A wide variety of plant and animal species can synthesize vitamin C from carbohydrate precursors including glucose and galactose. Ascorbic acid is synthesized by tissues of mammals with the exception of primates (including humans) and guinea pig. Early researchers (Grollman and Lehninger, 1957) described the synthesis of ascorbic acid in different animal species. The missing step in the pathway of ascorbic acid biosynthesis in all vitamin C-dependent species has been traced to inability to convert L-gulono-gamma-lactone to 2-keto-L-gulonate, which is transformed by spontaneous isomerization into its tautomeric form, L-ascorbic acid. Dietary vitamin C-dependent species, therefore, lack the enzyme L-gulono-gamma-lactone oxidase (GLO). In swine the GLO activity is dependent on age (Ching and Mahan, 1998).
Domestic animals such as swine, poultry, ruminants, horses, dogs and cats have the ability to biosynthesize vitamin C within their body. However, Jensen et al. (1983) observed that the ability to synthesize vitamin C in pigs is genetically affected. Likewise, Palludan and Wegger (1988) reported large differences in ascorbic acid content in tissues and fluids between normal pigs compared to a mutant strain of pigs that was unable to synthesize ascorbic acid. There appears to be no microbial synthesis of the vitamin in the intestine (Miller and Kornegay, 1983). In general, research studies have shown that healthy animals under ordinary conditions do not respond to supplemental vitamin C and hence there is no recommended requirement established by the NRC (1998) or the Agricultural Research Council (ARC, 1981). However, Marks (1975) proposed vitamin C requirements of 300 mg per kg (136 mg per lb) for starting pigs and 150 mg per kg (68 mg per lb) for finishing pigs. Recently, Mahan et al. (1994) and de Rodas et al. (1998) respectively, have reported that 50 and 75 ppm of dietary vitamin C from a stable source improved the performance of pigs during the first 14 days postweaning.