Fasting plasma total vitamin B6 (measured microbiologically), or more specifically pyridoxal phosphate, is widely used as an index of status. Conditions involving increased plasma activity of alkaline phosphatase may result in reduced plasma concentrations of pyridoxal phosphate, without affecting vitamin B6 nutritional status or tissue concentrations of pyridoxal phosphate. There is a compensatory increase in the circulating concentration ofpyridoxal, which, as discussed in Section 9.2, is the main form for extrahepatic uptake of vitamin B6. Barnard and coworkers (1987) have shown that, despite the fall in plasma pyridoxal phosphate in pregnancy, which has been widely interpreted as indicating vitamin B6 depletion or a greatly increased requirement for the vitamin, the plasma concentration ofpyridoxal phosphate plus pyridoxal is unchanged. Plasma pyridoxal phosphate is also affected by acute phase responses (Bates et al., 1999a). This suggests that determination of plasma pyridoxal phosphate alone may not be a reliable index of vitamin B6 nutritional status.
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