cytes is mediated by both high- and low-affinity transporters, and the vitamin is localized mostly in the cytosol. Intracellularly and in plasma, vitamin C exists predominately in the free reduced form as ascorbate monoanion, as shown in Figure 5-1 (Levine et al., 1994).
Since the immediate oxidized forms of vitamin C are readily reduced back to ascorbic acid, relatively small amounts of the vitamin are lost through catabolism. The primary products of oxidation beyond DHA include oxalic and threonic acids, L -xylose, and ascorbate 2-sulfate (Jacob, 1999). With large intakes of the vitamin, unabsorbed ascorbate is degraded in the intestine, a process that may account for the diarrhea and intestinal discomfort sometimes reported by persons ingesting large doses (see section on “Adverse Effects”).
Besides dose-dependent absorption, a second primary mechanism for regulation of body ascorbate content is renal action to conserve or excrete unmetabolized ascorbate. Recent studies have shown that little unmetabolized ascorbate is excreted with dietary intakes up to about 80 mg/day and that renal excretion of ascorbate increases proportionately with higher intakes (Blanchard et al., 1997; Melethil et al., 1986).
Dose-dependent absorption and renal regulation of ascorbate allow conservation of the vitamin by the body during low intakes and limitation of plasma levels at high intakes. Tissue-specific cellular transport systems allow for wide variation of tissue ascorbate concentrations. High levels are maintained in the pituitary and adrenal glands, leukocytes, eye tissues and humors, and the brain, while low levels are found in plasma and saliva (Hornig, 1975). Due to homeostatic regulation, the biological half-life of ascorbate varies widely from 8 to 40 days and is inversely related to the ascorbate body pool (Kallner et al., 1979). Similarly, catabolic turnover varies widely, about 10 to 45 mg/day, over a wide range of dietary intakes due to body pool size. A total body pool of less than 300 mg is associated with scurvy symptoms (Baker et al., 1971), while maximum body pools are limited to about 2 g (Kallner et al., 1979). At very low ascorbate intakes, essentially no ascorbate is excreted unchanged and a minimal loss occurs.