Vitamin C

Vitamin C, also known as ascorbic acid, is a water-soluble micronutrient that humans must obtain from food because they cannot synthesize it. Its fully reduced form, L-ascorbic acid, works mainly as an electron donor. It is a cofactor for a family of enzymes that hydroxylate collagen, carnitine, hormones and neurotransmitters, and it also acts as a major small-molecule antioxidant in plasma and tissues. Its historical importance is that its absence causes scurvy, a disease that once killed large numbers of sailors on long voyages. The discovery of its chemical identity in the 1930s was one of the first clear links between a dietary factor and a specific deficiency syndrome, and it helped establish the modern concept of vitamins as organic compounds that the body cannot make in sufficient amounts.

Vitamin C matters for medicine and public health for several reasons. Its physiological roles include wound healing, the maintenance of blood vessels and bone, and support for immune cell function. It also plays a part in gene regulation through enzymes that remove methyl groups from DNA and histones. Routine deficiency is uncommon in high-income countries but remains a concern for certain populations, while the health effects of high-dose supplements, including claims about colds and cancer, are still debated. Industrially, the compound is produced at large scale for food, pharmaceutical and cosmetic uses.

This entry first describes the chemistry of vitamin C and how the body absorbs, transports and stores it. It then examines its biological functions, including its roles as an enzyme cofactor, an antioxidant and a regulator of gene expression. Next it covers dietary sources, deficiency and scurvy, toxicity, and the evidence for supplementation. The entry ends with industrial production and the questions that remain open.

Chemical nature and forms

Structure and reducing power

The biologically active form of vitamin C is L-ascorbic acid, with the formula $\mathrm{C_6H_8O_6}$. It is a small organic molecule built on a six-carbon chain that closes into a five-membered cyclic ester, a lactone. The feature that governs its chemistry is an enediol group: two adjacent carbons (C2 and C3) carry hydroxyl groups and are joined by a carbon–carbon double bond. Because each of these hydroxyl hydrogens is unusually easy to release, the first one has an acid dissociation constant near 4.2, so at physiological pH the compound exists mostly as the ascorbate anion.

The same arrangement makes the molecule an effective reducing agent. Removing a hydrogen atom from each enol group releases electrons, and the enediol donates them in two successive one-electron steps. Each step passes through a short-lived ascorbyl (monodehydroascorbate) radical. Donating electrons lowers the oxidation-state of whatever it reacts with, which is why ascorbate can keep metal centers in enzymes and small-molecule oxidants in their reduced form.

Reversible and irreversible oxidation

Ascorbate's first oxidation product is dehydroascorbic acid (DHA), in which C2 and C3 have become ketone groups. This step is reversible: enzymes and small thiol-containing molecules such as glutathione can reduce DHA back to ascorbate, which is why the two forms together act as a redox pair in tissues. Some authors treat DHA as having comparable biological value to ascorbate, so analyses of foods often measure the sum of both forms.

The lactone ring of DHA is far less stable. In aqueous solution it can open by hydrolysis to 2,3-diketogulonic acid, and this ring opening is effectively irreversible. Further breakdown gives products such as furfural, 2-furoic acid and 3-hydroxy-2-pyrone, and the route depends on conditions. In acidic solution under aerobic conditions, ascorbate passes through DHA to 2-furoic acid and 3-hydroxy-2-pyrone, whereas under anaerobic conditions furfural dominates. Recent work has also questioned whether DHA is the main product of degradation in some alkaline systems, so the simple scheme should be applied with care. Once the ring is opened, the vitamin cannot be regenerated, so this step determines the practical loss of vitamin activity in foods and stored solutions.

Factors that accelerate loss

In foods and solutions, several factors speed up the conversion. Heat is a major factor: degradation generally follows first-order kinetics, and in one storage study a sample lost about 23 percent of its ascorbic acid after seven days at 25 °C in the dark, compared with about 56 percent at 35 °C. Cold storage slows the process considerably. Light promotes photo-oxidation, and oxygen drives autoxidation, with dissolved oxygen causing a rapid initial loss in aqueous systems. Alkaline conditions favour rapid breakdown, and at around pH 10 the main product is not DHA-derived furfural but an unidentified compound. Trace metal ions, particularly copper(II) and iron(III), catalyse the oxidation with oxygen. Because the reaction rate depends on the balance of reducing and oxidizing species in the medium, the same food can lose its vitamin at very different rates depending on packaging, pH and processing.

Naming and stereochemistry

"Vitamin C" is a generic term that covers ascorbic acid, its salts (such as sodium ascorbate and calcium ascorbate) and its esters (such as ascorbyl palmitate). The name "ascorbic" reflects the compound's antiscorbutic action, with the prefix a- meaning "without" and the root referring to scurvy. The molecule has two chiral centres, C4 and C5, giving four possible stereoisomers. Only the L-configuration at C5, which corresponds to the L-series of sugar acids, has full vitamin activity, because the transporters and enzymes that handle the vitamin in the body recognise this shape specifically. The D-enantiomer has very little vitamin activity. Erythorbic acid (D-isoascorbic acid) is a stereoisomer that retains strong antioxidant capacity and is widely used as a food additive, but it does not satisfy vitamin requirements. The distinction therefore matters for nutrition and for food labelling, even though the stereoisomers have similar chemical reactivity.

Discovery and history of identification

Citrus and the first controlled trial

Scurvy was long familiar to sailors, but its cure was found through comparison rather than theory. In 1747 the British naval physician James Lind compared six treatments in twelve sailors with the disease, and only oranges and lemons were effective in treating scurvy. Lind's result did not change naval practice at once. Gilbert Blane (1795) managed to persuade the Admiralty to use citrus juice as a daily ration on board British naval vessels. The science stayed incomplete for another century and a half: the active agent was unknown, and no animal model of the disease existed. In 1907 Axel Holst and Alfred Fröhlich, two Norwegians, posited the existence of vitamin C on the basis of its biological effects, and an international competition ensued to isolate the vitamin. Their work, which fortuitously produced scurvy in the guinea pig, gave researchers a laboratory model for testing purified fractions.

Hexuronic acid and the race to isolate it

Albert Szent-Györgyi, who was studying biological oxidation, obtained a crystalline substance from adrenal tissue. Sources differ on the date: one records that in 1926, during the course of his studies on biological oxidation process, A. Szent-Gyorgyi, obtained a crystalline substance from the adrenal cortex, which he first called Cx11. In 1928, he renamed the substance "hexuronic acid", while others give 1928 as the year of isolation. He later found the same reducing agent in plant juices and adrenal gland extracts, and he proved that hexuronic acid, which he had previously isolated, is identical with vitamin C and that it could be extracted in kilogram quantities from paprika.

Charles Glen King, working at the University of Pittsburgh, reached a parallel conclusion. King came to the same conclusion and published an announcement in Science magazine on April 1, 1932, two weeks before Szent-Györgyi's note appeared in Nature. A fuller report followed in the Journal of Biological Chemistry by Waugh and King in 1932. The credit for the discovery has been disputed ever since; controversy remains over whether both men deserve equal credit. The confirmation that hexuronic acid was the antiscorbutic factor depended on Szent-Györgyi's collaborator, J. L. Svirbely, an American-born chemist of Hungarian descent, who had previously worked with Charles King, and the guinea pig tests that the two of them carried out.

