Vitamin C, Scurvy and the Antioxidant-Supplement Myth

What vitamin C actually does

Ascorbic acid is not a vague "immune booster" — it is a precise chemical tool, an electron donor. Its headline job is collagen synthesis. Collagen — the scaffolding protein of skin, blood-vessel walls, gums, tendon and bone — is only strong because its strands are cross-linked, and those cross-links are forged by two enzymes, prolyl hydroxylase and lysyl hydroxylase. Both enzymes need vitamin C to keep their iron atom in the working state. Without vitamin C the hydroxylation fails, the collagen produced is weak and under-cross-linked, and any tissue that leans on collagen — above all the walls of small blood vessels — begins to fail. That single biochemical fact explains almost every sign of scurvy. Beyond collagen, vitamin C is a water-soluble antioxidant that mops up reactive oxygen species; a cofactor for the synthesis of carnitine (needed to burn fat for energy) and of certain neurotransmitters such as noradrenaline; and, in a quietly important role for the gut, a reducing agent that aids iron absorption.

That last role is the one clinicians use every day. Non-haem iron — the iron in plants, pulses and fortified foods — is absorbed poorly because it arrives in the oxidised ferric (Fe3+) form, which the gut struggles to take up. Vitamin C donates an electron and reduces it to the ferrous (Fe2+) form, which the intestinal transporter accepts readily. This is why a glass of orange juice with an iron-rich meal, or vitamin C taken alongside an oral iron tablet, measurably increases how much iron is absorbed — a cheap, practical trick that matters most in vegetarians and in iron-deficiency anaemia. It is exactly the link the Haematology section draws between diet and iron status, and it explains one of scurvy's quieter features: a mild anaemia, partly from poor iron handling and partly from the bleeding itself.

Scurvy: the deficiency written across the body

Humans, uniquely among most mammals, cannot make their own vitamin C — we lost the final enzyme in the pathway — so we depend entirely on diet, and we keep almost no reserve. Body stores are small and run down within a few weeks to a couple of months of near-zero intake, which is why scurvy appears faster than most people expect. The classic signs all trace back to failed collagen. In the skin: perifollicular haemorrhages (pinpoint bleeds ringing the hair follicles) and corkscrew hairs, where the weakened hair coils instead of growing straight. In the mouth: swollen, spongy, bleeding gums that can loosen the teeth. Everywhere: easy bruising, bleeding into joints and muscle causing arthralgia, poor wound healing and the breakdown of old scars, and a background anaemia. In children the picture adds impaired bone growth and painful subperiosteal bleeds, because the growing skeleton depends on collagen too.

Scurvy is a disease of the diet, so it clusters where diets go wrong. It is far from a historical curiosity. It reappears in the socially isolated elderly living on tea and toast, in people with alcohol-use disorder whose intake is chaotic and whose needs are higher, in those on extreme or highly restrictive diets, in some psychiatric illness and in severe poverty. Cooking and prolonged storage are part of the problem too: vitamin C is heat-labile and water-soluble, so overcooking vegetables or boiling them in plenty of water leaches and destroys much of the vitamin before it reaches the plate. The good sources are fresh fruit and vegetables — citrus, berries, blackcurrants, kiwi, peppers, tomatoes, broccoli and potatoes — ideally raw or lightly cooked. Because there is no meaningful store, a steady daily supply matters more than an occasional large dose.

Megadoses: colds, sepsis and the limits of the evidence

The moment a nutrient is called "essential," a tempting fallacy follows: if a little is vital, more must be better. Vitamin C is the classic case. Popularised decades ago as a cure for the common cold, high-dose vitamin C has been tested exhaustively — and the honest verdict is that routine supplements do not prevent colds in the general population and, at best, trim their duration by a trivial amount. It was later trialled at high intravenous doses in sepsis, an appealing idea given its antioxidant chemistry, but the well-conducted trials showed no convincing benefit — and some hinted at harm. The pattern repeats across the antioxidant vitamins: a mechanism that sounds compelling at the bench rarely survives contact with a randomised trial. Correcting a deficiency is transformative; loading a replete person is not.

"Water-soluble, so the excess just washes out" is only half true. It is true that most surplus vitamin C is excreted in the urine — which is exactly why megadoses achieve little and produce, memorably, "expensive urine." But excreted does not mean harmless. At very high doses vitamin C is metabolised to oxalate, and the extra urinary oxalate can precipitate as calcium-oxalate kidney stones in susceptible people — a real, avoidable harm. High doses can also interfere with certain laboratory tests, notably some point-of-care blood-glucose meters and urine dipsticks, giving falsely misleading readings. And in people with G6PD deficiency, very high intravenous doses can trigger haemolysis, because their red cells cannot handle the oxidative flux — a direct tie to the G6PD story told in the Haematology and Toxicology sections. None of this makes ordinary dietary vitamin C dangerous; it means the megadose is where the risk lives.

The antioxidant-supplement myth

Vitamin C is the entry point to one of the most important evidence-based-medicine lessons in all of nutrition. Oxidative damage contributes to cancer and cardiovascular disease; antioxidant vitamins — C, E and beta-carotene — neutralise oxidants in the test tube; therefore, the reasoning went, antioxidant supplements should prevent those diseases. It is a clean, intuitive hypothesis — and large randomised trials have demolished it. Supplementing antioxidant vitamins in well-nourished populations has, again and again, failed to reduce cancer or heart disease. Worse, some trials found net harm: beta-carotene supplements increased lung-cancer incidence in smokers, and high-dose vitamin E was associated with increased mortality and other harms. The intervention meant to protect actually hurt. This is the same theme developed in the Vitamins A & E chapter and the Supplements chapter — the repeated, expensive discovery that isolated antioxidant megadoses do not deliver what their mechanism promises.

Why does the whole food beat the isolated pill? The likeliest explanation is that the benefit associated with fruit-and-vegetable-rich diets was never due to any single antioxidant taken out of context. Whole foods deliver a complex, balanced mixture of hundreds of compounds in physiological amounts, alongside fibre and a pattern of eating; a megadose capsule delivers one molecule at a supraphysiological level, which can tip redox chemistry the wrong way and even blunt useful adaptive responses. The practical rule that falls out of decades of trials is simple and worth teaching without hedging: treat genuine deficiency, and get antioxidants from a balanced diet rather than from megadose supplements. Supplements are for correcting a demonstrated lack — not an insurance policy the well-fed should buy.