Vitamin C

Vitamin C, chemically L-ascorbic acid, is a water-soluble essential nutrient. The human body cannot make vitamin C on its own because it lacks the final enzyme of the biosynthesis pathway that most mammals use. Daily intake from the diet is therefore necessary.

In the body, vitamin C acts as a cofactor for several enzymes. It is involved in collagen formation and works as an antioxidant in the aqueous phase. The European Food Safety Authority (EFSA) has confirmed that vitamin C contributes, among other roles, to the normal function of the immune system and to the reduction of tiredness and fatigue.

In Austria, daily requirements are usually met through a varied diet with fresh fruit and vegetables. Classic sources include red bell peppers, broccoli, sauerkraut, rosehips, and citrus fruit. Vitamin C is sensitive to heat and oxygen; brief cooking and fresh preparation preserve the content.

Scurvy, the clinical deficiency of vitamin C, was for centuries a serious illness among sailors on long voyages. In 1747, the Scottish naval surgeon James Lind conducted aboard HMS Salisbury one of the earliest controlled clinical experiments in medical history, showing that citrus fruit reversed scurvy in affected sailors.

The name ascorbic acid comes from the Latin scorbutus, the historical name for scurvy. The molecule itself was isolated in 1928 by the Hungarian-American biochemist Albert Szent-Györgyi from paprika. He received the 1937 Nobel Prize in Physiology or Medicine for his work on biological combustion and vitamin C. In the same year, Walter Norman Haworth was awarded the Nobel Prize for elucidating the chemical structure.

Vitamin C also became widely known in the second half of the 20th century through the American chemist and Nobel Peace laureate Linus Pauling, who recommended high doses to prevent the common cold. Subsequent studies have largely not confirmed that idea; the area is treated today as historical context, not as established use.

Vitamin C works in the body along two main routes. As a cofactor, it supports enzymes that add hydroxyl groups to proline and lysine residues in collagen. This hydroxylation is required for the stable triple helix of collagen and therefore for the function of connective tissue, bones, cartilage, gums, skin, and blood vessels. Vitamin C also acts as a cofactor in the synthesis of norepinephrine and in the activation of certain peptide hormones.

In the aqueous phase of cells and blood plasma, vitamin C acts as an antioxidant. It can scavenge free radicals and regenerate the reduced, biologically active form of vitamin E from its oxidised form. In the digestive tract, vitamin C reduces non-heme iron from plant sources from the ferric to the ferrous form, which helps absorption.

EU Regulation 432/2012 summarises these effects in permitted statements, including normal collagen formation, normal function of the immune system, reduction of tiredness and fatigue, regeneration of the reduced form of vitamin E, and increased iron absorption.

Vitamin C is one of the most-studied vitamins. The Cochrane review by Hemilä and Chalker from 2013 (29 trials, 11,306 participants) examined whether vitamin C prevents or shortens the common cold. Routine supplementation does not measurably reduce the incidence of colds in the general population. In a subgroup of people under short periods of severe physical stress, for example marathon runners or soldiers in winter training, the incidence of colds was substantially lower.

For this specific setting, EFSA has authorised a claim that attributes to vitamin C a contribution to the normal function of the immune system during and after intense physical exercise (conditional on an additional 200 mg per day).

In people with adequate intake, vitamin C is among the nutrients with established reference values. The DACH reference values (2015) list 110 mg per day for adult men and 95 mg per day for adult women. Higher values apply during pregnancy and lactation and for smokers.

For non-heme iron absorption from plant sources, taking 25 to 100 mg of vitamin C with the same meal is a well-documented way to substantially increase iron absorption (Hallberg and Hulthén 2000).

Forms and preparation

You can cover most of your vitamin C needs through everyday food. Fresh fruit and briefly cooked vegetables preserve the content better than long cooking. If you take vitamin C as a supplement, keep single doses around 200 mg or below, because the body absorbs a smaller share of larger single doses. At higher total intakes, you can split the amount across the day. Vitamin C is available as ascorbic acid powder, capsule, effervescent tablet, lozenge, in liquid form, or as a liposomal formulation. Buffered forms such as calcium ascorbate or magnesium ascorbate are sometimes described as easier on the stomach. Acerola, rosehip, and camu camu are common plant-extract sources at Austrian Reformhaus shops. You can take supplemental vitamin C with a meal, especially together with plant iron sources like legumes, wholemeal bread, or dark leafy greens, when you want to support iron absorption from that meal.

Dosage

The DACH reference values (2015) recommend 110 mg of vitamin C per day for adult men and 95 mg per day for adult women. From the fourth month of pregnancy, the reference value is 105 mg per day; for lactating women it is 125 mg per day. Smokers have a higher need of about 155 mg per day (men) and 135 mg per day (women). EFSA has set a tolerable upper intake level (UL) of 1000 mg per day for supplemental vitamin C in adults. This UL covers supplemental intake; there is no defined upper limit for vitamin C from ordinary foods. Cellular uptake of vitamin C is saturated when the plasma concentration reaches a certain range. A single dose of about 200 mg leads to near-complete absorption; at single doses in the gram range, the absorbed share drops. If you take a higher total dose, split it across the day.

Drug interactions

Iron preparations: vitamin C increases the absorption of iron from plant sources and from iron supplements. In iron supplementation, this can be intended and is part of the medical prescription. In iron overload, this should be avoided. Aluminium-containing antacids: high amounts of vitamin C can increase absorption of aluminium from such gastric medicines. Anticoagulants such as warfarin: a clinically significant interaction with vitamin C at usual doses is not established; at high doses, intake should be discussed with the prescribing physician.

Contraindications

Routine high-dose vitamin C is not recommended in iron-overload conditions such as haemochromatosis, in people receiving repeated blood transfusions, or with a known history of calcium-oxalate kidney stones. In impaired kidney function, vitamin C excretion is altered; supplemental intake in the high-dose range should be medically supervised.

Side effects

The most common complaints at higher vitamin C doses concern the gastrointestinal tract: mild diarrhoea, abdominal cramps, nausea, occasionally heartburn. These typically appear only at doses above one gram per day and usually resolve when the dose is reduced or taken with a meal.

1. EFSA NDA Panel. Tolerable Upper Intake Levels for Vitamins and Minerals. 2006.
2. NIH Office of Dietary Supplements. Vitamin C — Fact Sheet for Health Professionals. 2021.
3. DGE, OEGE, SGE, SVE. DACH-Referenzwerte fuer die Naehrstoffzufuhr: Vitamin C (Update 2015). 2015.
4. Hemila H, Chalker E. Vitamin C for preventing and treating the common cold. 2013. doi:10.1002/14651858.CD000980.pub4
5. Hallberg L, Hulthen L. Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron. 2000. PMID:10799384