Person:

Amarasiriwardena, Chitra

Nursing infants may be exposed to lead from breast milk, but relatively few data exist with which to evaluate and quantify this relationship. This route of exposure constitutes a potential infant hazard from mothers with current ongoing exposure to lead as well as from mothers who have been exposed previously due to the redistribution of cumulative maternal bone lead stores. We studied the relationship between maternal breast milk lead and infant blood lead levels among 255 mother–infant pairs exclusively or partially breast-feeding through 1 month of age in Mexico City. A rigorous, well-validated technique was used to collect, prepare, and analyze the samples of breast milk to minimize the potential for environmental contamination and maximize the percent recovery of lead. Umbilical cord and maternal blood lead were measured at delivery; 1 month after delivery (± 5 days) maternal blood, bone, and breast milk and infant blood lead levels were obtained. Levels of lead at 1 month postpartum were, for breast milk, 0.3–8.0 μg/L (mean ± SD, 1.5 ± 1.2); maternal blood lead, 2.9–29.9 μg/dL (mean ± SD, 9.4 ± 4.5); and infant blood lead, 1.0–23.1 μg/dL (mean ± SD, 5.5 ± 3.0). Infant blood lead at 1 month postpartum was significantly correlated with umbilical cord (Spearman correlation coefficient rS = 0.40, p < 0.0001) and maternal (rS = 0.42, p < 0.0001) blood lead at delivery and with maternal blood (rS = 0.67, p < 0.0001), patella (rS = 0.19, p = 0.004), and breast milk (rS = 0.32, p < 0.0001) lead at 1 month postpartum. Adjusting for cord blood lead, infant weight change, and reported breast-feeding status, a difference of approximately 2 μg/L (ppb; from the midpoint of the lowest quartile to the midpoint of the highest quartile) breast milk lead was associated with a 0.82 μg/dL increase in blood lead for breast-feeding infants at 1 month of age. Breast milk lead accounted for 12% of the variance of infant blood lead levels, whereas maternal blood lead accounted for 30%. Although these levels of lead in breast milk were low, they clearly have a strong influence on infant blood lead levels over and above the influence of maternal blood lead. Additional information on the lead content of dietary alternatives and interactions with other nutritional factors should be considered. However, because human milk is the best and most complete nutritional source for young infants, breast-feeding should be encouraged because the absolute values of the effects are small within this range of lead concentrations.

Despite the many well-recognized benefits of breast-feeding for both mothers and infants, detectable levels of lead in breast milk have been documented in population studies of women with no current environmental or occupational exposures. Mobilization of maternal bone lead stores has been suggested as a potential endogenous source of lead in breast milk. We measured lead in breast milk to quantify the relation between maternal blood and bone lead levels and breast-feeding status (exclusive vs. partial) among 310 lactating women in Mexico City, Mexico, at 1 month postpartum. Umbilical cord and maternal blood samples were collected at delivery. Maternal breast milk, blood, and bone lead levels were obtained at 1 month postpartum. Levels of lead in breast milk ranged from 0.21 to 8.02 microg/L (ppb), with a geometric mean (GM) of 1.1 microg/L; blood lead ranged from 1.8 to 29.9 microg/dL (GM = 8.4 microg/dL); bone lead ranged from less than 1 to 67.2 microg/g bone mineral (patella) and from less than 1 to 76.6 microg/g bone mineral (tibia) at 1 month postpartum. Breast milk lead was significantly correlated with umbilical cord lead [Spearman correlation coefficient (r(_S)) = 0.36, p less than 0.0001] and maternal blood lead (r(_S)= 0.38, p less than 0.0001) at delivery and with maternal blood lead (r(_S) = 0.42, p less than 0.0001) and patella lead (r(_S)= 0.15, p less than 0.01) at 1 month postpartum. Mother's age, years living in Mexico City, and use of lead-glazed ceramics, all predictive of cumulative lead exposure, were not significant predictors of breast milk lead levels. Adjusting for parity, daily dietary calcium intake (milligrams), infant weight change (grams), and breast-feeding status (exclusive or partial lactation), the estimated effect of an interquartile range (IQR) increase in blood lead (5.0 microg/dL) was associated with a 33% increase in breast milk lead [95% confidence interval (CI), 24 to 43%], whereas an IQR increase in patella lead (20 microg/g) was associated with a 14% increase in breast milk lead (95% CI, 5 to 25%). An IQR increase in tibia lead (12.0 microg/g) was associated with a 5% increase in breast milk lead (95% CI, -3% to 14%). Our results indicate that even among a population of women with relatively high lifetime exposure to lead, levels of lead in breast milk are low, influenced both by current lead exposure and by redistribution of bone lead accumulated from past environmental exposures.