis and in transfusional iron overload, where it can also mobilize iron into a redox-active pool.

The interference nobody expects is with the meter. High plasma ascorbate interferes with several point-of-care glucose monitors, with fecal occult blood testing, and with some urine dipstick chemistries. A falsely high home glucose reading in somebody taking grams of vitamin C is a documented and preventable confusion.

Who should be careful, and one thing to avoid. Anyone with a history of calcium oxalate stones, anyone with hemochromatosis, and anyone in a training block who wants the adaptation Paulsen 2014. Stopping a very high dose abruptly after months has been reported to produce a transient rebound in requirement as renal clearance stays upregulated; tapering costs nothing. Nothing here is medical advice or diagnosis, and these statements have not been evaluated by the Food and Drug Administration.

Sources read for this page

• Dosedel M, et al. Vitamin C: Sources, Physiological Role, Kinetics, Deficiency, Use, Toxicity, and Determination.. Nutrients 2021 · PMID 33668681
• Hemila H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews, 2013 · PMID 23440782
• Paulsen G, et al. Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial.. Journal of Physiology 2014 · PMID 24492839
• Bjornsen T, et al. Vitamin C and E supplementation blunts increases in total lean body mass in elderly men after strength training.. Scandinavian Journal of Medicine and Science in Sports 2016 · PMID 26129928
• Wyckelsma VL. Antioxidant supplementation blunts the proteome response to 3 weeks of sprint interval training preferentially in human type 2 muscle fibres. J Physiol 2025 · PMID 40433923
• Li S. Potential harms of supplementation with high doses of antioxidants in athletes. J Exerc Sci Fit 2022 · PMID 35812825
• Kemble JP. Public Interest in Vitamin C Supplementation During the COVID-19 Pandemic as a Potential Risk for Oxalate Nephrolithiasis. Cureus 2025 · PMID 40130101

How you would know if it worked

Vitamin C — safety & side effects

Bloodwork to run alongside Vitamin C

Baseline first, then again at 8–12 weeks.

The Full Micronutrient Screen panel covers these in one order — 11 markers, $363.60 with the discount applied.

Check results you already have → · All 103 markers A–Z

Vitamin C — frequently asked questions

Vitamin C inside a finished plan

One arm of 2 Protocol Blueprints, free to read in full.

What Vitamin C is used for

Vitamin C appears under 8 goals in the goal router.

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