Vitamin C is an essential
micronutrient for humans, with pleiotropic functions related to its ability to donate electrons. It is a potent
antioxidant and a cofactor for a family of biosynthetic and gene regulatory
enzymes.
Vitamin C contributes to immune defense by supporting various cellular functions of both the innate and adaptive immune system.
Vitamin C supports epithelial barrier function against pathogens and promotes the
oxidant scavenging activity of the skin, thereby potentially protecting against environmental oxidative stress.
Vitamin C accumulates in phagocytic cells, such as neutrophils, and can enhance chemotaxis, phagocytosis, generation of
reactive oxygen species, and ultimately microbial killing. It is also needed for apoptosis and clearance of the spent neutrophils from sites of
infection by macrophages, thereby decreasing
necrosis/NETosis and potential tissue damage. The role of
vitamin C in lymphocytes is less clear, but it has been shown to enhance differentiation and proliferation of B- and T-cells, likely due to its gene regulating effects.
Vitamin C deficiency results in impaired immunity and higher susceptibility to
infections. In turn,
infections significantly impact on
vitamin C levels due to enhanced
inflammation and metabolic requirements. Furthermore, supplementation with
vitamin C appears to be able to both prevent and treat respiratory and systemic
infections. Prophylactic prevention of
infection requires dietary
vitamin C intakes that provide at least adequate, if not saturating plasma levels (i.e., 100-200 mg/day), which optimize cell and tissue levels. In contrast, treatment of established
infections requires significantly higher (gram) doses of the
vitamin to compensate for the increased inflammatory response and metabolic demand.