A new perspective in the assessment of vitamin B-6 nutritional status during pregnancy in humans.

Plasma pyridoxal-5'-phosphate (PLP) concentration has been suggested as a valid indicator to assess vitamin B-6 nutritional status. Animal and human studies have shown that plasma PLP concentrations decrease progressively during pregnancy and large doses of vitamin B-6 supplementation are required to maintain plasma PLP at early or prepregnant levels. PLP is known to be hydrolyzed to pyridoxal (PL) by alkaline phosphatase (ALP), resulting in an inverse relationship between PLP and ALP. The object of this study was to compare the PLP-PL equilibrium in a group of healthy pregnant females with that of an age-matched nonpregnant control group from a similar socioeconomic background. The mean plasma PLP level was 37% lower, (P less than 0.0001), whereas the mean PL level was almost 90% higher (P less than 0.001) in the pregnant group than in the nonpregnant control group. The total amount of plasma PLP and PL levels, however, did not differ significantly (P greater than 0.24) between the two groups. Because the PL vitamer is regarded as the ultimate transport form of vitamin B-6, it may serve as a readily available source of vitamin B-6 to meet possible increased metabolic demands. Therefore, the estimation of plasma PLP alone does not permit an accurate assessment or understanding of the nutritional status and the physiology of vitamin B-6 in conditions associated with altered vitamin B-6 homeostases.