Structure, synthesis and naming

The chemical identity was settled in 1933. The structure of hexuronic acid (vitamin C) was elucidated in 1933 by Hirst and Haworth using material isolated by Szent-Gyorgyi. Norman Haworth and Szent-Györgyi then proposed a new name that described the compound's function, based on the Latin roots of its clinical association: "a-scorbic acid" («a» meaning no in Latin and «scorbutus» referring to scurvy) because of its antiscorbutic function. The compound was made in the laboratory in the same year: it was synthesized by Haworth, and independently by Tadeus Reichstein in 1933. Synthetic ascorbic acid was soon available commercially.

Nobel recognition

The 1937 prizes recognised different parts of this work. Szent-Györgyi received the Nobel Prize in Physiology or Medicine for "discoveries in connection with the biological combustion processes, with especial reference to vitamin C and the catalysis of fumaric acid." Haworth's award was in chemistry, for his investigations on the chemistry of carbohydrates and VitC. Haworth and another vitamin researcher Paul Karrer, shared that year's prize in chemistry, also in part for work on vitamin C. Accounts of the Nobel deliberations describe a contest over whether the physiology prize should go to Szent-Györgyi alone, and the prize was given to Szent-Györgyi alone, but the deliberations were reportedly long and acrimonious.

Effect on the concept of a vitamin

The discovery reshaped what a vitamin was. Casimir Funk had introduced the term in 1912 for essential dietary factors, and the so-called anti-scorbutic factor was soon considered as «water-soluble C». Once the substance was a defined compound with a known structure, its biological activity could be tied to a measurable chemical quantity rather than to a crude food extract, which made standardisation possible. The name ascorbic acid also reflected function rather than chemical class, a departure from the amine-based expectations behind Funk's original term. These features established vitamin C as a model case for defining nutrients by a pure compound and a specific deficiency disease, a framework that later shaped nutritional standards.

Biosynthesis and dependence on diet

Most mammals, and many other animals, can make their own ascorbate. In these species the pathway begins with glucose and passes through glucuronate, which is one of the carbohydrate intermediates of the glucuronate pathway. Glucuronate is converted to l-gulonate by aldehyde reductase, and l-gulonate is then converted to l-gulonolactone by a lactonase known as SMP30 or regucalcin. The final step is the oxidation of l-gulonolactone to L-ascorbic acid by L-gulonolactone oxidase (GULO), an enzyme associated with the endoplasmic reticulum membrane. The resulting 2-keto-L-gulonolactone then enolizes spontaneously to ascorbate. Early work in 1957 found that the pathway operated in the liver of mammals and in the kidney of birds and reptiles, and that the terminal enzyme was missing in some mammals, which is how the gap was first recognized.

Synthesis is not universal. Within the deuterostomes, humans, other non-human primates, the guinea pig, numerous bats, various birds and teleost fish have all lost the ability to make the vitamin, owing to the complete or partial loss of the GULO gene. The loss is not a single event. Anthropoid primates (the group that includes humans, apes and monkeys) and guinea pigs carry GULO sequences that are highly mutated and no longer code for a working enzyme. The anthropoid lineage has lost seven of the twelve exons found in functional vertebrate copies of the gene, while the guinea pig has lost its first and fifth exons along with part of the sixth. Comparative dating based on these differences places the inactivation at roughly 61 million years ago in anthropoid primates and about 14 million years ago in guinea pigs, which is consistent with the two losses being independent. The relic sequence that remains in the human genome is a pseudogene, a gene copy that has accumulated mutations and no longer produces a functional protein. Some bat GULO pseudogenes appear to have been reactivated during evolution, so the loss is not uniformly irreversible across all lineages.

The reason this gene in particular is so often lost is thought to be that its loss affects only one function, the production of vitamin C itself. Other genes in the pathway, such as regucalcin, are involved in calcium homeostasis and other processes, so their disruption would be costly, whereas damage to the terminal enzyme has few other consequences. Once the enzyme was no longer required for survival, natural selection would not act to preserve it, and genetic degradation could accumulate over many generations. A proposed explanation for why this was tolerated is that the ancestral diets of these animals were rich in fresh plant foods such as fruit and leaves, which supplied substantial amounts of ascorbate and so reduced the penalty for losing internal synthesis. A separate hypothesis, sometimes called the uric acid hypothesis, suggests that urate, which is abundant in primates, could have taken over part of the antioxidant role. These explanations remain plausible but are not firmly established, and the evolutionary history of the trait is still debated.

The practical consequence is that humans cannot replace the vitamin internally. The body pool is maintained only by intake from food or supplements, so the requirement is continuing rather than occasional. A diet that provides too little ascorbate for long enough will deplete the body's stores, and the resulting deficiency (scurvy) is described in a later section. This dependence on the diet is also the reason the vitamin is classified as an essential nutrient for humans, even though it is synthesized by most other mammals.

Absorption, transport and tissue distribution

Intestinal uptake

Dietary vitamin C reaches the blood mainly through the small intestine, and the two oxidation states use different routes. The reduced form, L-ascorbate, is carried across the apical membrane of enterocytes by sodium-dependent vitamin C transporter 1 (SVCT1), which couples its movement to the sodium gradient, so the process is active and energy-dependent. Ascorbate was taken up along the entire length of the small intestine with a threefold higher initial uptake rate in distal than proximal segments. Absorption is therefore not confined to one stretch of bowel. Because the carrier is saturable, the proportion of an intake that is absorbed falls as the dose rises, which is one reason high oral doses do not translate into proportionally higher plasma levels.

Entry into cells and recycling of the oxidized form

The oxidized form, dehydroascorbic acid (DHA), is handled by a different set of carriers. SVCT1 and SVCT2 show an absolute specificity for reduced vitamin C and are unable to transport dehydroascorbic acid. Instead, the glucose transporters GLUT1, GLUT3, and GLUT4 (but not GLUT2, GLUT5, or SGLT-1) are able to transport this form of vitamin C. This matters because although dehydroascorbic acid concentrations are low in plasma under normal conditions, the oxidized molecule is absorbed at much higher rates across GLUT1 and 3 than the reduced form is across the SVCTs. Once inside a cell, DHA is reduced back to ascorbate, which keeps the reduced form trapped and accumulating against the concentration gradient. Together, the SVCT and GLUT routes allow tissues to retain vitamin C in a form that can be used in enzymatic and antioxidant reactions.

Distribution from the bloodstream to organs relies largely on SVCT2. Distribution from the bloodstream to target organs is mediated by SVCT2, a transporter that is widely expressed in all tissues including the choroid plexus. Erythrocytes are an exception, since red blood cells lose SVCT proteins during maturation.

Body pool and tissue distribution

The classical estimate for the whole-body pool is about 1500 mg. At saturation the whole body content of ascorbate in adult males is approximately 20 mg/kg, or 1500 mg. Clinical signs of scurvy appear when the whole body content falls below 300-400 mg. This figure comes from isotope and tracer studies carried out several decades ago, and some recent reviews argue that it may be an underestimate. The body pool of VitC in humans is currently not known in detail, but the original estimates of 1500 mg based on tracer studies performed about 6 decades ago are very likely underestimated.

