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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
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).
*It is advisable for most of this amount to be obtained by consuming foods fortified with B12 or a B12-containing supplement.
Evidence Considered in Estimating the Average Requirement
Absorption and Utilization of B12. There is some evidence that the absorption of B12 may increase during pregnancy. An increase in the number of intrinsic factor-B12 receptors was observed in pregnant mice and found to be regulated by placental lactogen (Robertson and Gallagher, 1983). A greater absorption of oral B12 was reported from the single study of pregnant women (Hellegers et al., 1957), but the methods used do not permit quantification of the increase.
Serum total B12 concentrations begin to decline early in the first trimester. In a longitudinal Dutch study of 23 subjects, serum B12 fell significantly by the end of the first trimester, more than could be accounted for by hemodilution (Fernandes-Costa and Metz, 1982). There were further decreases through the sixth month to about half of nonpregnancy concentrations. Some of the later decrease was due to hemodilution. However, transcobalamin I and III increase during the second and third trimesters, and transcobalamin II increases sharply in the third trimester to about one-third more than in nonpregnant, nonlactating control subjects (Fernandes-Costa and Metz, 1982).
Transfer to the Fetus. The serum B12 concentration of the newborn is twice that of the mother, decreasing to adult concentrations at about 6 to 7 months postpartum (Luhby et al., 1958). The placenta