Vitamin C (ascorbic acid) - Human Nutrition

Vitamin C (ascorbic acid)

Vitamin C is a vitamin for only a limited number of vertebrate species: humans and the other primates, the guinea pig, bats, the passeriform birds, and most fishes. Ascorbate is synthesized as an intermediate in the gulo-nolactone pathway of glucose metabolism; in those vertebrate species for which it is a vitamin, one enzyme of the pathway, gulonolactone oxidase, is absent.

The vitamin C deficiency disease, scurvy, has been known for many centuries and was described in the Ebers papyrus of 1500 BC and by Hippocrates. The Crusaders are said to have lost more men through scurvy than were killed in battle, while in some of the long voyages of exploration of the fourteenth and fifteenth centuries up to 90% of the crew died from scurvy. Cartier’s expedition to Quebec in 1535 was struck by scurvy; the local native Americans taught him to use an infusion of swamp spruce leaves to prevent or cure the condition.

Recognition that scurvy was due to a dietary defi-ciency came relatively early. James Lind demonstrated in 1757 that orange juice and lemon juice were pro-tective, and Cook kept his crew in good health during his circumnavigation of the globe (1772–1775) by stopping frequently to take on fresh fruit and vegeta-bles. In 1804 the British Navy decreed a daily ration of lemon or lime juice for all ratings, a requirement that was extended to the merchant navy in 1865.

 The structure of vitamin C is shown in Figure 8.19; both ascorbic acid and dehydroascorbic acid have vitamin activity. Monodehydroascorbate is a stable radical formed by reaction of ascorbate with reactive oxygen species, and can be reduced back to ascorbate by monodehydroascorbate reductase. Alternatively, 2 mol of monodehydroascorbate can react together to yield 1 mol each of ascorbate and dehydroascorbate. Dehydroascorbate may either be reduced to ascorbate or undergo hydration to diketogulonate and onward metabolism.

Vitamin C is found in fruits and vegetables. Very significant losses occur as vegetables wilt, or when

Figure 8.19 Vitamin C (ascor-bic acid, monodehydroascor-bate and dehydroascorbate).

they are cut, as a result of the release of ascorbate oxidase from the plant tissue. Significant losses of the vitamin also occur in cooking, both through leaching into the cooking water and also atmospheric oxida-tion, which continues when foods are left to stand before serving.