Response: Keeping abreast of new science.

Sinks and Jackson take strong issue with a recent paper in EHP by Gulson et al. (1) on the biokinetics of lead transfer from maternal bone and other body lead stores to nursing infants via human breast milk. I wrote an invited Research Highlights paper (2) that accompanied and discussed the larger context and implications of Gulson et al. While Sinks and Jackson did not include my paper in their letter, their criticisms apply to a number of issues addressed in my artide. Sinks and Jackson would have us believe Gulson et al. (1) were guilty of a public health heresy when the latter noted that there may be potential risks for nursing infants if lead intakes via breast milk are elevated because of elevated long-term maternal lead exposures. My quantitative lead exposure risk calculations for nursing infants in Table 1 make it clear that the potential impact of maternal lead burdens for nursing infants across a range of maternal blood lead (BPb) values is not a trivial matter. The clearly indicated goals of Gulson et al. (1) were to examine and quantitatively characterize 1) in vivo lead movement in nursing mothers, specifically bone lead resorption during lactation and nursing, and 2) the toxi-cokinetic interplay between endogenous (bone) and exogenous (diet) lead in the bodies of these mothers as they relate to transport of maternal lead to breast milk and then to nursing infants. Their findings document that bone lead releases can contribute significantly to breast milk lead and ultimately to infant lead intake in terms of lead source fractional input. The breast milk study was the latest in a published peer-reviewed series by Gulson et al. that used the method of stable lead isotopic ratio analysis to quantify the contribution of bone lead to BPb and lead in other metabolic compartments, and the temporal character of such inputs. The findings revealed by Gulson et al., when examined with the many studies of breast milk lead levels in lactating and nursing women, indicate that the toxicokinetic parameters governing lead transfer at low concentrations of BPb to breast milk apply to other cases where there were or are high maternal lead exposures. This especially applies to the ratio of breast milk lead to BPb concentrations, which appears to increase at higher maternal BPb levels. Neither the Gulson et al. paper nor my perspective article engaged in undue speculation about risks to …