Vitamin C

Vitamin C is the common name for L-ascorbic acid, a water-soluble organic acid that readily gives up electrons, which is the basis of both its antioxidant behavior and its role as an enzyme cofactor ^[1]. Unlike most animals, humans and other primates have lost the ability to make ascorbic acid and depend entirely on dietary intake ^[1]. It is taken up by specific sodium-dependent transporters, and its oxidized product dehydroascorbic acid can enter cells through glucose transporters, with the body regulating vitamin C levels through absorption, tissue uptake, and renal excretion ^[1].

As an electron donor, ascorbic acid serves as a cofactor for a set of mammalian enzymes, most familiarly the hydroxylases that stabilize and mature collagen, the structural protein of skin, blood vessels, and connective tissue ^[1][3]. It also supports the synthesis of certain hormones and neurotransmitters and participates in antioxidant defense by scavenging reactive oxygen species and helping regenerate other antioxidants ^[1]. In addition, vitamin C enhances the absorption of non-heme iron from plant foods ^[1].

A prolonged lack of vitamin C impairs collagen formation and produces scurvy, whose features include bleeding and swollen gums, easy bruising, poor wound healing, joint pain, and fatigue ^[1]. Scurvy was historically notorious among sailors on long voyages without fresh produce, and the recognition that citrus fruit could prevent it was an early landmark in nutritional science ^[1]. The disease is now uncommon but still occurs where diets lack fruit and vegetables ^[1].

Beyond preventing deficiency, vitamin C is one of the most widely taken supplements, often in the belief that it wards off the common cold ^[2]. Pooled analysis of controlled trials indicates that routine supplementation does not reduce how often adults in the general population catch colds, although it modestly shortens their duration and may help people undergoing brief periods of intense physical stress ^[2]. High-dose intravenous vitamin C has been studied in other settings, and research continues into its physiology and its actions at different concentrations ^[1].

Vitamin C is regulated as a food, dietary supplement, and food additive rather than a prescription medicine, and it is also used industrially as an antioxidant preservative ^[1]. It is sold as plain ascorbic acid and as buffered mineral ascorbates and other formulations, in tablets, capsules, and powders ^[1]. Because the body tightly controls its vitamin C levels and excretes the excess, very high oral doses mainly cause digestive upset rather than serious toxicity ^[1].

Vitamin C is very safe; excess is excreted, and the main side effect is GI upset and diarrhea at high grams. High doses can raise oxalate and may matter for people prone to kidney stones. Otherwise it is well tolerated across a wide dose range.

Vitamin C's most consequential interaction is not with a drug but with a measurement. At high plasma concentrations ascorbic acid reduces the mediator used in glucose oxidase and glucose dehydrogenase test strips, so point-of-care meters return falsely high readings, and on some devices falsely low ones or an error. Bench testing across meter models shows bias well beyond clinically acceptable limits at the concentrations reached during intravenous vitamin C, and a falsely high reading invites unnecessary insulin. It also interferes with fecal occult blood tests and several colorimetric assays.

Ascorbic acid sharply increases non-heme iron absorption, which is useful alongside iron salts and unhelpful in hemochromatosis or transfusional iron overload. Combined with deferoxamine, high doses have been linked to cardiac decompensation, apparently through mobilized free iron.

Gram quantities are excreted partly as oxalate, so alongside other nephrotoxins or reduced renal function the risk of oxalate nephropathy rises.

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The history of vitamin C is inseparable from scurvy, the scourge of long sea voyages, and from the landmark 1747 experiment of the Scottish naval surgeon James Lind, who showed that citrus fruit could cure and prevent the disease long before its chemical basis was understood. The vitamin itself was isolated between 1928 and 1932 by the Hungarian biochemist Albert Szent-Gyorgyi, who first obtained the substance he called hexuronic acid from adrenal glands and plant tissues; he received the Nobel Prize in Physiology or Medicine in 1937 for this and related work, while the American biochemist Charles Glen King independently identified it as the antiscorbutic factor.

Its structure was established by Norman Haworth, and a practical industrial synthesis was devised by Tadeus Reichstein in the early 1930s, making the vitamin cheaply available worldwide. In later decades the chemist Linus Pauling controversially championed very high doses, helping to spark enduring scientific interest in the vitamin's pharmacological, as opposed to nutritional, effects.

Vitamin C is one of the most familiar and trusted nutrients, valued as a potent antioxidant and as the essential cofactor for the enzymes that build collagen, which is why it is so closely tied to healthy skin, blood vessels, and wound healing. It reliably prevents and cures scurvy and enhances the absorption of iron from plant foods, giving it a firm place in everyday nutrition.

In recent years it has attracted serious scientific attention at a very different scale: intravenous infusions can achieve blood levels far beyond what diet allows, at which point ascorbate behaves as a pro-oxidant that selectively stresses certain tumor cells, an effect now under active investigation in oncology. This research is genuinely promising yet still preliminary, and results in areas such as sepsis have been mixed, so enthusiasm is tempered by honest uncertainty. As a dietary nutrient, however, its value is beyond dispute.