Vitamin C is absorbed in a manner similar to carbohydrates (monosaccharides). Intestinal absorption in vitamin C-dependent animals appears to require a sodium-dependent active transport system. It is assumed that those species that are not scurvy prone do have an absorption mechanism by diffusion (Spencer et al., 1963). Ascorbic acid is readily absorbed when quantities ingested are small, but limited intestinal absorption occurs when excess amounts of ascorbic acid are ingested. Bioavailability of vitamin C in feeds is limited, but apparently 80% to 90% appears to be absorbed (Kallner et al., 1977). Site of absorption in the guinea pig is in the duodenal and proximal small intestine, whereas the rat showed highest absorption in the ileum (Hornig et al., 1984).
In its metabolism, ascorbic acid is first converted to dehydroascorbate by several enzymes or nonenzymatic processes and can then be reduced back to ascorbic acid in cells (Rose et al., 1986). Absorbed vitamin C readily equilibrates with the body pool of the vitamin. No specific binding proteins for ascorbic acid have been reported, and it is suggested that the vitamin is retained by binding to subcellular structures.
Ascorbic acid is widely distributed throughout the tissues, both in animals capable of synthesizing vitamin C as well as in those dependent on an adequate dietary amount of the vitamin. In experimental animals, highest concentrations of vitamin C are found in the pituitary and adrenal glands, and high levels are also found in the liver, spleen, brain and pancreas. Vitamin C also tends to localize around healing wounds. Tissue levels are decreased by virtually all forms of stress, which also stimulates the biosynthesis of the vitamin in those animals capable of synthesis.
Ascorbic acid is excreted mainly in urine, with small amounts in sweat and feces. In guinea pigs, rats, and rabbits, CO2 is the major excretory mechanism for vitamin C. Primates do not normally utilize the CO2 catabolic pathway, with the main loss occurring in the urine. Urinary excretion of vitamin C depends on body stores, intake and renal function.