Definition: 

Vitamin C is represented by two compounds : ascorbic acid (or ascorbate), and dehydroascorbic acid (or dehydroascorbate). The second is an oxidation product of ascorbic acid. Chemical formula of these two compounds are are very similar to glucose formula, and many animal can synthesize vitamin C from glucose. Thus ascorbic acid is not a vitamin for many rodent species.

Metabolism:

Vitamin C is absorbed in the upper part of the small gut, but also in ileon. Once absorbed, vitamin C rapidly diffuses in all body tissues. In blood, there is much more ascorbate than dehydroascorbate. Kidneys quickly eliminate vitamin C as ascorbate, dehydroascorbate, oxalic acid and some other metabolites. Vitamin C is not stored in the organism, and, as urine excretion is fast, frequent intakes of vitamin C are needed in humans.

Like vitamin E, vitamin C plays major roles in free radical scavenging. But while vitamin E acts in the lipid parts of cells, ascorbate plays this role in water. When scavenging free radicals, ascorbate transforms to ascorbyle radical, which is rapidly converted into dehydroascobate. Then, dehydroascorbate can be recycled into ascobate.

Vitamin C converts oxidized vitamin E (tocopheroxyle) into vitamin E thus maintaining full anti-radical defenses.

Ascorbic acid is a cofactor of certain hydroxylases. These are enzyme protein that are implicated in collagen synthesis, and in hormone and neuronal mediators.

Collagen has important roles in bone and skin structure, it gives them strength. Vitamin C is indispensable to proto-collagen polymerisation. Polymerisation can only be done by hydroxylation of proto-collagen lysyl- and prolyl- residues.

Adrenalin and nor-adrenalin are two important neuronal mediators. Both require hydroxylase and vitamin C for normal synthesis.

In foods, iron may exist under different forms. Reduced form of iron is much more absorbed in gut than the oxidized. In the digestive tract, vitamin C keeps iron in its reduced form, thus facilitating iron absorption

Vitamin C has anti-histaminic properties, moreover it is necessary to polymorphonuclear cell mobility (chemotactism), to T cell proliferation, and to complement activation.

Ascorbate is 100 fold more concentrated in immune cells than in plasma. This fact would suggest that it plays major roles in the functions of these cells.

Typically, vitamin C deficiency leads to scurvy. Depending on dietary intakes, plasma vitamin C changes very quickly. For this reason, leukocyte vitamin C concentrations are considered as better index to assess vitamin C status. However, leukocyte vitamin C measurement is only performed by a few laboratories.

Thus plasma vitamin C lower than 10 µM (1.76 mg/l), found in repeated samples, is considered as the threshold below which a vitamin C deficiency is possible

In industrialized countries vitamin C deficiency is not common, however it is regularly observed in persons who do not eat enough fresh fruits and vegetables.

When starting, vitamin C deficiency causes drowsiness, then, if intake are still too low, joint pain appears, gum inflammation and bleeding develops, and finally teeth become loose and mishappen.

In infants and children growth is delayed.

Kiwis, oranges, citrus, fruits and fresh vegetables contain high vitamin C levels

Vitamin C is not toxic, even at high dosage.