Concentrations differ widely between organs. The adrenal gland and the eye rely heavily on SVCT2, and SVCT2 is the main isoform in the adrenal gland and in the eye. White blood cells accumulate the vitamin strongly: in one pharmacokinetic study, neutrophils, monocytes, and lymphocytes saturated at 100 mg daily and contained concentrations at least 14-fold higher than plasma. Brain tissue also takes up ascorbate, mainly at the choroid plexus, which is where the transport barrier between blood and cerebrospinal fluid lies.

Renal handling and the plasma plateau

The kidney is the main regulator of how much vitamin C the body keeps. Filtered ascorbate is largely reclaimed in the proximal tubule, and if the body stores and plasma concentration are low, SVCT1 in the kidneys will almost quantitatively reabsorb ascorbate from the urine to prevent loss of vitamin C from the body. This reabsorption is itself saturable. When intake is high, tubular reabsorption reaches its limit and the surplus is excreted. In healthy people, the body becomes saturated, resulting in plateau plasma steady state concentrations of about 65–80 μmol/L, and any further excess of vitamin C is quantitatively excreted.

The shape of the intake–plasma curve follows from these two saturable steps. Saturable gastrointestinal absorption and nonlinear renal clearance act additively to produce the ceiling effect in plasma concentrations. In the classic steady-state study, the steep portion of the curve occurred between the 30- and 100-mg daily dose, and complete plasma saturation occurred at 1000 mg daily. Plateau concentrations are reached at intakes above roughly 200 mg per day, so further increases in dose add little to plasma levels and mostly raise urinary losses.

Cofactor roles in enzymatic reactions

In these reactions ascorbate works as a cofactor in the sense that an enzyme requires it, but it is also used up stoichiometrically. It donates electrons, and the oxidized product that results is dehydroascorbate, so the vitamin is not regenerated within the enzyme itself. Ascorbate serves as an electron donor to eleven enzymes, three of which are found in fungi but not in humans or other mammals. That leaves eight human enzymes, several of which are monooxygenases or dioxygenases that depend on the vitamin's reducing power.

The central mechanism is protection of metal ions at the active site. Many of these enzymes carry iron or copper, and the catalytic cycle requires the metal to switch between oxidation states. For copper-dependent enzymes, copper alternates between its oxidized (Cu²⁺) and reduced (Cu⁺) states, enabling the enzyme to maintain its catalytic activity. Ascorbate supplies the electrons that return the metal to its active reduced form, and in iron-dependent enzymes it helps keep iron in the ferrous state so that it can bind oxygen and substrate. The two enzyme classes differ in how they use oxygen: the monooxygenases dopamine β-monooxygenase and peptidyl-glycine α-monooxygenase incorporate a single oxygen atom into a substrate, whereas the dioxygenases incorporate two. Both reactions are redox reactions in which ascorbate is the reductant.

Dopamine β-hydroxylase and catecholamine synthesis

The clearest example of an ascorbate-dependent step in the body is the conversion of dopamine to noradrenaline by dopamine β-hydroxylase (DBH). The enzyme is a copper-containing protein with four identical subunits, and its activity requires ascorbate. The products are norepinephrine, dehydroascorbate, and H2O. The reaction takes place inside the catecholamine storage vesicles of noradrenergic neurons and adrenal chromaffin cells, and noradrenaline is one of the neurotransmitters and hormones formed by this route. Experimental work on cultured chromaffin cells showed that ascorbate-depleted cells had low dopamine beta-hydroxylase activity, and adding ascorbate restored both intracellular levels and the rate of the reaction. The vesicular pool is maintained by electron transport across the vesicle membrane, so the reduced cofactor is continuously regenerated from cytosolic ascorbate. A rare hereditary defect in the vesicle protein CYB561 has been linked to a shortage of ascorbate inside these vesicles, producing a functional DBH deficiency.

Carnitine biosynthesis

Ascorbate is also a cofactor for enzymes in the carnitine pathway. Carnitine is an essential cofactor in the transport of long-chain fatty acids into mitochondria in order to produce ATP via beta-oxidation. In the pathway, four enzymatic steps are required to synthesize carnitine, and the first and last are catalyzed by a trimethyllysine dioxygenase and a γ-butyrobetaine dioxygenase. Earlier biochemical work suggested that the vitamin was indispensable for these steps. However, a study in senescence marker protein-30/gluconolactonase knockout mice, which cannot synthesize vitamin C, reported that the total carnitine levels of both groups were similar in all tissues and serum, leading the authors to conclude that the vitamin is not essential for carnitine biosynthesis in vivo. The question therefore remains open, and the role of ascorbate in carnitine formation is less settled than in the catecholamine pathway.

Peptide amidation

The third major cofactor role is in peptide amidation, the process that completes many neuropeptides and peptide hormones. The enzyme involved, peptidylglycine α-amidating monooxygenase (PAM), is the only known enzyme able to catalyze the reaction of amidation of the terminal carboxyl (C-terminal α-amidation) as the final and essential step in the biosynthesis of neuropeptides and peptide hormones. The reaction proceeds in two steps. In the first, this first partial reaction is catalyzed by the Cu and Ascorbate dependent PHM, which hydroxylates the α-carbon of the C-terminal glycine. In the second, a zinc-dependent lyase removes glyoxylate to leave the C-terminal amide. Substrates include adrenomedullin (ADM), calcitonin gene-related peptide (CGRP), amylin, neuropeptide Y (NPY), substance P, and many others, and the amide is often essential for receptor binding and biological activity. Because of this dependence, dysregulation of amidated peptide hormones has been associated with multiple pathologies, including neurodegenerative diseases, cardiovascular conditions, metabolic disorders. Some researchers have speculated that vitamin C administration could influence this pathway in sepsis, but this remains a hypothesis rather than an established effect.

Collagen synthesis and connective tissue

Collagen as the target of ascorbate

Collagen is built from three polypeptide chains wound into a triple helix. Its stability depends on a composition rich in glycine, proline, and the modified amino acids hydroxyproline and hydroxylysine. Glycine occupies every third position in the repeating sequence, which allows the chains to pack tightly. Proline and lysine residues must be hydroxylated while the chains are still unfolded, in the endoplasmic reticulum, because some of the enzymes can only modify unfolded chains, not triple-helical collagen. Vitamin C is required for this step.

The hydroxylation reaction

The modification is carried out by prolyl and lysyl hydroxylases, which are iron-dependent enzymes. Vitamin C keeps the iron in these enzymes in its active ferrous state. During catalysis the iron is oxidised to the ferric form, which inactivates the enzyme, and ascorbate reduces it back so that activity is restored. Because the vitamin is gradually oxidised in this cycle, a continuing supply is needed. Without it, hydroxylation of proline and lysine does not occur, and the chains cannot assemble into the normal helix.

The hydroxyl groups have distinct roles. Hydroxyproline stabilises the triple helix, while hydroxylysine provides the sites for later covalent linkage between fibres and for glycosylation. After the chains are secreted, extracellular enzymes cleave the terminal propeptides, and the resulting molecules self-assemble into fibrils. Lysyl oxidase catalyses covalent cross-links between lysine and hydroxylysine residues, which strengthen the fibrils.

Effects of impaired collagen formation

When collagen is structurally weak, the tissues that depend on it fail first. Blood vessel walls are especially affected. Defective collagen weakens the vessel walls, which leads to ruptured capillaries and bleeding under the skin, producing easy bruising and larger purple patches. Small pinpoint haemorrhages form around hair follicles, a pattern known as perifollicular haemorrhage (these often appear where weakened connective tissue is most vulnerable).

