Illustration 6-1 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 (Johnston, 2006).
Under normal conditions, dogs and cats can synthesize vitamin C within their body.
Because of de novo synthesis, vitamin C is not technically a dietary required vitamin for healthy dogs and cats.
It is proposed that the collagen matrix produced by ascorbic acid-treated cells provides a permissive environment for tissue-specific gene expression.
A common finding in all studies is that vitamin C can alter the expression of multiple genes as cells progress through specific differentiation programs (Ikeda et al., 1997).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.In guinea pigs, rats, and rabbits, CO2is the major excretory mechanism for vitamin C.Primates do not normally utilize the CO2 catabolic pathway, with the main loss occurring in the urine.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.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.Vitamin C is the least stable and, therefore, most easily destroyed of all the vitamins.Metabolic need for ascorbic acid is a general one among species, but a dietary need is limited to humans, subhuman primates, guinea pigs, fruit-eating bats, some birds (including the red-vented bulbul and related Passeriformes species), insects, fish (such as coho salmon, rainbow trout, and carp), and perhaps certain reptiles (Mc Dowell, 2000).