Requirements of vitamin B6 have been found generally to depend on species, age, physiological function, dietary components, intestinal flora and other factors that are not yet fully understood. Quantity of dietary protein affects requirement for vitamin B6, and nutrients other than protein influence the B6 requirement. Niacin and riboflavin are needed for interconversions of different forms of vitamin B6. Excessive thiamin intake was reported to produce vitamin B6 deficiency in rats (Driskell, 1984). Conversely, vitamin B6 is required for synthesis of niacin from tryptophan.
Microorganisms of the intestinal tracts of animals produce vitamin B6, but whether significant quantities are absorbed and utilized is in doubt. For ruminants, far more important than large intestinal synthesis is the considerable quantities of vitamin B6 synthesized by ruminal microorganisms. The amounts of vitamin B6 normally synthesized by bacteria in the rumen are sufficient to prevent outward signs of deficiency. Prior to full rumen development, young ruminants require dietary vitamin B6 to prevent deficiency. An exact vitamin B6 requirement has not been determined, but calves fed 65 µg of vitamin B6 per kg (29.5 µg per lb) of body weight did not develop deficiency signs (Johnson et al., 1950). This requirement is similar to that determined in humans and swine of similar body weight. Whether ruminal synthesis provides optimal levels of vitamin B6 for high- producing dairy cows is unknown.
