ABSTRACT
2Breast milk is unique as a matrix for biomonitoring, providing information about the metal body burden of women as well as the exposure of infants. The heavy metals mercury, lead, cadmium, and arsenic are widespread and persistent agents with significant dose-related toxicological implications at high exposure levels. However, the interrelationships under conditions of chronic exposure are not fully known. Metal in breast milk originates from exogenous sources, i.e., uptake via contaminated air, food, and drinking water, and endogenous release along with essential trace elements, which is characteristic for the reproductional period. Metal transfer into breast milk depends on the chemical form and the distribution of the metal in maternal blood fractions. Methylmercury is strongly bound to erythrocytes. A small quantity of methylmercury passes into breast milk and is easily absorbed by the suckling infant. Inorganic mercury is readily transferred into breast milk, but is not well absorbed by infants. Lead transfer is associated with casein. Human milk has a very low casein content; therefore, the excretion rate of lead is low. Because cadmium binds to metallothioneins, the mammary gland, like the placenta, is considered to serve as a barrier for cadmium and to protect the infant. Inorganic arsenic is not excreted in breast milk to any significant extent. The suckling infant may be exposed to toxic influences in a period of highest susceptibility. Metal toxicity is dependent on the chemical form involved, which determines the bioavailability, absorption rate, and retention time. The brain is regarded as the most important target organ of toxic impairment even at low doses. There is some epidemiological evidence that prenatal metal exposure (in particular, methylmercury exposure) correlates with neurodevelopmental deficits. Yet, it remains unclear whether and to what extent postnatal metal exposure through breastfeeding impairs the infant’s health. The toxicokinetics of arsenic among neonates and infants has been scarcely reported. As environmental and maternal conditions lead to significant differences in milk metal levels, all measures must be taken to avoid additional metal exposure of infants via breastfeeding.