The gums and teeth are also affected. Swelling and bleeding of the gums is an early mucosal symptom, and progressive gum disease can lead to tooth loss. Defective collagen synthesis impairs dentine formation and causes bleeding into the gums. In growing children, bone is also affected. Weak bone matrix leads to fractures and bleeding beneath the periosteum. Haemorrhage is a hallmark of scurvy and can occur in any organ.

Wound healing, bone, teeth and burns

Impaired repair is one of the most consistent features of deficiency. Old wounds may reopen, and new cuts may fail to heal properly. Because repair of skin, bone and dentine depends on newly synthesised collagen, the same defect limits each of these processes. Recovery from extensive burns, which requires rapid deposition of new dermal collagen, is expected to be affected in the same way, although specific clinical evidence on vitamin C in burn care is outside the scope of this section.

Why these signs appear first

Changes in collagen-rich tissues are among the earliest signs of deficiency because these tissues depend most on continuous collagen turnover and have fragile capillary support. The pattern of change depends on the growth rate of the affected tissue, so bone changes are often seen mainly in infants during rapid growth. Clinical signs appear when the total body pool drops below about 300 mg, which is below the level at which clinical scurvy is seen. Treatment reverses these changes quickly: bleeding stops within 24 hours with proper treatment, and perifollicular petechiae resolve within about two weeks.

Antioxidant function and redox biology

Reactions with reactive oxygen species

Ascorbate readily gives up electrons, which lets it neutralise several reactive oxygen species (ROS). Superoxide radicals, hydrogen peroxide, hydroxyl radicals and singlet oxygen are commonly defined as ROS, and biological systems generate them as metabolic by-products. With superoxide and related radicals, ascorbate undergoes one-electron oxidation, and the product is an ascorbate radical that forms when both the neutral and anionic forms of ascorbate are oxidised by superoxide, hydroperoxyl and hydroxyl radicals. The measured rate constants for these reactions differ widely between studies. One determination for ascorbate and superoxide gave 3.3 × 10⁴ M⁻¹ s⁻¹, nearly three orders of magnitude lower than a value reported by an earlier group.

The reaction with singlet oxygen is much faster. Its rate constant is about 3 × 10⁸ M⁻¹ s⁻¹ (reported as rapid), and the stoichiometry is 1:1, with ascorbate and singlet oxygen producing hydrogen peroxide and dehydroascorbic acid. This conversion has a biological consequence: a short-lived oxidant becomes one that persists longer and can travel further, so that singlet oxygen with a half-life of nanoseconds to microseconds becomes an oxidant able to diffuse to distant targets. Oxidation products are not inert either. In plant and animal systems, dehydroascorbic acid and 2,3-diketogulonate can also quench reactive oxygen species.

Regeneration of vitamin E

Vitamin E (α-tocopherol) protects membranes and lipoproteins by reacting with lipid peroxyl radicals, which leaves a tocopheroxyl radical behind. Ascorbate is thought to repair this intermediate. Ascorbic acid reacts rapidly with the tocopherol radical, reducing it back to its native form. The two vitamins work in different compartments: vitamin C scavenges oxygen radicals in the aqueous phase, whereas vitamin E scavenges oxygen radicals within the membranes. Because ascorbate is hydrophilic, the regeneration step takes place at the membrane–cytosol boundary. In model systems, ascorbate regenerates vitamin E by a nonenzymic mechanism, whereas glutathione regenerates vitamin E enzymatically.

Cell studies have supported this idea. In primary rat hepatocytes, adding 2 to 6 mM ascorbate after oxidant exposure prevented further loss of cellular vitamin E, and the accumulation of the oxidised tocopherol derivative fell by 35%, which the authors took as evidence of recycling rather than simple sparing. Whole-body evidence is less clear. A review of the field noted that in vivo experiments have yielded equivocal results concerning this recycling pathway. In cigarette smokers, the rate of blood vitamin E oxidation caused by increased oxidative stress is substantially attenuated by vitamin C supplementation, which is consistent with a recycling role, though it does not prove one.

Metal ions and pro-oxidant behaviour

The same chemistry can turn ascorbate into a pro-oxidant. Its reducing power extends to transition metals, and these interactions involve redox reactions including the reduction of Fe(III) to Fe(II), through the Fenton reaction. The reduced metal then reacts with hydrogen peroxide, forming reactive hydroxyl radicals or peroxide ions. At high concentrations, vitamin C catalyzes the reduction of free transition metal ions, which causes the formation of oxygen radicals. Ascorbate also reduces copper, converting Cu²⁺ to Cu⁺, which can likewise drive the Fenton chemistry (Fenton reaction).

Whether this happens in the body depends on the availability of free metal. In healthy individuals, iron is largely sequestered by iron binding proteins such as transferrin and ferritin, and this iron is essentially redox inactive. Free iron appears mainly in disease, for example in thalassemia or hemochromatosis, where non-transferrin-bound iron is present. Some researchers propose three factors determine whether ascorbate acts as an antioxidant or pro-oxidant: the redox potential of the cellular environment, the presence or absence of transition metals, and the local concentration of ascorbate. The ascorbyl radical is also affected by metals: it is normally very stable, but becomes more reactive especially in the presence of metal ions, including iron. The pro-oxidant effect is also proposed to explain some results from high-dose intravenous vitamin C against metal-rich tumours, a question addressed in the cancer context.

How much of the effect is antioxidant?

The extent to which radical scavenging explains vitamin C's biological effects remains contested. Pro-oxidant reactions are well established by in vitro experiments, yet while there is extensive evidence of vitamin C pro-oxidative abilities in the presence of transition metals in vitro, there is no convincing and unambiguous evidence of such in vivo activity. Some authors argue that the antioxidant function remains dominant under normal physiological conditions where free metal ions are scarce. Others point out that the in vivo effects of ascorbate cannot be cleanly separated from its role as an enzyme cofactor, and that its oxidation products are themselves active. Antioxidant supplementation trials, discussed in the supplementation section, have produced inconsistent results, and a randomised controlled trial design has not settled how much of the vitamin's benefit, if any, comes from antioxidant action.

Epigenetic and gene-regulatory roles

Ascorbate affects gene expression mainly as a cofactor for a large family of enzymes that need Fe(II) and 2-oxoglutarate (2-OG). These dioxygenases use molecular oxygen and 2-OG to hydroxylate their substrates, and they need iron in the reduced ferrous state to work. Ascorbate's reducing power helps keep that iron active, which links the vitamin's chemistry to several epigenetic and signalling pathways.

TET enzymes and DNA demethylation

The ten-eleven translocation (TET) enzymes, TET1, TET2 and TET3, hydroxylate 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Further oxidation produces 5-formylcytosine and 5-carboxylcytosine, which can be removed from DNA and replaced, so this pathway is the main route for active demethylation of DNA. Adding vitamin C to mouse embryonic stem (ES) cells promotes TET activity, leading to a rapid and global increase in 5hmC. This is followed by DNA demethylation of many gene promoters and upregulation of demethylated germline genes. The effect is specific: vitamin C, but not other antioxidants, enhances the activity of recombinant Tet1 in a biochemical assay. Imprinted regions and intracisternal A-particle retroelements, which resist demethylation in the early embryo, also resisted this effect . A parallel study found that the vitamin promoted the Tet-mediated generation of 5-hmC in mouse and human cells.

