Vitamin C, explained properly.

What vitamin C actually is

Vitamin C is the common name for L-ascorbic acid, a water-soluble vitamin that humans cannot synthesize. Almost all other mammals make their own vitamin C from glucose in the liver — humans (and other primates, plus guinea pigs and a few fruit bat species) lost the gene encoding L-gulonolactone oxidase about 61 million years ago, so we depend entirely on dietary intake.

This single genetic deletion is why scurvy — the historical hallmark of vitamin C deficiency — was such a devastating disease for sailors, polar explorers, and anyone without access to fresh fruit for months at a time. Today, scurvy is rare in developed countries but marginal deficiency (plasma levels < 23 µmol/L) is still common in smokers, the elderly, and people on restrictive diets.

How vitamin C works in the body

Vitamin C is a cofactor for 8 human enzymes. Most of its clinically-relevant effects trace back to those 8 reactions, not to the popular antioxidant framing:

1. Collagen synthesis. Vitamin C is required for proline and lysine hydroxylation, the two reactions that make collagen fibers stable enough to hold your skin, blood vessels, tendons, and bones together. This is why severe deficiency causes bleeding gums, poor wound healing, and eventually death.
2. Carnitine synthesis. Required to transport long-chain fatty acids into mitochondria for energy. Low vitamin C → fatigue, exercise intolerance.
3. Neurotransmitter synthesis. Dopamine β-hydroxylase converts dopamine to norepinephrine using vitamin C. Adequate intake supports mood and stress response.
4. Iron absorption. Reduces dietary ferric iron (Fe³⁺) to the ferrous form (Fe²⁺) that the gut can absorb. A 50 mg vitamin C dose roughly doubles non-heme iron absorption.
5. Immune function. Concentrates in neutrophils and lymphocytes, supports phagocytosis, and shortens cold duration by ~8% in adults (1.4 days) per Hemilä 2013 Cochrane. Does NOT prevent colds in the general population.
6. Antioxidant recycling. Regenerates oxidized vitamin E back to its active form in lipid membranes. Also scavenges free radicals directly in plasma.
7. Carnitine + cholesterol metabolism. Affects bile acid synthesis and LDL clearance.
8. Peptide hormone processing. Required for amidation of several hormones including vasopressin and oxytocin.

The 5 forms of vitamin C, compared

All vitamin C supplements are eventually converted to L-ascorbic acid in the body, but the starting form affects absorption, gut tolerance, and price.

How much vitamin C do you actually need?

Per the US Institute of Medicine (IOM 2000) and the Australian NHMRC. The body absorbs less and less of each additional mg above ~200 mg, so dose splitting matters more than dose size for plasma levels.

Source: NIH Office of Dietary Supplements · UL = tolerable upper intake level. The UL of 2000 mg/day is conservative — diarrhea thresholds are usually 3000-5000 mg/day in healthy adults. Smokers need ~35 mg more than non-smokers because smoking oxidizes vitamin C faster.

The dosing strategy that actually works

Most vitamin C supplements are dosed wrong. The body tightly regulates plasma levels through intestinal absorption (SVCT1 transporter) and renal excretion. Here is what actually moves the needle:

Smokers, sick days, and the kidney-stone myth

What to pair with vitamin C (and what to avoid)

Frequently asked questions

References & sources

1. NIH Office of Dietary Supplements — Vitamin C Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/
2. Padayatty SJ, et al. (2004). Vitamin C pharmacokinetics: implications for oral and intravenous use. Annals of Internal Medicine 140(7):533-537. PMID: 15068921
3. Hemilä H, Chalker E. (2013). Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews. PMID: 23440782
4. Levine M, et al. (1996). Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. PNAS 93(8):3704-3709. PMID: 8623000
5. Carr AC, Frei B. (1999). Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans. American Journal of Clinical Nutrition 69(6):1086-1107. PMID: 10357726
6. Linus Pauling Institute — Vitamin C. lpi.oregonstate.edu
7. Examine.com — Vitamin C research review. examine.com
8. Chemist Warehouse AU, iHerb, Amazon, Swisse AU, Blackmores AU, BioCeuticals AU, Ethical Nutrients, Herbs of Gold, Nature's Own, Vistra TH, Mega We Care, Thorne, Interpharma — product pages and price snapshots cited per row in the vitamin C comparison table.