Requirement for vitamin B6 has been found generally to depend on species, age, physiological function, dietary components, the intestinal flora and other factors that are not yet fully understood. Vitamin B6 is produced by microorganisms in intestinal tracts of animals, but whether significant quantities are absorbed and utilized is in doubt. Animals practicing coprophagy (e.g., the dog) would obviously be receiving vitamin B6 from this source.
Breed of animal and environmental temperature have been shown to influence vitamin B6 requirements for some species (e.g., poultry and rats). Regarding ambient temperature, when rats are housed at 33°C they needed twice as much vitamin B6 as when they were housed at 19°C (Bräunlich, 1974).
Quantity of dietary protein affects requirement for vitamin B6 in both animals and humans. Vitamin B6 requirement is increased when high-protein diets are fed. For example, when feed contained 60% casein instead of 20%, the level of pyridoxine required by mice was three times as high (Miller and Baumann, 1945). Research findings indicate that the requirement of growing cats is positively related to the level of protein in the diet (Axelrod et al., 1945; Bai et al., 1991). For a 30% casein diet, the B6 requirement was between 1 and 2 mg per kg (0.45 and 0.91 mg per lb) of diet, but for a 60% casein diet the requirement was 2.0 or more mg per kg (0.91 mg per lb) of diet (Morris and Rogers, 1994).
A number of studies have suggested that amino acid imbalance has an adverse effect on vitamin B6 status, in that weight gain was depressed and survival was decreased when large amounts of a single amino acid were added to rat diets limited in the vitamin. High tryptophan, methionine and other amino acids increase the need for vitamin B6 (Scott et al., 1982).
Certain feed antagonists, bioavailability of B6 in feeds and nutrients other than protein influence the B6 requirement. Niacin and riboflavin are needed for interconversions of different forms of vitamin B6, with an overdose of thiamin reported to produce vitamin B6 deficiency in rats (LeKlem, 1991). Roth-Maier and Kirchgessner (1993) suggested that adult sows are able to maintain optimal metabolic functions over eight weeks by utilizing bacterially synthesized vitamin B6. Cellulose supplement to these pigs increased total vitamin B6 excretion from 3.4 to 5.2 mg and 25% to 50% was excreted in urine. In rats administered sulfasalazine, a vitamin B6 deficiency was aggravated, suggesting that the intestinal synthesis of the vitamin was affected (Trumbo and Raidi, 1991). A large amount of literature for humans (McDowell, 2000) has shown that vitamin B6 requirements are elevated as a result of drugs and inborn errors of metabolism. It is likely that the vitamin B6 requirements of dogs and cats are likewise altered by unrecognized inborn errors of metabolism and by drugs.
Flesh-eaters, such as cats, derive considerable energy from dietary protein. Because these animals have high transaminase activity, it is logical to expect that their vitamin B6 turnover, and therefore requirement, would be higher than that of omnivores. The vitamin B6 requirement of the cat is about four times higher than that of the dog (AAFCO, 1992).