In vivo evidence is more limited. In a mouse model that cannot synthesize ascorbate, offspring showed DNA hypermethylation in the liver, possibly because plasma and hepatic ascorbate levels remained low during the suckling period, despite dosing of the dams. The same process operates in immune cells: as a cofactor for epigenetic enzymes, ascorbate facilitates TET2/3-mediated DNA modification and demethylation of multiple elements at the Prdm1 locus.

Histone demethylases

Jumonji C-domain histone demethylases use the same cofactor set, and vitamin C serves as a cofactor for members of the α-ketoglutarate-dependent dioxygenase family such as TETs, KDMs, HIF prolyl hydroxylases and ALKBHs. Members of the JMJD2 (KDM4) family all have the ability to demethylate H3K9me2/3 and/or H3K36me2/3, and the JMJD6 enzyme removes dimethyl marks from arginine, for example at H4R3, at promoter regions of target genes. Chemically, these enzymes oxidize the methyl group, which then breaks down and releases the mark from the histone. Their activity matters for cell identity: JMJD2 family members are involved in the regulation of stem cell self-renewal and differentiation.

Hypoxia-inducible factor

Ascorbate also acts on the hypoxia-inducible factor (HIF) pathway. In normoxia, three HIF-1α prolyl hydroxylases (PHD1, PHD2, PHD3) recognize a conserved sequence present twice in HIF-1α, and two distinct proline residues, Pro402 and Pro564, are hydroxylated. Hydroxylated HIF-1α binds the von Hippel–Lindau protein (pVHL), leading to ubiquitination and rapid proteasomal degradation. When oxygen is scarce, this hydroxylation slows, HIF-1α binds ARNT to form a functional transcriptional activator that turns on genes with hypoxic response elements, such as VEGF, EPO and glycolytic enzymes. Ascorbate also enhances the asparaginyl hydroxylase FIH-1, which inhibits the function of the C-terminal transactivation domain of HIF-1α.

Stem cell differentiation and cancer

The strongest data on developmental effects come from cultured cells. Vitamin C shifted mouse ES cells toward a state that more closely approximates that of the inner cell mass of the blastocyst. In reprogramming experiments, the vitamin is thought to help erase DNA methylation and the epigenetic memory it encodes, improving reprogramming of differentiated cells to an embryonic-like state.

Links to cancer are proposed rather than established. TET genes are highly expressed in embryonic tissues, and TET dysfunction is associated with oncogenesis and metastasis. Supplementation has been shown to promote DNA demethylation in the promoter regions of tumor suppressor loci encoding p16, p21, and p53 in the cited studies. Some reviewers have suggested that restoring TET function by vitamin C administration, in combination with DNA methyltransferase inhibitor therapy, may help erase DNA hypermethylation at tumor suppressor loci to promote differentiation and cell death. This remains a hypothesis. Most of the evidence comes from cell lines and animal models, and whether comparable chromatin changes occur in human tissues at ordinary intakes is not settled.

Immune function

Accumulation in immune cells

Vitamin C is concentrated in several types of white blood cell far above its level in plasma. Mean plasma vitamin C concentrations are typically around 50 µmol/L, yet neutrophils accumulate millimolar intracellular concentrations against a concentration gradient, which is thought to indicate an important role for the vitamin in these cells. Neutrophils, mononuclear phagocytes, and lymphocytes accumulate vitamin C to high concentrations, which can protect these cells from oxidative damage. Once inside, the reduced form is regenerated from its oxidised partner, which keeps the internal pool high. Once inside the cell, dehydroascorbic acid is immediately reduced to ascorbate, maintaining the concentration gradient and allowing further influx, which can raise intracellular ascorbate by up to 20-fold.

Antimicrobial activity and clearance of spent cells

In phagocytes, the best-characterised roles are in microbial killing. Ascorbate can enhance chemotaxis, phagocytosis, generation of reactive oxygen species, and ultimately microbial killing. The vitamin also appears to protect the cell during this process. The antioxidant function of ascorbate is thought to be important in protecting the neutrophil during microbial killing, which involves both enzymatic and oxidative mechanisms. Resolution of inflammation also depends on it. It is needed for apoptosis and for the clearance of spent neutrophils from sites of infection by macrophages, which reduces necrosis and potential tissue damage. The effect on lymphocytes is less well defined. It has been shown to enhance differentiation and proliferation of B- and T-cells, likely through its gene-regulating effects.

Evidence of deficiency comes mainly from animal and cellular work. Neutrophils isolated from scorbutic guinea pigs show attenuated chemotaxis, phagocytosis, oxidant production and microbial killing, and supplementation reversed these defects. In people, vitamin C deficiency results in impaired immunity and higher susceptibility to infections. The relationship runs in both directions, since infections significantly impact on vitamin C levels due to enhanced inflammation and metabolic requirements.

Epithelial barriers and skin

The immune role also extends to the body's outer surfaces. Vitamin C supports epithelial barrier function against pathogens and promotes the oxidant scavenging activity of the skin, which may protect against environmental oxidative stress. Skin contains millimolar concentrations of vitamin C, with higher levels in the epidermis than in the dermis. Most of the barrier evidence comes from cell culture and animal studies. Supplementing keratinocytes in culture enhances their differentiation and barrier function. In cell culture models, vitamin C promotes the synthesis of barrier lipids, which would support a functioning stratum corneum with low water permeability. Human data are sparser; one topical study reported decreased skin roughness with use of the product.

Clinical evidence on infection

Observational data and trials have not produced consistent conclusions. Reviews have argued that supplementation with vitamin C appears to be able to both prevent and treat respiratory and systemic infections. Controlled trials are less uniform. A systematic review of randomised trials on neutrophil function found that 44% of the RCTs assessed showed effects of vitamin C supplementation on neutrophil functions, but the studies were very heterogeneous, comprising different participant cohorts and different dosing regimens. Another review reported that nine of the 16 RCTs included reported no effect of supplementation, alone or combined with other micronutrients or antioxidants, on various neutrophil functions.

Inflammation and tissue damage

Whether vitamin C limits harmful inflammation in severe illness is the most contested question. Preclinical work suggests that vitamin C supplementation may reduce endothelial injury in the pulmonary and systemic vasculature, oxidative damage, and harmful inflammation. Clinical results in sepsis have been mixed. In a 2019 trial of high-dose infusion in patients with sepsis-induced severe respiratory failure, infusion did not reduce organ failure scores at 96 hours or improve biomarkers of inflammation and vascular injury at 168 hours. A larger trial in adults with sepsis receiving vasopressors went the other way. At 28 days, the composite of death or persistent organ dysfunction occurred more often in the vitamin C group than in the placebo group. The investigators reported that the secondary analyses, which included five biomarkers of tissue dysoxia, inflammation, and endothelial injury, did not identify a mechanism for the harm. Earlier, smaller work in burn patients found that vitamin C decreased endothelial leakage, which reduced fluid requirements and increased urine output.

The inflammation pathway also shows why timely clearance matters. In chronic non-healing wounds, such as those in people with diabetes, neutrophils persist and instead undergo necrotic cell death, which can perpetuate the inflammatory response and hinder wound healing. Because vitamin C supports apoptosis and clearance, its role in this cycle is one of the reasons it remains under study in infection and critical illness.

