L-ascorbic acid (ascorbate or vitamin C) is a required nutrient for humans. Absorption, transport, and disposition of ingested ascorbate involve the following: (1) bioavailability and absorption in the gastrointestinal tract; (2) presence in the circulation; (3) tissue distribution; (4) excretion; and, (5) metabolism. Fundamental to each of the above are ascorbate chemistry and mechanisms of transport of ascorbate across membranes. Ascorbate can be reversibly oxidized to dehydroascorbic acid, which can be irreversibly degraded. Both reduced and oxidized forms cross cell membranes. Differences in transport kinetics, tissue specificity, and Na+ and energy dependence strongly support the existence of separate transport mechanisms. An important consideration in the analysis of ascorbate transport is that of substrate availability. Reduced ascorbate is by far the most predominant form found in plasma and tissues. Dehydroascorbic acid is rapidly reduced intracellularly to ascorbate by both enzymatic and chemical mechanisms. Despite constitutively low levels of dehydroascorbic acid, conditions that promote oxidation of ascorbate can profoundly alter both the nature and availability of substrate. Elucidation of mechanisms that modulate the delivery of ascorbate to tissues and its utilization under different metabolic conditions will be invaluable for making recommendations for ascorbate ingestion.
