As described earlier, the average amount of B₁₂ secreted in the milk of mothers with adequate B₁₂ status is approximately 0.33 µg/ day during the first 6 months of lactation. During the second 6 months, the average amount of B₁₂ secreted is slightly less: 0.25 µg/ day.

The concentration of B₁₂ in milk is usually similar to that in maternal plasma. In some studies, human milk and maternal plasma concentrations are strongly correlated (Srikantia and Reddy, 1967) but in others they are not (Casterline et al., 1997; Donangelo et al., 1989). The correlation appears to be stronger when maternal B₁₂ status is marginal (Fréry et al., 1992).

Current maternal intake of the vitamin may have an important influence on secretion of the vitamin in milk. In several studies of infants with clinical signs of B₁₂ deficiency caused by low maternal intake or absorption of the vitamin, maternal plasma concentrations of the vitamin were found to be normal or low normal, suggesting that maternal B₁₂ stores are less important than current maternal intake (Hoey et al., 1982; Johnson and Roloff, 1982; Kuhne et al., 1991; Sklar, 1986). This is also indicated by the observation that the length of time that mothers had been strict vegetarians was not correlated with the urinary MMA concentrations of their infants (Specker et al., 1988).

Low B₁₂ concentrations in human milk occur commonly in two situations involving inadequate intake: when the mother is a strict vegetarian and in developing countries where the usual consumption of animal products is low. When the B₁₂ status of the mother is marginal, further maternal depletion may occur as reflected in decreasing concentrations of maternal plasma B₁₂ (Black et al., 1994; Shapiro et al., 1965).

To estimate the EAR for lactation, 0.33 µg/day of B₁₂ is added to the EAR of 2 µg/day for adolescent girls and adult women; the result is rounded up.