17.3: Absorp­tion and metabolism of vitamin C (19.3)

Vitamin C is absorbed in the small intestine by an active sodium-dependent vitamin C trans­porter (SVCT‑1) that is dose dependent and can become saturated (Savini et al., 2008). About 70%–90% of vitamin C is absorbed when daily intakes range from 30 to 180mg, but absorp­tion falls to ≤ 50% at intakes above 1g/d (Kallner et al., 1979; Levine et al., 1996). Indeed, the osmotic diarrhea and intestinal discomfort some­times experienced by persons ingesting large gram doses of vitamin C is due to the presence of a large amount of unabsorbed vitamin C. Regulation of the whole body vitamin C content is achieved in part by this dose-dependent intestinal absorp­tion of vitamin C. Renal action to conserve vitamin C (via SVCT‑1) or excrete unmeta­bolized vitamin C also plays an important part. Tissue uptake occurs primarily by the SVCT‑2 isoform (Savini et al., 2008). Different tissues uptake and retain vitamin C to variable extents, with neuronal and glandular tissues containing the highest concen­trations (Hornig, 1975), indicating vital roles for vitamin C in these tissues. Catabolism of ascorbic acid in humans occurs through oxidation to dehydro­ascorbic acid and, in the absence of sufficient reducing equivalents, subsequent hydrolysis to diketo­gulonic acid. This can then be oxidised to smaller molecules, such as oxalate, that are excreted in urine. However, it should be noted that dehydroascorbic acid is readily taken up via cell membrane glucose transporters (GLUTs) and rapidly reduced back to ascorbic acid via various chemical and enzymatic pathways (Rumsey et al., 1997; Washko et al., 1993), thus limiting dehydro­ascorbic acid concen­trations in vivo. With large intakes, most of the vitamin C is excreted in its unmetabolized form (Levine et al., 1996).