A wide variety of plant and animal species can synthesize vitamin C (ascorbic acid) from carbohydrate precursors including glucose and galactose. The missing step in the pathway of ascorbic acid biosynthesis in all vitamin C-dependent species has been traced to inability to convert L-gulonolactone to 2-keto-L-gulonate, which is transformed into L-ascorbic acid. Vitamin C dietary-dependent species, including poultry, therefore lack the enzyme L-gulonolactone oxidase.
Domestic animals such as poultry have the ability to biosynthesize ascorbic acid within their body. 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. However, Marks (1975) proposed vitamin C requirements (mg per kg; mg per lb of diet) for poultry and swine as follows: poultry (50 to 60; 23 to 27), starting pigs (300; 136), and finishing pigs (150; 68).
Although vitamin C can be synthesized by poultry, the synthesis is reduced or the requirements for vitamin C are increased during times of stress. During times of environmental, nutritional or pathological stress, the addition of ascorbic acid to the birds' feed or to their drinking water appears to alleviate many of the undesirable physical consequences of exposure (e.g., chronic adrenocortical activation, immunosuppression, weight loss and reduced egg production) to single or multiple concurrent stressful stimuli such as high environmental temperature, beak trimming, coccidiosis challenge and transportation (Pardue and Thaxton, 1986; Satterlee et al., 1989; Kutlu and Forbes, 1993; McKee and Harrison, 1995; Jones, 1996). Thyroid status may be a factor affecting ascorbic acid requirement, with supplemental vitamin C improving chick performance from experimentally induced hypothyroidism (Takahashi et al., 1991).
