Folic acid requirements for monogastric species would be dependent on degree of intestinal folic acid synthesis and utilization by the animal. The levels of antibacterials added to the feed will affect microbial synthesis of folic acid. Sulfa drugs are folic acid antagonists. In the chicken, sulfa drugs have been shown to increase the requirement (Scott et al., 1982) for this vitamin. Moldy feeds have also been shown to contain antagonists (e.g., mycotoxins) that inhibit microbial intestinal synthesis of folic acid in swine (Purser, 1981).
In dogs and cats, sulfonamides have been shown to severely reduce intestinal microbial synthesis of folic acid. Deficiency signs can occur in dogs after extended oral administration of sulfonamides (NRC, 1985). Carvalho da Silva et al. (1955) were not able to induce folic acid deficiency in two- to three-month-old kittens with a purified diet containing no added folic acid or vitamin B12 unless 0.6% to 2.0% of sulfaguanidine or sulfathalidine was included. Dogs and cats being treated with sulfonamides would have a need for dietary sources of folic acid.
Self-synthesis of folic acid is dependent on dietary composition. For poultry, some research has indicated higher folic acid requirements for very high protein diets or when sucrose was the only source of carbohydrates (Scott et al., 1982). Keagy and Oace (1984) reported that dietary fiber had an effect on folic acid utilization; xylan, wheat bran and beans stimulated folic acid synthesis in the rat, reflected as higher fecal and liver folic acid. Animals that practice coprophagy would have a lower dietary need for folic acid, as feces are a rich source of the vitamin (Abad and Gregory, 1987). Dogs would therefore receive part of their folic acid requirement via coprophagy, while cats would not have this source of the vitamin.
Folic acid requirements are dependent on the form in which it is fed and concentrations and interrelationships of other nutrients. Deficiencies of choline, vitamin B12, iron and vitamin C all have an effect on folic acid needs. Although most folic acid in feedstuffs for pets is present in the conjugated form, apparently dogs and cats are capable of utilizing it. Folic acid requirements are related to type and level of production. The more rapid the growth or production rates, the greater the need for folic acid due to its role in nucleic acid synthesis.
Folic acid requirements are related to type and level of production. Growth rate, age and pregnancy influence folic acid requirements; however, this information is not available for dogs and cats. The requirement decreases with age because diminished growth rate reduces the need for DNA synthesis. Increased catabolism of folic acid is a feature of pregnancy. Studies with both rats (McNulty et al., 1993) and humans (McPartlin et al., 1993) demonstrated an enhanced folic acid catabolism that was a feature of pregnancy per se and not simply due to increased weight. All dietary forms of folic acid are available to the dog since it has the conjugase enzyme to break the polyglutamates down to the monoglutamate form that is required for absorption (Baugh et al., 1975).