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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
trations in the second and third trimester and at term are much higher than in the mother, and significant fetal sequestration of the vitamin has been demonstrated by cord vein and artery differences (Cleary et al., 1975; Contractor and Shane, 1970; Shane and Contractor, 1980). Barnard and colleagues (1987) reported that much of the decrease in plasma PLP concentration during pregnancy is offset by increases in plasma PL, but other investigators have not observed this compensation (Contractor and Shane, 1970). Studies in animals suggest direct transport of PLP to the fetus (Contractor and Shane, 1971). In the pregnant rat about 15 percent of an intraperitoneal dose of PN is initially taken up by the uterus, placenta, and fetus.
Maintenance of plasma PLP concentrations at nonpregnant values requires about 2 mg/day of supplemental PN in the first trimester and between 4 and 10 mg/day in the third trimester (Cleary et al., 1975; Hamfelt and Tuvemo, 1972; Lumeng et al., 1976). Maintenance of other status indicators such as tryptophan metabolites at nonpregnant levels after a tryptophan load requires even higher intakes, but this test may be affected by hormonal changes (Shane and Contractor, 1980). It is not clear whether these changes in status indicators during pregnancy reflect poorer vitamin status or represent normal physiological changes during pregnancy. The latter is more reasonable. There is no a priori reason to use laboratory values for nonpregnant women as controls for pregnant women. There is also no evidence of significant problems in B6 status during pregnancy despite the reduced levels of status markers. Schuster and coworkers (1981) examined the relationship between α-EAST ratios and Apgar scores in low-income mothers receiving an average of 1.3 mg/day of B6 in the diet. A small but significant effect of B6 status was noted. However, the possibility could not be eliminated that other variables unrelated to B6 may have been responsible.
For an assumed body store of 1,000 µmol and a fetal, uterine, and placental accumulation of 15 percent, the fetus and placenta would accumulate approximately 25 mg of B6. This would be about 0.1 mg/day averaged over gestation. With additional allowances made for the increased metabolic needs and weight of the mother and about 75 percent bioavailability of food B6, an additional average need in pregnancy of 0.25 mg can be estimated. This increased need would be concentrated more in the second half of gestation. Unlike nutrients such as iron, B6 is not stored in the body to any substantial extent, so it is unlikely that a surplus in early gestation would satisfy the increased need in the latter stages of gestation.