Vitamin B12 requirements are exceedingly small; an adequate allowance is only a few µg per kg of feed. The vitamin B12 requirements of various species depend upon the levels of several other nutrients in the diet. Excess protein increases the need for vitamin B12 as does performance level. The vitamin B12 requirement seems to depend on the levels of choline, methionine and folic acid in the diet; vitamin B12 is also interrelated with ascorbic acid metabolism (Scott et al., 1982). The requirements for both vitamin B12 and folic acid are reduced when the diet contains an abundance of compounds that can supply methyl groups (Dyer et al., 1949). Sewell et al. (1952) showed that vitamin B12 has a sparing effect on the methionine needs of the pig. A reciprocal relationship occurs between vitamin B12 and pantothenic acid in chick nutrition, with pantothenic acid having a sparing effect on the vitamin B12 requirement. Dietary ingredients may also affect the requirement, as wheat bran has been shown to reduce availability of vitamin B12 in humans (Lewis et al., 1986).
Dietary need depends on intestinal synthesis and tissue reserves at birth. Intestinal synthesis probably explains frequent failures to produce a vitamin B12 deficiency in pigs and rats on diets designed to be vitamin B12 free. The deficiency can be readily produced in rats, however, when coprophagy is prevented completely (Barnes and Fiala, 1958). The dog's inclination toward coprophagy will supply part of the vitamin B12 requirement. Both dogs and cats likely also obtain some vitamin B12 by direct absorption of the vitamin produced by bacterial synthesis in the intestine. However, the amount from this source is not reliable. Some intestinal microorganisms reportedly may compete with the intrinsic factor for vitamin B12, thus preventing absorption of the vitamin in sufficient quantities (Giannella et al., 1971; 1972). In the dog, hookworm infections can also increase vitamin B12 requirements (Corbin and Kronfeld, 1972).
Genetic factors have been shown to increase the vitamin B12 requirements of both dogs and cats. Fyfe et al. (1989; 1991a) recently described a family of dogs with the clinical, genetic and laboratory features of selective intestinal vitamin B12 malabsorption seen in humans. The malabsorption was caused by inefficient brush-border expression of intrinsic factor-vitamin B12 receptor due to a mutation of this complex and its retention (Fyfe et al., 1991b). Vitamin B12 deficiency associated with methylmalonic acidemia has been demonstrated in cats (Vaden et al., 1992). Apparently this defect in vitamin B12 absorption is the result of an inborn error of metabolism.
Research has shown that vitamin B12 is needed by dogs and cats but a quantitative requirement has not been determined in detail. The stated requirements are based on some research conducted in dogs and cats, and on data from other animals.