Dietary sources and intake recommendations

Food sources

Plants supply nearly all of the vitamin C in ordinary diets. Among common foods, the richest include citrus fruits, red and green peppers, kiwifruit, broccoli, strawberries, and Brussels sprouts, and fruit and vegetable juices are concentrated sources. In the United States, citrus fruits, tomatoes and tomato juice, and potatoes are major contributors. Animal foods add little, and cereals contain the vitamin only when it is added during fortification: although vitamin C is not naturally present in grains, it is added to some fortified breakfast cereals. Per serving, some tropical fruits are especially rich; one cup of raw guava has been listed at about 377 mg, far above the daily reference value.

Losses during cooking, storage and processing

Because the vitamin is both water-soluble and heat-sensitive, the way food is prepared strongly affects how much reaches the plate. Losses on cooking are attributed to high temperatures and to leaching of the vitamin into the water used for boiling. In a study of indigenous leafy vegetables in Uganda, boiling or steaming removed 95-99% of vitamin C, although such extreme figures are not typical of every food. A review of thermal processing reports boiling losses of 26–100% across the samples examined, and frying can produce larger losses still. Mechanical handling matters too: peeling, cutting or crushing can activate ascorbic acid oxidase, which speeds oxidation of L-ascorbic acid.

Storage and freezing have their own effects. Prolonged storage and cooking can both reduce the vitamin C content of food. Blanching and freezing cause fewer losses at the start, but frozen products lose more nutrients during storage owing to oxidation. Canned foods are comparatively stable afterwards because oxygen is largely absent. Practical advice from the sources is consistent: steaming or microwaving may lessen cooking losses, cooking liquids can be reused in soups or gravies, and many of the best food sources of vitamin C, such as fruits and vegetables, are usually eaten raw.

Recommended intakes and why they differ

In the United States, the National Academies set Recommended Dietary Allowances for adults at 90 mg daily for men and 75 mg for women. The adult value is calculated from the intake needed to nearly saturate plasma and leukocytes while keeping urinary loss low, and it is adjusted for body mass. Values differ between countries because authorities choose different endpoints and models. Australia and New Zealand, for example, set an adult reference intake of 45 mg/day, derived from an estimated average requirement of 30 mg/day. That estimate rests on assumptions about absorption efficiency and the daily rate at which the body's pool is catabolised. Differences in the choice of deficiency-prevention versus tissue-saturation criteria, in the absorption and turnover values assumed, and in the reference population explain most of the spread.

Groups with higher requirements

Smoking increases the requirement. Smoking can deplete vitamin C levels in the body, so an additional 35 mg beyond the RDA is suggested for smokers. Pregnancy raises the allowance to 85 mg per day for adults, and lactation to 120 mg per day for adults, reflecting the vitamin transferred into breast milk. Observed intakes often fall short of these figures. In an analysis of US survey data, pregnant and lactating women were consuming 40% to 55% less vitamin C than recommendations. Smoking was also linked to lower intake and lower status in these women.

Illness is not assigned a separate numerical allowance in the standard tables. The higher turnover of the vitamin during infection and inflammation is discussed in the immune function section. Overall, vitamin C deficiency is rare in developed countries but may occur with a limited diet that provides less than 10 mg daily for one month or longer.

Deficiency and scurvy

Course of deficiency

Because humans cannot synthesise vitamin C, the body depends on a steady supply from food, and its reserves can run down. The body's pool of vitamin C can be depleted in 1-3 months. Estimates of the timeline differ between sources. One review puts the total body pool at about 1500 mg, and a separate account gives 4-12 weeks for depletion. The earliest changes are nonspecific. In adults, symptoms of vitamin C deficiency develop after weeks to months of vitamin C depletion. Lassitude, weakness, irritability, weight loss, and vague myalgias and arthralgias may develop early. The connective-tissue damage that defines scurvy usually appears later. Symptoms of scurvy (related to defects in connective tissues) develop after a few months of deficiency.

Clinical signs

The characteristic signs reflect impaired collagen formation in the skin, gums, vessels and bone (see collagen). Physical exam often displays specific cutaneous signs including follicular hyperkeratosis associated with coiled corkscrew hairs, perifollicular hemorrhages, petechiae, purpura, and ecchymoses. In the mouth, gums may become swollen, purple, spongy, and friable; they bleed easily in severe deficiency. Eventually, teeth become loose and avulsed. Other manifestations include gingival bleeding, joint pain, hemarthrosis, and anemia. Children may show additional problems, since in infants and children, bone growth may be impaired. Poor wound healing and bleeding into internal tissues are also common. If left untreated, the disease can progress to jaundice, neuropathy, hemolysis, seizures, and death.

Diagnosis

Diagnosis rests mainly on the clinical picture and on the response to treatment. Scurvy should be considered on the clinical presentation and confirmed with recovery after Vitamin C supplementation. Serum levels of Vitamin C reflect recent intake rather than body stores, so are unreliable. Plasma measurements have limits, because signs of scurvy can occur with low-normal serum levels of vitamin C. Leukocyte measurements are considered a better guide to tissue status, since the level of vitamin C in leukocytes more accurately correlates to tissue stores compared with serum levels, because these cells are not affected acutely by circadian rhythm or dietary changes. Radiographs have a narrower role. Skeletal radiographs can help diagnose childhood (but not adult) scurvy.

Treatment and recovery

Treatment is replacement with oral or intravenous ascorbate. One adult regimen is oral Vitamin C 1000 mg/d in divided doses for at least one week, then 300-500 mg/d for a further week, followed by the recommended daily allowance. Dividing the dose matters because the body can only absorb about 100 mg of vitamin C at a time through the gut. Intravenous treatment is used when rapid or complete correction is needed. Improvement often begins in a few days with complete recovery in a few weeks. Individual symptoms resolve at different rates. Most symptoms of scurvy resolve rapidly within 24–72 hours of starting Vitamin C supplements. Bone changes take longer, since bone changes may take several weeks to resolve. Full recovery is usual within 1–3 months. Loss of teeth and sequelae from haemorrhage are the only permanent disabilities.

Groups at risk

Vulnerability is linked to diet, absorption and substance use. Older adults, people on restricted diets, those with malabsorption and those with chronic illness are consistently identified as higher-risk groups. These groups include individuals with alcohol use disorder, smokers, patients with malabsorption syndromes, those on restrictive diets, older adults, patients undergoing dialysis, and children with selective eating behaviors. Smoking matters for biochemical reasons as well as dietary ones: smokers carry 30 to 40% lower vitamin C blood levels than non-smokers. Malabsorption can also be caused by inflammatory bowel disease, Whipple disease, or dyspepsia, and by other conditions that impair uptake. Population data show that deficiency is not confined to extreme cases. In one study of non-hospitalised patients in Paris, 5% of women and 12% of men were deficient; in those older than 65 years, this proportion increased to 15% of women and 20% of men. Infants fed only cow's milk or plant-based drinks during the first year of life are also at risk, and this is a recognised cause of deficiency in young children.

