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concentrates B12, which is then transferred to the fetus down a concentration gradient. Fetal and maternal B12 serum concentrations are quite strongly correlated (Fréry et al., 1992). It appears that only newly absorbed B12 is readily transported across the placenta and that maternal liver stores are a less important source of the vitamin for the fetus (Luhby et al., 1958). This implies that current maternal intake and absorption of the vitamin during pregnancy have a more important influence on the B12 status of the infant than do maternal B12 stores. The importance of adequate maternal intake during pregnancy is supported by the appearance of B12 deficiency in infants at 4 to 6 months when their mothers have been strict vegetarians for only 3 years (Specker et al., 1990).

Fetal Accumulation. The human fetus accumulates an average of 0.07 to 0.14 nmol/day (0.1 to 0.2 µg/day) of B12, a range based on three studies of the liver content of infants born to women who were adequate in B12 (Baker et al., 1962; Loria et al., 1977; Vaz Pinto et al., 1975) and an assumption that the liver contains half the total body B12 content. Placental B12 is negligible (0.01 nmol/L [14 ng/L]) (Muir and Landon, 1985). The low body content of B12 in the newborn implies that pregnancy is unlikely to deplete maternal stores.

B12EAR and RDA Summary, Pregnancy

On the basis of a fetal deposition of 0.1 to 0.2 µg/day throughout pregnancy and evidence that maternal absorption of the vitamin becomes more efficient during pregnancy, the EAR is increased by 0.2 µg/day during pregnancy. No distinction is made for the age of the mother.

EAR for Pregnancy

14–18 years

2.2 µg/day of vitamin B12

19–30 years

2.2 µg/day of vitamin B12

31–50 years

2.2 µg/day of vitamin B12

The RDA for B12 is set by assuming a coefficient of variation (CV) of 10 percent (see Chapter 1) because information is not available on the standard deviation of the requirement for B12; the RDA is defined as equal to the EAR plus twice the CV to cover the needs of 97 to 98 percent of the individuals in the group (therefore, for B12 the RDA is 120 percent of the EAR).



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