Samples of maternal and foetal blood and of human milk, cow's milk, and dried cow's milk have been analysed for cobalamins by chromatography and bioautography. Plasma cobalamins in late pregnancy and after delivery differed quantitatively from those in healthy non-pregnant women. Differences in the proportions of plasma cobalamins were also observed between mothers and their newborn infants. The findings suggest that methylcobalamin may be a more labile plasma form of the vitamin than 5-deoxyadenosyl cobalamin. Breast milk from healthy mothers contained more deoxyadenosylcobalamin and proportionately less methylcobalamin than their plasma. Cyanocobalamin given parenterally to lactating women was detected in the plasma, and high concentrations appeared unchanged in the milk. The B12 in fresh cow's milk was found to be largely 5-deoxyadenosyl cobalamin whereas dried milk contained only hydroxocobalamin and a little cyanocobalamin. The nutritional implications of the findings, and their bearing on problems of cobalamin transport, are discussed.
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