of 50 mg pyridoxal (PL) or the equivalent dose of pyridoxal phosphate (PLP) can be accounted for in the urine within 24 hours, demonstrating that the phosphorylated form is effectively hydrolyzed and absorbed in the gut (Shane, 1978; Snell, 1958). Under the same conditions, about 40 percent of an equivalent dose of PN can be accounted for in the urine, but PN at high doses raises the plasma PLP concentration and is retained more effectively than is PL (Shane, 1978).

Similarly, dietary pyridoxamine (PM) and PL are about 10 percent less effective than PN in raising the plasma PLP concentration, and slightly more of these vitamins is excreted in the urine as 4-pyridoxic acid (4-PA) (Wozenski et al., 1980). Most controlled B₆ studies have used PN as the added B₆ source, but requirements calculated from these studies would underestimate the B₆ requirement by only 5 percent or less for individuals deriving most of their B₆ as PLP and PMP from animal sources.

Because of PLP’s role as a coenzyme for many enzymes involved in amino acid metabolism, it has been proposed that B₆ requirements are influenced by protein intake. Many studies have demonstrated that increased protein intake causes a relative decrease in B₆ status as judged by a variety of B₆ status indicators (Baker et al., 1964; Hansen et al., 1996b; Linkswiler, 1978; Miller et al., 1985; Sauberlich, 1964). This had led some to define B₆ requirements in terms of protein intake. A number of other studies, however, have failed to demonstrate an effect of protein intake on B₆ status parameters. A study in young and elderly men and women found little effect of dietary protein levels (12 and 21 percent of total energy) on B₆ status as measured by plasma PLP and erythrocyte aspartate aminotransferase (Pannemans et al., 1994).

Almost all studies investigating the effects of different protein intakes have assessed the effects of graded levels of PN on status indicators to obtain a B₆ requirement in milligrams. They have then adjusted this value based on the protein intake to obtain a value per gram of protein. This approach assumes a linear relationship between B₆ requirements and protein intake for which there is little experimental justification. This approach may also overestimate the requirement for B₆ because the requirement has been set by assuming a protein intake of 100 g for men. Thus, the approach cannot be used for setting an Estimated Average Requirement (EAR). Increased protein may cause a relative decrease in B₆ status indicators