Ascorbic acid is a simple sugar and a powerful reducing agent, its main role being to control the redox potential within cells. It is involved in the hydroxylation of proline to hydroxyproline, which is necessary for the formation of collagen. The failure of this biochemical pathway in vitamin C deficiency accounts for virtually all of the clinical effects seen.

Humans, along with a few other animals (e.g. primates and the guinea-pig), are unusual in not being able to synthesize ascorbic acid from glucose.

Vitamin C is present in all fresh fruit and vegetables. Unfortunately, ascorbic acid is easily leached out of vegetables when they are placed in water and it is also oxidized to dehydro-ascorbic acid during cooking or exposure to copper or alkalis. Potatoes are a good source as many people eat a lot of them, but vitamin C is lost during storage.

It has been suggested that ascorbic acid in high dosage (1-2 g daily) will prevent the common cold. While there is some scientific support for this, clinical trials have shown no significant effect. Vitamin C supplements have also been advocated to prevent atherosclerosis and cancer, but again a clear benefit has not been demonstrated.

Vitamin C deficiency is seen mainly in infants fed boiled milk and in the elderly and single people who do not eat vegetables. In the UK it is also seen in Asians eating only rice and chapattis, and in food faddists.