Toxicity and safety

Gastrointestinal effects and the upper limit

Vitamin C is water-soluble, and the body excretes what it does not need, so acute toxicity at ordinary doses is low. The most frequent adverse effects of large oral doses are gastrointestinal. Doses above the usual range tend to provoke loose stools, nausea and cramping, which occur because unabsorbed ascorbate remains in the intestinal lumen and draws water into it. An upper intake level of 2 g/day was set to avoid diarrhea and gastrointestinal disturbances with high supplemental intakes. The limit was first assigned with the 2000 dietary reference intake review, which fixed the figure for adults aged 19 and older at 2,000 mg per day. For vitamin C, it covers food and supplements together, so the ceiling applies to the total intake from all sources.

The reasoning behind the number matters for interpreting it. The 2,000 mg value is a tolerability threshold rather than a point at which organ damage has been demonstrated. Reviews of the evidence have found no reliable scientific evidence that oral doses of vitamin C up to 10 g/day in adults are toxic or detrimental to health. The limit is therefore best read as a comfort ceiling for the general adult population, and individual tolerance varies.

Oxalate and kidney stones

The principal concern about high doses concerns the kidney. Part of ascorbate is metabolised to oxalate, which is excreted in urine, and oxalate forms the main component of the most common type of kidney stone, calcium oxalate. Some, but not all, studies have reported that supplemental vitamin C increases urinary oxalate concentrations. Whether that increase translates into a greater risk of stones remains unresolved. Large scale observational studies have not shed light on the role of vitamin C supplements in the development of kidney stones, largely due to conflicting evidence.

Because of this uncertainty, guidance is cautious for people who already form stones. The Linus Pauling Institute summary advises that those predisposed to kidney stone formation may consider avoiding high-dose (≥1 g/day) vitamin C supplementation. Some provincial guidance from Alberta goes further, stating that for people with a history of stones, high dose vitamin C supplementation of more than 1000 mg per day is not recommended.

Iron overload

Vitamin C increases the absorption of dietary non-haem iron, and this effect becomes a hazard when the body already stores excess iron. In hemochromatosis, a hereditary disorder of iron accumulation, large doses can aggravate the condition. Safety protocols for clinical trials reflect this. A sepsis trial protocol lists diarrhea/abdominal bloating, increased oxalate excretion, iron overload in participants with hemochromatosis, and hemolysis in participants with G6PD deficiency as recognised high-dose harms and excludes people with these conditions.

G6PD deficiency and hemolysis

The most serious well-documented harm is hemolysis in people with glucose-6-phosphate dehydrogenase (G6PD) deficiency, an inherited enzyme deficiency that leaves red blood cells vulnerable to oxidative stress. Ascorbate given at high concentrations can act as an oxidant in these cells. A 75-year-old woman presented with acute onset of jaundice, dark urine and shortness of breath after receiving 30 g of vitamin C infusion, and her condition was later attributed to a previously unrecognised G6PD deficiency. A meta-summary of published cases found that about 71.4% (10/14) were diagnosed with glucose-6-phosphate dehydrogenase (G6PD) deficiency, and that most reactions began early in therapy. Hemolysis is not confined to intravenous use, since oral vitamin C, if consumed in large doses for a prolonged period, may also cause hemolysis.

The picture is complicated by a dose-dependent reversal. In G6PD-deficient patients, moderate doses have been used to treat methemoglobinemia, whereas supraphysiological doses can cause it. Estimates of the threshold vary: in vitro data suggest protection at plasma concentrations up to about 5 mmol/L, while a recent review reported that vitamin C doses of 4 to 6 g may propagate hemolysis. For this reason G6PD testing is commonly used to exclude people from high-dose protocols.

Interactions with medicines

Clinically documented interactions are fewer than the list of supplement-related concerns might suggest. The best-established is with aluminium: vitamin C increases the intestinal absorption of aluminium, which is relevant for people taking aluminium-containing antacids or phosphate binders over long periods. Evidence for other drug interactions is limited and often comes from case reports, so clinicians generally weigh high-dose therapy against the individual's renal function, iron status and G6PD status rather than applying a fixed rule.

Supplementation and clinical research

Common cold

Respiratory infection is the most studied outcome in vitamin C trials, and the findings are modest. In the Cochrane review of placebo-controlled trials, regular supplementation did little to lower the risk of catching a cold in the general community (pooled risk ratio 0.97). It did shorten colds by about 8% in adults and 14% in children. In a subgroup of marathon runners, skiers and soldiers exposed to extreme physical stress, incidence fell by roughly half. When treatment began only after symptoms appeared, seven trials found no consistent effect on duration or severity, although one large trial that gave a single 8 g dose on the first day recorded a shorter illness. A 2023 meta-analysis of trials using at least 1 g per day in healthy people reported a 15% reduction in overall severity. Its effect was concentrated in severe symptoms, with no clear effect on mild ones.

Cardiovascular disease

Observational cohort studies have linked higher circulating vitamin C with lower rates of hypertension, coronary heart disease and stroke, but randomized trials have not confirmed a protective effect. The largest is the Physicians' Health Study II, which assigned 14,641 male physicians to 500 mg per day of synthetic ascorbic acid or placebo for a median of about eight years. Major cardiovascular events did not differ between the groups. A 2017 systematic review of eight trials with 15,445 participants found no reliable reduction in cardiovascular events or mortality, but the evidence was of low quality and depended heavily on that single large trial. A Mendelian randomization study found that genetically higher vitamin C levels were not associated with most cardiovascular outcomes, with one exception for cardioembolic stroke. Its authors concluded that raising levels through supplements may not help prevent cardiovascular disease.

Cancer

Observational studies of cancer incidence mostly show no association or a modest protective one, and randomized trials of oral supplementation have reported no effect on cancer risk. Intravenous high-dose ascorbate is a separate question. It was first proposed as a treatment in the 1970s, when Ewan Cameron and Allan Campbell reported responses in patients with advanced cancer, and the idea later drew in Linus Pauling. The clinical base remains thin. A systematic review of 23 phase I and II trials with 385 patients identified only one randomized trial, in ovarian cancer, which compared ascorbate with standard chemotherapy and reported a longer progression-free survival. Another systematic review judged the evidence limited and of low quality, while noting possible improvements in quality of life and chemotherapy-related fatigue. A 2024 phase 2 trial in 34 patients with metastatic pancreatic cancer reported a rise in median survival from about eight to 16 months, a result that drew wide attention but rests on a small study. Pooled analyses that mix randomized and observational data have reported longer survival, but such designs are open to confounding and do not settle the question. Randomized, placebo-controlled trials remain the main missing evidence. The same pattern appears in sepsis, where a 2020–2025 systematic review found short-term physiological improvements but no proven mortality benefit.

Why results vary

Several factors explain the inconsistency across studies. Dose matters: daily supplements of a few hundred milligrams, oral therapeutic doses of several grams, and intravenous infusions produce very different blood concentrations, so the trials are not directly comparable. Timing matters too, since prophylaxis and treatment started after symptoms answer different questions. Outcomes differ, and incidence is much harder to change than duration or severity. Population is critical. People who are already well nourished, such as the physicians in the largest heart trial, have less room to benefit than people under physical stress or with low baseline status. Study design also varies, from weeks-long cold trials to multi-year cardiovascular trials, and the quality of individual trials ranges widely. Together these differences mean that a null result in one population does not rule out an effect in another.

