The most clearly defined function of vitamin C is to maintain collagen formation; it appears to serve as a coenzyme in the hydroxylation of proline to hydroxyproline, which is an important step in collagen synthesis (FAO/WHO, 1970). It is also involved in other hydroxylation reactions, although the specific mechanisms are not known. Clinical observations show that vitamin C may be necessary for the normal metabolism of tyrosine in both children and adults. Vitamin C is considered to be effective in healing wounds. Absence of vitamin C, particularly in newly formed tissue, is associated with marked impairment of collagen formation. Nevertheless, massive doses do not hasten wound healing. Vascular reactivity is abnormal in scorbutic conditions.
Studies of experimental scurvy show that the haemorrhagic phenomenon can appear even though all tests for blood clotting activity as well as biopsies of capillaries remain normal (Hodges et al., 1969). Vitamin C plays an important role in converting the vitamin folacin to folimin acid in connection with the manufacture of blood, in addition to regulating the respiratory cycle in the mitochondria and corpuscles. The evidence is unclear whether large amounts of vitamin C play a role in preventing infections and protecting against the effects of stress and detoxifying chemicals. Also, there is no reliable evidence that large doses of vitamin C protect against the common cold (Carpenter, 1986). However, evidence is accumulating that vitamin C has certain reasonably clearly defined metabolic functions other than the prevention of scurvy.
Vitamin C increases the bioavailability of both non-haem and haem iron in foods by increasing its absorption by 200-600% (see Fig. 1). The effect depends on the simultaneous presence of vitamin C and iron in the gut, e.g. the amount of vitamin C-containing food consumed with a meal and the amount of vitamin C ingested. Vitamin C can interact with several metallic elements of nutritional significance. Due to its activity as a reductant, vitamin C lowers the toxicity of elements whose reduced forms are poorly absorbed or more rapidly excreted, e.g. selenium, nickel, lead and cadmium (Combs, 1992).
High utilization rates for vitamin C have been found in many psychiatric patients and an improved mental state has been found in controlled trials with high doses of vitamin C (Marks, 1975). Vitamin C is involved in histamine metabolism. Blood histamine and vitamin C concentrations are negatively correlated in women in pre-term labour (Combs, 1992). Vitamin C has also been found to affect immune function in several different ways, e.g. stimulating the production of interferon (Combs, 1992). Other roles of vitamin C mentioned in the literature include as an anti-stress factor and as a necessary substance in the regulation of cholesterol. Many properties of vitamin C are related to its being an antioxidant and reacting rapidly with organic free radicals, which is an important feature of protecting living tissues against the detrimental effect of free radicals.
Figure 1: Effect of increasing quantities of ascorbic acid on the absorption of non-haem iron in a meal
Source: De Maeyer et al., 1989