Define Criteria for Assessment of Pyridoxine Status?
Vitamin B6 status is most appropriately evaluated by using a combination of indicators, namely plasma PLP concentration, urinary excretion, erythrocyte aminotransferases activity coefficients, tryptophan catabolites, erythrocyte and whole blood PLP concentration, and plasma homocysteine concentration, including those considered as direct indicators (e.g. vitamer concentration in cells or fluids) and those considered to be indirect or functional indicators (e.g. erythrocyte aminotransferases saturation by PLP or tryptophan metabolites). Plasma PLP may be the best single indicator because it appears to reflect tissue stores. A plasma PLP concentration of 20 m mol/M has been proposed as an index of adequacy based on recent findings. Plasma PLP levels have been reported to fall with age. Urinary 4-pyridoxic acid level responds quickly to changes in vitamin B6 intake and is therefore of questionable value in assessing status. However, a value higher than 3 m mol/day, achieved with an intake of approximately mg/day, as been suggested to reflect adequate intake. Erythrocyte aminotransferases for aspartate and alanine are commonly measured before and after addition of PLP to ascertain amounts of apoenzymes, the proportion of which increases with vitamin B6 depletion. Values of 1.5-1.6 for the aspartame aminotransferase and approximately 1.2 for the alanine aminotransferase have been suggested as being adequate. Catabolites from tryptophan and methionine have also been used to assess vitamin B6 status. In a review of the relevant literature, Leklem suggested that a 24-hour urinary excretion of less than 65 normal xanthurenate after a 2g oral dose of tryptophan indicates normal vitamin B6 status.