Industrial production and uses

From the Reichstein process to fermentation

Industrial vitamin C is made from glucose, but the route has changed considerably since the 1930s. The starting step is catalytic hydrogenation of glucose to sorbitol. A microbial oxidation then converts sorbitol to L-sorbose, usually with Gluconobacter oxydans, and the remaining steps determine the process. Tadeus Reichstein's method, which was patented in 1936 and later modified with Grüssner, used a sequence of chemical reactions to turn L-sorbose into 2-keto-L-gulonic acid (2-KLG). The sorbose was protected with acetone, oxidized, and then converted by lactonization to L-ascorbic acid. This process required large amounts of organic solvents, several reaction stages and a high capital investment, and it recovered only a modest share of the glucose as the vitamin. Acetone use and waste disposal added further cost.

From the late 1960s and early 1970s, Chinese researchers developed a two-step fermentation process that replaced most of the chemical stages. In the second step, a mixed culture, typically Ketogulonicigenium vulgare together with a companion Bacillus species, converts L-sorbose directly into 2-KLG. Reported yields are about 98% for the first step and 97% for the second, and the process is now reported to be used by Chinese manufacturers as the main route to industrial vitamin C. Research continues on one-step methods that would make 2-KLG directly from glucose using engineered bacteria, but these have not replaced the established routes. In all variants, the final conversion of 2-KLG to ascorbic acid still involves an acid- or base-mediated lactonization, which is one reason why the chemistry of the molecule is central to production.

Food, feed, pharmaceutical and cosmetic uses

Industrial L-ascorbic acid serves several markets. Its reducing power makes it useful as a food antioxidant and preservative: it slows the browning of cut fruit and helps protect the colour and quality of processed meats. It is also added to beverages and baked goods as a fortificant. In the pharmaceutical sector it is used as an active ingredient in tablets, injections and dietary supplements, and in animal feed, where it supplements diets for species that cannot synthesize enough of it. Cosmetic formulations use derivatives of the vitamin as antioxidants and skin-care actives. Feed-grade and food-grade material account for much of the volume, while pharmaceutical-grade material commands higher prices per tonne.

Scale and economic importance

Estimates of global output vary widely because most producers are private and market reports use different definitions. A chapter on bioprocessing has estimated annual world production at around 80,000 tonnes, consuming more than 140,000 tonnes of sorbitol, while some market reports put recent volume at roughly 148,000 tonnes. Market-value estimates for 2024–2026 range from about USD 1.5 billion to USD 2.2 billion, depending on the source. Production is heavily concentrated in Asia: China is generally described as the largest producer, and a small number of companies control a large share of supply. Because of this concentration, price and supply are sensitive to Chinese manufacturing capacity, environmental regulation and trade. The industry's economic weight therefore rests less on the vitamin's market size than on its role as an essential ingredient across food, feed and medicine.

Open questions and ongoing research

Optimal intake and tissue saturation

Current intake recommendations rest on a definition of "optimal" status that remains unsettled. According to the NIH health professional fact sheet, at daily intakes of 100 mg or higher, cells appear to be saturated, and at intakes of at least 200 mg, plasma concentrations increase only marginally. A review of randomised trials notes that the definition of optimal vitamin C status remains a matter of controversy. Some authors favour the saturation point of about 70 μmol/L in plasma, which would require a daily intake of about 200 mg. Work in healthy men, by contrast, places tissue saturation at ≈100 mg/d, and a plasma concentration of 50 μmol/L is reached at about the same intake. These figures differ enough that the same intake range can look adequate or marginal depending on the chosen biomarker.

Groups outside the healthy young adults on whom most kinetic work has been done are less well characterised. A UK trial registry entry observes that due to lack of robust pharmacokinetic data in older people, the intake required to optimise the vitamin C status of this cohort is not yet known and may be higher than that for healthy adults. A pilot feasibility study in care-home residents was designed to assess the intake requirements of care home residents for optimal vitamin C status, with a larger trial on respiratory infections planned to follow. Whether current recommendations are sufficient for saturating tissues in smokers, people with chronic inflammation, and other high-need groups therefore remains open.

Epigenetic mechanisms and therapeutic potential

The link between vitamin C and DNA demethylation is supported mainly by laboratory and animal work. In the vast majority of in vitro, animal and clinical studies included in one review, vitamin C increased the hydroxylation of 5-methylcytosine to 5-hydroxymethylcytosine catalysed by TET enzymes, which is the first step in active demethylation. Human data are much sparser. In a single-family study, a non-randomized, open-label clinical trial of vitamin C on DNA methylation enrolled only a handful of people. Its authors reported that vitamin C reinforces the DNA demethylation cascade, reduces the proportion of hypermethylated loci and diminishes gene expression differences between TET2 mutation carriers and control individuals, but the sample size makes generalisation difficult.

Therapeutic use is being tested in higher-risk groups. A multi-centre phase II study is examining whether oral vitamin C can reverse the epigenetic changes characteristic of these disease entities in low-risk myeloid malignancies, and its investigators have noted that plasma levels are often very low in patients with haematological cancers. Mechanistic questions also remain. A 2025 preprint suggests that TET activity is not the only or predominant source of this modification in human DNA, which would mean the pathway is more complex than the simple TET model implies. Whether vitamin C can be used to modify epigenetic states in a clinically useful way is still undetermined.

The gut microbiota

The role of the gut microbiota in vitamin C handling is one of the least explored areas. Absorption is normally concentrated in the small intestine, but at high doses some vitamin is not absorbed and can reach the distal gut and directly impact on the complex symbiotic relationships within the gut microbiota. A randomised, placebo-controlled trial in healthy young adults with suboptimal status gave 1000 mg daily for four weeks and reported that the supplementation increased gram-positive bacteria, while reducing LPS-producing gram-negative bacteria, particularly Desulfovibrio. The same study linked these changes to lower serum spermidine, which suggests altered microbial metabolism. These findings are still preliminary, and their relevance to absorption efficiency and to whole-body status has not been established.

Causal evidence is limited. A Mendelian randomisation study examined the relationship between gut microbiota and deficiencies of vitamins A, B12 and D, but not vitamin C. Reviews have proposed that the microbiota may contribute to the vitamin C pool, but the human evidence for a meaningful microbial contribution is weak. Distinguishing the effects of the microbiota from the effects of the vitamin itself will require controlled studies that measure both microbial composition and vitamin C concentrations in plasma and tissues.

Supplements, chronic disease and the need for trials

Observational cohorts have repeatedly associated higher vitamin C intake with lower risk of cardiovascular disease and some cancers, but supplementation trials have often been disappointing. One review observes that with one exception, studies investigating consumption of supplemental vitamin C, including the Linxian trial, did not show a protective effect against cancer, possibly because the dietary intake of vitamin C was already sufficient for tissue saturation. A recurring weakness is that investigators did not measure vitamin C concentrations before or after supplementation, so it is unclear whether participants could benefit at all. If participants start near saturation, supplementation would be expected to have made little or no difference to measured outcomes.

The same saturation logic affects laboratory biomarkers. If tissues are already saturated, supplementation cannot further lower oxidative damage, which may explain some inconsistent results in biomarker studies. Because the evidence base is heavily dependent on baseline status, future work needs to report plasma or leukocyte concentrations, stratify by baseline intake, and follow participants long enough to capture chronic disease endpoints. Researchers also point out that clinical trials to examine this possibility are warranted and necessary, not least due to the differences in vitamin C metabolism between mice and men, which underscores why animal findings cannot substitute for well-designed human trials.

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