Biosynthesis

he vast majority of animals and plants are able to synthesize their own vitamin C, through a sequence of four enzyme-driven steps, which convert glucose to vitamin C.[6] The glucose needed to produce ascorbate in the liver (in mammals and perching birds) is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process.[23] In reptiles and birds the biosynthesis is carried out in the kidneys.

Among the animals that have lost the ability to synthesise vitamin C are simians (specifically the suborder haplorrhini), guinea pigs, a number of species of passerine birds (but not all of them), and in apparently many major families of bats and perhaps all of them. Humans have no enzymatic capability to manufacture vitamin C. The cause of this phenomenon is that the last enzyme in the synthesis process, L-gulonolactone oxidase, cannot be made by the listed animals because the gene for this enzyme, Pseudogene ΨGULO, is defective.[24] The mutation has not been lethal because vitamin C is abundant in their food sources. It has been found that species with this mutation (including humans) have adapted a vitamin C recycling mechanism to compensate.


Most simians consume the vitamin in amounts 10 to 20 times higher than that recommended by governments for humans.[26] This discrepancy constitutes the basis of the controversy on current recommended dietary allowances.

It has been noted that the loss of the ability to synthesize ascorbate strikingly parallels the evolutionary loss of the ability to break down uric acid. Uric acid and ascorbate are both strong reducing agents. This has led to the suggestion that in higher primates, uric acid has taken over some of the functions of ascorbate.[27] Ascorbic acid can be oxidised (broken down) in the human body by the enzyme ascorbic acid oxidase.

An adult goat, a typical example of a vitamin C-producing animal, will manufacture more than 13,000 mg of vitamin C per day in normal health and the biosynthesis will increase "many fold under stress".[28] Trauma or injury has also been demonstrated to use up large quantities of vitamin C in humans.[29] Some microorganisms such as the yeast Saccharomyces cerevisiae have been shown to be able to synthesize vitamin C from simple sugars.