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Abstract
Vitamin C in humans must be ingested for survival. Vitamin C is an electron donor, and this property accounts for all its known functions. As an electron donor, vitamin C is a potent water-soluble antioxidant in humans. Antioxidant effects of vitamin C have been demonstrated in many experiments in vitro. Human diseases such as atherosclerosis and cancer might occur in part from oxidant damage to tissues. Oxidation of lipids, proteins and DNA results in specific oxidation products that can be measured in the laboratory. While these biomarkers of oxidation have been measured in humans, such assays have not yet been validated or standardized, and the relationship of oxidant markers to human disease conditions is not clear. Epidemiological studies show that diets high in fruits and vegetables are associated with lower risk of cardiovascular disease, stroke and cancer, and with increased longevity. Whether these protective effects are directly attributable to vitamin C is not known. Intervention studies with vitamin C have shown no change in markers of oxidation or clinical benefit. Dose concentration studies of vitamin C in healthy people showed a sigmoidal relationship between oral dose and plasma and tissue vitamin C concentrations. Hence, optimal dosing is critical to intervention studies using vitamin C. Ideally, future studies of antioxidant actions of vitamin C should target selected patient groups. These groups should be known to have increased oxidative damage as assessed by a reliable biomarker or should have high morbidity and mortality due to diseases thought to be caused or exacerbated by oxidant damage.
Key teaching points:
• Vitamin C is essential for life and is a powerful water-soluble antioxidant.
• Antioxidant actions of vitamin C have been shown by in vitro experiments.
• Oxidant damage of biological molecules result in oxidation products that can be measured. These assays have not been fully validated.
• Diet rich in fruits and vegetables are associated with lower risk of cardiovascular disease and cancer. It is not known whether vitamin C contributes to these benefits.
• When vitamin C is given by mouth, the relationship between oral dose and plasma concentration is sigmoidal. Plasma concentrations are tightly controlled and excess vitamin C is excreted.
• Other than preventing scurvy, vitamin C has no proven benefits. In humans, vitamin C treatment has not resulted in changes in biomarkers of oxidation or in clinical outcome.
Key teaching points:
• Vitamin C is essential for life and is a powerful water-soluble antioxidant.
• Antioxidant actions of vitamin C have been shown by in vitro experiments.
• Oxidant damage of biological molecules result in oxidation products that can be measured. These assays have not been fully validated.
• Diet rich in fruits and vegetables are associated with lower risk of cardiovascular disease and cancer. It is not known whether vitamin C contributes to these benefits.
• When vitamin C is given by mouth, the relationship between oral dose and plasma concentration is sigmoidal. Plasma concentrations are tightly controlled and excess vitamin C is excreted.
• Other than preventing scurvy, vitamin C has no proven benefits. In humans, vitamin C treatment has not resulted in changes in biomarkers of oxidation or in clinical outcome.