This overview of vitamin B12 metabolism and requirements during the continuum of pregnancy and lactation has identified several gaps in our knowledge. More information is needed concerning the roles of the different transcobalamins during pregnancy and lactation, including their impact on placental and mammary transfer of cobalamin and their effect on intestinal absorption in the infant. Knowledge is needed about the relative importance of maternal stores and current dietary intake on fetal storage of the vitamin, and on its concentration in breast milk. Because there is some evidence that infant's urinary methylmalonic acid excretion is reduced by intakes slightly higher than the current RDA, the adequacy of the current RDA for vitamin B12 during infancy should be verified. Finally, it is possible that vitamin B12 deficiency is more common in pregnant and lactating women and their young children in developing countries than has been recognized previously, due primarily to malabsorption. It is important to confirm whether or not this is the case, in view of its potential impact on infant neurobehavioral development and the relative ease with which supplements of the vitamin could be provided.
PIP: Vitamin B12 metabolism and requirements during pregnancy and lactation are reviewed. Pregnant women who have been strict vegetarians for only a few years, and even those who consume low amounts of animal products, are more likely to become vitamin B12 deficient during pregnancy and lactation, to give birth to an infant who develops clinical or biochemical signs of B12 deficiency, and/or to have low levels of this vitamin in their breast milk. Populations that consume large amounts of animal products ingest 3-32 mcg/day, compared to 0-0.25 mcg/day for strict vegetarians. Changes in B12 metabolism during pregnancy affect intestinal absorption, changes in plasma concentrations, and placental transport. The recommended dietary allowance (RDA) during pregnancy is an increase from 2.0 mcg/day to 2.2 mcg/day to cover fetal storage. The World Health Organization (WHO) advises an increase of 0.4 mcg/day to a total of 1.4 mcg/day. Vitamin B12 metabolism during lactation involves the mechanism of secretion and forms in milk. For lactating women the WHO recommends that intake be increased by 0.3 mcg/day to a total of 1.3 mcg/day, while the RDA is increased from 2.0 to 2.6 mcg/day. There is some evidence that the infant's urinary methylmalonic acid excretion is reduced by intakes slightly higher than the current RDA, therefore the adequacy of the current RDA for vitamin B12 during infancy should be verified. More information is needed concerning the roles of the different transcobalamins during pregnancy and lactation, including their impact on placental and mammary transfer of cobalamin and their effect on intestinal absorption in the infant. Knowledge is also needed about the relative importance of maternal stores and current dietary intake on fetal storage of the vitamin, and on its concentration in breast milk. It is possible that vitamin B12 deficiency is more common in pregnant and lactating women and their young children in developing countries than has been recognized previously, primarily because of malabsorption.